Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. U. S. DEPARTMENT OF AGRICULTURE, BUREAU OF PLANT INDUSTRY—BULLETIN NO. 166. B. T. GALLOWAY, Chief of Bureau. THE MISTLETOE PEST IN THE SOUTHWEST. BY WILLIAM L. BRAY, SPecIAL AGENT, INVESTIGATIONS IN ForEsT PATHOLOGY IssuED Fresruary 2, 1910. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1910. BUREAU OF PLANT INDUSTRY. Chief of Bureau, BEVERLY T. GALLOWAY. Assistant Chief of Bureau, ALBERT F. WOODS. Editor, J. E. ROCKWELL. Chief Clerk, JAMES E. JONES. E INVESTIGATIONS IN FOREST PATHOLOGY. SCIENTIFIC STAFF. Haven Metcalf, Pathologist in Charge. George G. Hedgecock and Perley Spaulding, Pathologists. Carl Hartley and Clarence J. Humphrey, Assistants. 166 © LETTER OF TRANSMITTAL. U.S. DEPARTMENT OF AGRICULTURE, BurEAvU OF PLANT INDUSTRY, OFFICE OF THE CHIEF, Washington, D. C., September 24, 1909. Sm: I have the honor to transmit herewith a paper entitled ‘‘The Mistletoe Pest in the Southwest,” by Prof. William L. Bray, formerly a special agent in the Office of Investigations in Forest Pathology. The paper embodies the results of several years of investigation of the various phases of the mistletoe pest and an account of the meth- ods of combating it that are known at present. The subject of con- trol of the mistletoe pest is still under investigation in this Bureau. This paper, however, marks the completion of Professor Bray’s work on the subject. I recommend that it be published as Bulletin No. 166 of the special series of this Bureau. Respectfully, A. F. Woops, Acting Chief of Bureau. Hon. James Witson, 7 Secretary of Agriculture. 166 3 So ee Se OC eae > a CORT EN LS: ee Oe 2 ee es Development of flowers and fruit.............----- Eee es aaa a Distribution of seed by birds and otherwise.........----------- Structure and vitality of the seed........ See ee eee Germination and establishment of the seedling...........--.-- we) De EL ses DoS 2 ce WL eS SSE Sa ee ee OULD St ae oe ee ee 2s Ee ee aS ee a Trees most susceptible to mistletoe infection........-.....-.------- East of hosts of the mistletoe in Texas......-........----..-.----- oS Te Ee ee a UIRIREE EC SSUES eet a oe ee aihsn ewe --- Methods of combating the misiletoe..................-....---.... Possibility of eliminating old infections................-..---- Tree surgery as applied to mistletoe infection.............----- ES ie ee eee Sle as ae Re ea ae pee miteed cuore i Wise Care OF trees-. 2-222. =. 222... 222 22 ee esi The commercial importance of mistletoe..................-------- Prate I. iD ErGeuds LLED Ses ROas: PLATES. Fig. 1.—An isolated hackberry tree near Belton, Tex., with innu- merable bunches of mistletoe. Fig. 2.—A cedar elm tree on a vacant lot in Austin, Tex., showing its winter condition.........- Page. 34 Fig. 1.—A water oak tree in a creek-bottom field near Bryan, Tex. | Fig. 2.—A deformed branch of a hackberry tree which has been infected by mistletoe for ten to twelve years...........-....--.-.-- TEXT FIGURES. A seed of mistletoe pasted on a branch by berry pulp and ready to PETMINATS ss. eos ace = See ee eee en ee nee I ea . Sectional view of a germinating mistletoe seed...............--.--- . Diagram showing the method of penetrating a branch by a mistletoe seedling 12.08 Rate 5, Maik 2 Dany ataiags Bae ce gto ne ae el me . Sectional view, partly diagrammatic, of a branch infected with mis- tletoe, showing the relation between parasite and host........---- . Mistletoe on a branch of a cedar elm tree, showing the starved end of the branch and how mistletoe comes to be terminal on a branch. - . A branch of an Osage orange tree infected with mistletoe, showing deformity and multiplication of branches at the point of infection. . The trunk of a mesquite tree at Waco, Tex., deformed by long-stand- ing mistletoe infections. [295 .2.%.- sees n ee eee 166 6 34 13 13 14 16 NG 18 19 B. P. I.—515. THE MISTLETOE PEST IN THE SOUTHWEST. INTRODUCTION. The name “‘mistletoe’’ was long ago applied to the mysterious parasitic European shrub to which, centuries later, Linneus gave the technical name Viscum album, by which name it is at present dis- tinguished from other related species also called mistletoe. These other more or less closely related species have been discovered from time to time in almost all parts of the world—certainly in most of the warm countries—until at the present time a large kinship circle or family of mistletoes is known,-@mbracing more than 600 species. In the course of botanical explorations in the Western Hemisphere numerous representatives of this family were found, and among them one which so closely resembled the original mistletoe that it was given the generic name Viscum, with the specific designation flavescens; indeed, one early botanical explorer, Thomas Walter, called specimens found in the Carolinas Viscum album, under the impression that they were identical with the European plant. Later, the pioneer botanist, Nuttall, suggested that the American plant differed from the European one widely enough to deserve separate generic distinction, and so proposed the name Phoradendron (tree thief). The name then became Phoradendron flavescens. Interest- ingly enough, the specimen upon which this name was based was from Texas. In spite of the fact that there are some scores of species of mistletoe in the Americas (including the West Indies), this Phoradendron flavescens enjeys the distinction of being the American mistletoe, and has been accepted in this country in lieu of the genuine mistletoe of Europe and invested with much of its traditional and historical setting. DISTRIBUTION AND HARMFUL OCCURRENCE OF MISTLETOE. The American mistletoe is a leafy, green, parasitic shrub, com- monly found growing upon various species of broad-leaved trees throughout the Southern States and extending in more or less modi- fied forms across Texas, southern New Mexico, and Arizona to southern California, and thence northward in the coast region to 166 7 8 THE MISTLETOE PEST IN THE SOUTHWEST. Oregon and Washington. Eastward its northern limit is in New Jersey, southern Pennsylvania, southern Ohio, Indiana, Illinois, Missouri, and eastern Oklahoma. Speaking generally, mistletoe is held in rather high regard everywhere because of its inherently interesting mode of life, because of the traditional and _ historical atmosphere which invests the original mistletoe of Europe, and because of its desirability, for the above and other reasons, as a decorative Christmas shrub. The fact that it is a parasite implies a possible injury to the tree upon which it grows, but, again speaking generally, the injury is regarded as relatively slight and more than offset by the feeling of regard for the parasite. Among people who see it rarely, and, especially among those living in northern cities who purchase small sprigs or bunches of it at a good price for Christmas decoration, the idea of mistletoe being a harmful parasite upon trees is quite lost sight of. Nevertheless, there are localities in which mistletoe becomes so abundant upon trees and so harmful to them as to make the control of it or its extermination a serious practical question. © | This is notably the case in parts of Texas. Between the ninety- sixth and ninety-seventh meridians in Texas is a belt of country in which mistletoe is a serious parasite in many localities. The area most harmfully infected would be comprised in a circle of 100 miles, more or less, in radius, having its center at Austin. The north- . south extension of this area is really greater than its east-west dimen- sions, for to the eastward one passes presently into the more humid and more heavily timbered country, and westward the available hosts for the parasite become fewer. The climatic conditions of this area have a more or less direct bearing upon the question. Between the ninety-sixth and ninety- seventh meridians in Texas and Oklahoma lies a zone of transition from the humid climate of the Gulf States to the arid climate of the Southwest. The heavy forest growth of the South does not enter this belt, except as it follows the moist soils of river and creek bottoms, and even in these favorable situations the luxuriant growth becomes checked by the decreasing rainfall and drier atmosphere. Upland tree growth is stunted; the trees occur more sparsely, are broader of crown, less tall, and more frequently of imperfect growth. The trees bear evidence of a struggle against unfavorable conditions. In this region it becomes a difficult matter to secure umbrageous trees for shading and beautifying streets and parks and private grounds. Not many native species are suitable for these purposes. Very little has been done toward the solution of the whole question of selection, planting, and care of shade trees. In spite of the less inviting conditions for forest growth, mistletoe appears to flourish better in this belt than elsewhere; at least, it is 166 “7 Bt Peat DISTRIBUTION AND HARMFUL OCCURRENCE. 9 relatively more abundant than it is eastward. It is a curious fact, whose explanation is not very obvious, that mistletoe is more varied in form and relatively more abundant in the arid districts of the Southwest (e. g., portions of New Mexico and southern California) than it is in the Gulf States east of the ninety-sixth meridian. It is not that the growth of mistletoe as a shrub is more vigorous than in the eastern belt; as a matter of fact, in the more humid climate it makes a most luxuriant and symmetrical growth, but fewer of the trees relatively are infected and these evidently in more restricted localities.* In river or creek bottoms or in swamps one sees the taller trees with bunches of mistletoe far up on the remote branches. In middle Texas, on the contrary, mistletoe is by no means confined to bottom-land trees, but it infests those of upland prairies also. Stunted native growths and transplanted trees are especially apt to be infected, and not merely by isolated bunches of mistletoe on remote branches, but throughout the tree on old as well as young branches, and not infrequently upon the main trunk itself, so that the whole tree is in- fected, weakened and disfigured, and finally killed. (PI. I, fig. 1.) Thus it comes about that just in the region where trees in perfection are especially difficult to find they are more than elsewhere subject to harmful infection by this parasite. The explanation may be suggested that mistletoe, like a good many other plants of arid situa- tions, requires much sunlight for its best growth and especially for the development of flowers, and thereby of numerous and vigorous seeds, and is at a disadvantage in competing with the heavy shade- casting foliage of forests in humid climates. The necessity for light might explain why in bottom-land forests of the East mistletoe is confined to the highest branches of the tallest trees (as shown by observations made in the case of bottom-land timber in parts of Arkansas, southeastern Oklahoma, and northeastern Texas), and how with increasing intensity of sunlight and the more meager foliage and open stand of trees incident to the drier climate of the Southwest mistletoe is enabled to spread over the entire tree. That more arid environmental conditions have acted in the nature of a stimulus to mistletoe in its growth and reproduction may be alleged from the fact that in the arid Southwest there is much more variation in the habit of the plant than there is in the Atlantic and Gulf forest regions. Thus between middle Texas and southern Cali- fornia there are several distinct types or varieties of Phoradendron flavescens (macrophyllum, villosum, orbiculatum, and pubescens), besides several more distinct species, notably Phoradendron californi- 2 Exceptions to this statement are not unusual. Tuus, at Houston, Tex., at various points in Louisiana, and also in some localities in Mississippi and Tennessee, mistletoe has been reported as harmfully abundant in isolated trees or clumps of trees in door- yards and parks. 12579—Bul. 166—10——2 10 THE MISTLETOE PEST IN THE SOUTHWEST. cum, which infests many species of desert trees and shrubs, and Pho- radendron bolleanum and P. juniperum, which last two are especially noteworthy as being parasitic upon coniferous trees, especially desert junipers. BIOLOGY AND LIFE HISTORY. A brief account of the life history and of the habits of nutrition of mistletoe will assist in making clear the reason for its harmfulness to trees. The plant is a parasite. It fastens itself upon its host, the tree, penetrates its tissues, and draws nourishment from it, deforming it and sapping its vitality. Yet the mistletoe is a green, leafy plant; that is, it possesses the pigment chlorophyll, which gives the green color to normal vegetation. Some of the tropical species of mistletoe bear leaves as large as those of an American elm. The presence of green leaves indicates that the mistletoe has the power, which inde- pendent green plants everywhere possess, of constructing organic foodstuffs, such as starch, out of morganic compounds (carbon dioxid and water), utilizing sunlight as the source of energy in the process. It is therefore only partly a parasite so far as dependence upon a host for food is concerned, but apparently none the less harmful on that account. . It secures from its host apparently only what the normal, shrubby plant derives from the soil, namely, water and certain necessary mineral constituents. Imagine a grapevine or trumpet creeper, while retaining its foliage, to sever connection with the soil and to thrust root-like outgrowths into the body of the tree to which it clings, in order to absorb from the tree what before it absorbed from the soil. This would represent the relation which mistletoe sustains to its host. In this connection it is instructive to observe that the mistletoe family, viewed as a whole, shows a progressive development of para- sitism. Thus at one end of the series stands the Australian genus Nuytsia, whose single species 1s a nonparasitic tree. At the opposite extreme is the degenerate, absolute parasite Phrygilanthus aphyllus, parasitic upon a cactus of the genus Cereus in Chile. This plant is said to possess neither cotyledons nor foliage leaves, nor does it develop the vigorous shrubby habit characteristic of mistletoes gen- erally. The more familiar mistletoes are sometimes called ‘‘half - parasites,” but they also show great variation in habit from the very broad-leaved forms above mentioned down to those which are yellow- green and quite leafless; e. g., Phoradendron juniperum and Arceutho- bium pusillum, the latter of which barely emerges from its host. In the general sequence of events the life history of mistletoe is just like that of any flowering woody plant; for example, the hack- berry, upon which it preys. It bears flowers; in due time the berries follow, each with its inclosed seed; the berries are deposited by birds 166 BIOLOGY AND LIFE HISTORY. 11 or beaten down by rains upon the branches, where under favorable conditions the seeds germinate, and if the seedling becomes estab- lished upon the branch it grows again to the age of producing flowers and seed, and so on from generation to generation. Of course the mistletoe seed is more limited than the hackberry in its choice of a substratum upon which to grow, since the only situation in which it has any prospect of devéloping a plant is upon some part of a living ‘tree. DEVELOPMENT OF FLOWERS AND FRUIT. The flowers of the American mistletoe are minute and incon- spicuous, although in some tropical mistletoes they are relatively large and showy. The flowering season in Texas falls usually in December, which is approximately the date of flowering for the species throughout its range. The plant is diccious; that is, any one individual is wholly male or wholly female. Manifestly some plants never produce berries. This separateness of pollen-produc- ing and ovule-producing plants makes it a matter of importance whether female and male plants grow in close proximity and whether there are any special means by which pollination is effected. Sev- eral of the conspicuous-flowered tropical mistletoes are regularly pollinated by insects. It is claimed also that the European mistletoe (Viscum album) is pollinated by insects, but so far as has been deter- mined the American mistletoe apparently depends upon the wind to carry its pollen. After the flowering period and pollination, the development of seed and berry goes forward very slowly. With the approach of the following winter the berry begins’to enlarge and by December the seeds are ripe and the berries pulpy and white. Thus it happens that the flowering season of this year coincides with the ripening of fruits begun a year ago. DISTRIBUTION OF SEED BY BIRDS AND OTHERWISE. The seed when ripe is inclosed in a clear, sticky pulp covered by a tough, semitransparent skin, the whole constituting the mistletoe berry.* This pulp appears to be in some degree an object of food to certain birds, notably to mocking birds, cedar birds or waxwings, and to robins in Texas,” and the seeds are distributed in considerable 4 In the case of the European mistletoe the sticky pulp of the berry is utilized in the preparation of bird lime. © Doubtless most berry-eating birds feed more or less upon mistletoe berries, according to the abundance or scarcity of other, choicer kinds. Thus in Bulletin No. 120 of the University of Texas on the American mistletoe, page 7, Mr. York reports that sparrows and cardinals are the principal birds, next to the mocking birds, which feed upon mistletoe berries. 166 12 THE MISTLETOE PEST IN THE SOUTHWEST. numbers by these birds either in their excrement or by being wiped from the beak against a branch in the birds’ efforts to remove the adhesive pulp. In either case the pulp still remaining about the seed causes it to stick to the branch and by drying to become firmly pasted there. It is the conclusion of most observers that the mocking bird is the chief distributer of mistletoe seed, but perhaps the cedar birds actually distribute more, for in March and April these birds appear in flocks of hundreds in search of berry mast—especially hack- berries—and during the brief visits of a few days or a week or two all the berry-laden trees are visited repeatedly until the berries are gone. During these flights, mistletoe berries are also eaten, though probably not much noticed until the hackberry crop is exhausted. Robins also are reported to be common distributers of mistletoe seed. In the vicinity of Austin large flocks of robins spend the winter, or part of it, in the cedar brakes, where they feed largely on cedar mast; but at times they appear in numbers about farmyards and in towns, feeding upon hackberries, and during these visits also upon mistletoe berries. . : Mistletoe is, however, not wholly dependent upon birds for the distribution of its seeds, for as the spring advances and the berries grow softer they fall away easily, becoming crushed and adhering to the branch below. Especially are they beaten off in heavy downpours of rain and washed against the branches, where they are left adhering in favorable situations for germination. The char- acter of infection of certain cedar elm trees illustrates very plainly this method of infection. Pendent branches become laden with -bunches of mistletoe from base to tip, showing how the seeds were washed down the branch during a heavy rainfall. STRUCTURE AND VITALITY OF THE SEED. Of course, mistletoe seeds become planted in the ways just de- scribed in all sorts of places and in favorable weather are induced to germinate in almost any situation—upon branches, upon the main trunk, upon leaves, dead twigs, fences, and even upon mis- tletoe itseli—but the only situation in which there is any prospect of this germination resulting in the establishment of a mistletoe plant is, as previously stated, upon the living branch or trunk of a tree, and only then where the bark is thin or cleft, or otherwise in a condition to permit the penetration of the root axis or sinker of the seedling. Jn any event, the bark or epidermis of a tree would seem to be barren soil, since it dries so quickly. This is particu- larly true in central Texas and westward, where the atmosphere is habitually dry. The seeds have need to be strongly drought resistant, and this quality they seem to possess in a marked degree. 166 BIOLOGY AND LIFE HISTORY. 13 They begin to be distributed upon branches in December and Janu- ary, but favorable conditions for germination are not likely to occur before March, and perhaps not before April or May, depending upon the temperature, and when this is sufficiently high upon the appearance of a period of ramy, humid weather. Whether the seeds placed upon branches in midwinter remain capable of germination as late as April or May is not known, but of course many berries remain to be distributed as late as May. [If it be true that birds eat mistletoe ber- ries only when other choicer kinds are no longer avail- able, then the chief season of distribution would about coincide with the more fa- vorable conditions for 2 ie Fig. 1.—A seed of mistletoe pasted on a branch by berry mination. pulp and ready to germinate: a, Beak of seed coat; 3, ere a arte on truch. (All very svuch calarzed) which seeds are embedded upon a branch is a protection to them against extreme drying out and doubtless useful also in absorbing water before germination, thus allow- ing the seed to he in a moist, slimy matrix. (See fig. 1.) The seed structure proper is also adapted to the arid conditions upon a branch. The seed is invested in a fibrous coat (endocarp) quite unlike the cus- tomary hard shell of berry seeds, which is peculiarly active in absorb- ing water and transmitting it to the living parts withm. The peculiar thickening of the walls of the cells in this fibrous coat suggests the | -water-absorbing tracheid cells of sphagnum moss and of the velamen in the air roots of certain orchids, both of which types are noteworthy because of their water-absorbing prop- Fic. 2.—Sectional view of a germinating mistletoe seed: erties. ag. a, Embryo, showing knob-like enlargement of root Within the fibrous coat lies eee ee @; ¢,endospermor food » body of food material or endosperm in which the mi- nute embryo lies embedded, except for its slightly protruding root tip (fig. 2). Both embryo and endosperm retain chlorophyll pigment throughout the dormant period, but previous to germi- nation the green color becomes intensified and the presence of a gas (oxygen?) in the interspaces shows that the seed is already a carbon-assimilating body, able to proceed with its development within the moment of arrival of favorable conditions or of lying dormant and uninjured. with the return of arid conditions. It 166 14 THE MISTLETOE PEST IN THE SOUTHWEST. appears capable of repeating this behavior of alternate activity and dormancy throughout the germination period and until the seed- ling is fully established, and therefore less influenced by sudden fluctuations of moisture and dryness. In short, the seed in its germination phase is a well-adapted drought plant or xerophyte. GERMINATION AND ESTABLISHMENT OF THE SEEDLING. The first apparent movement in germination consists in the elon- gation of the axis (hypocotyl) of the embryo, whereby the root tip is thrust beyond the beak of the seed jacket, thus becoming directly exposed to the air and sunlight. Upon becoming exposed the axis bends so as to bring the blunt end of it into contact with the underlying branch. This prompt bending in its growth appears to indicate a sensitiveness of the tip toward the branch, but it has been shown that it is on account of its sensitiveness to light that the axis bends, and the bending has for its object the avoidance of too intense illumination. This, however, results in bringing the root tip of the embryo against the branch. The root end of the embryo is blunt or even knob- shaped while still within the seed jacket, and as it emerges and turns toward the branch the end further enlarges until it becomes promi- nently knob-formed even before it Fia. 3.—Diagram showing the method of pene- COMe€S into contact with any under- trating a branch by a mistletoe seedling: a, lying object (fig. 2 When, how- Corky covering of branch; b, cortex zone; c, : bast fibers; d, cambium or growth ring; e, CVE, it touches the branch, the knob wood zone; f, sinker of mistletoe penetrating becomes broader and is flattened on to wood zone; g, disk; h, body of mistletoe seed. 2 : the contact side, as when a plastic ob- ject is pressed firmly against a hard surface. Thus it behaves like an adhering disk or sucker, such as one observes in the Boston ivy, where the tips of the clinging tendrils flatten against the wall. At the center of the flattened disk the cells continue to push forward, with the result that a point of tissue, the primary sinker, bores its way, or, perhaps more accurately, dissolves its way, into the tissues of the branch (fig. 3), for the cells of this puncturing point are able to secrete a substance (enzyme) capable of dissolving the walls of cells lying in its path. Thus it really dissolves and absorbs the tissue of the host. This primary sinker pushes downward through the soft bark zone (cortex, phloem, and cambium) until it reaches the younger wood cells. Meanwhile an axial strand of cells in the pri- mary sinker becomes differentiated into water-transporting tracheids, 166 BIOLOGY AND LIFE HISTORY. 15 of which those formed in the wood zone of the host become applied to the water-carrying cells of the host, thus establishing an efficient means of conducting water and mineral nutrients from the host into the parasite. The mistletoe seedling is now virtually a tiny bud graft (fig. 3). Having thus established its connection with the source of some of its food supply, the upper end of the hypocotyl with the minute cotyledons gradually withdraws from the inclosing and partly di- gested endosperm, and becoming erect the cotyledons slowly expand as the first-pair of green leaves. Perhaps this is as far as develop- ment goes during the first season. In some species of mistletoe the - cotyledons remain coveted by the seed jacket and endosperm (if there is any) during all of the first season. Apparently the progress of development at this period depends upon weather conditions. In a dry climate these are apt to be such as to interrupt repeatedly the progress of germination and of becoming established. Existence becomes largely a question of endurance during this period, and it is probable that in a season of unusual warmth and humidity the seed- ling progresses in its development farther than merely to expand the cotyledons; whereas, in an unfavorable season, if the seedling sur- vives at all it may approach the winter with germination, in the popu- lar sense, still incomplete. This seems to be a reasonable interpreta- tion of facts and conditions thus far observed in the field, and it points with renewed emphasis to the peculiarly resistant qualities of the mistletoe at this period of its existence. In spite of these qualities, however, the great majority of mistletoe seedlings perish, or the seeds never germinate at all in the face of such extreme arid condi- tions as frequently prevail. It should be noted that the mistletoe in establishing itself as pre- viously described, with its primary sinker in contact with the wood cells, has the problem of adjusting itself to the season’s growth in thickness. This it does by maintaining a zone of embryonic tissue in the sinker corresponding to the cambium zone of the host. Some time after the seedling has established itself as described, probably not until the second season’s growth, there arise from the primary sinker in the zone of soft bark, or cortex, lateral outgrowths called cortical roots, or cortical haustoria, which spread along and around the host stem in this cortex zone, extending farther and far- ther from the original point of penetration (fig. 4). At intervals from the side of the cortical roots nearest the wood zone more sinkers arise and penetrate along the line of least resistance (medullary rays) into the wood, where they develop a connection with the water-carry- ing vessels as the primary sinker did. These cortical roots persist and increase in thickness; indeed, it appears to be the thickening of these at their junction with the parent axis which gives the enlarging 166 16 THE MISTLETOE PEST IN THE SOUTHWEST. mistletoe bush its strongly buttressed attachment to the branch (fig. 4). The presence of these persistent cortical roots interrupts the formation of host tissue exterior to them, and thus they come to lie more or less exposed along the surface of the branch. At these exposed places (possibly also from the younger cortical roots still within the cortex zone) adventitious buds are developed which may give rise to new mistletoe shoots. (See figs. 5 and 6.) They do this in considerable numbers if from any cause the original mistletoe plant is injured, or especially if it be broken off, as is most frequently done when the effort is made torid a tree of the para- site. In this way the cortical roots are stim- ulated to further erowth and to giving rise to repeated crops of mistletoe shoots. Scraping off the old plants stimulates the development of more adventitious buds and thus tends to enlarge the area of infection. It has been observed that in some trees, or at least in some situa- tions on the branch, there is no considerable spread of cortical roots and no development of Fig. 4.—Sectional view, partly diagrammatic, of a branch infected shoots from adventi- with mistletoe, showing the relation between parasite and host: : : a, Branch of host tree; 6, mistletoe; c, primary sinker; d, sinker tious buds. Thus, _ from cortical root e; f, cortex or soft bark; g, cambium or growth the case of the water ring; h, woodof branch. Thestarving and dwarfing of the branch ‘oak one may find huge beyond the mistletoe is shown at 7. : bushes of mistletoe a yard in diameter on branches eight or ten years old with not a single indication of the spread of the infection along the branch. This may be observed also in the mesquite and doubtless in any other tree where the primary mistletoe plant started on a very young branch and has developed unharmed. (See fig. 5.) In the course of time, however, the original plant is destroyed by some agency or other, but the cortical roots persist and thenceforth develop secondary shoots from adven- titious buds. In this way may be explained the characteristic occur- rence of mistletoe on old branches and even on trunks, as shown not 166 Ce a Se Se ee ee ee eee re BIOLOGY AND LIFE HISTORY. iy infrequently on mesquite and very commonly on hackberry. The cortical haustoria have persisted for years, In many cases producing an unsightly deformity of the branch or trunk (fig. 7). There is an obvious similarity between this behavior of mistletoe and that of cer- tain noxious weeds, such as Johnson grass, where from an original plant underground rootstocks spread in all directions, sending down more roots into the soil and sending up plant after plant into the air until a wide area is beset by it; also in the further respect that merely cutting off the tops only serves to stimulate the underground parts to greater activity in spreading and breaking out in new places. Fic. 5.—Mistletoe on a branch of a cedar elm tree, showing the starved end of the branch and how mistletoe comes to be terminal on a branch. RATE OF GROWTH. During the period in which the parasite is becoming established the rate of development, as already shown, is extremely slow. Dur- ing the whole of the first season the mistletoe plant -may not grow more than a quarter or half inch in length. After becoming thor- oughly established, however, growth is relatively rapid, depending, as does the host also, upon the character of the season. The shoots from adventitious buds have been observed to grow to the length of 4 to 6 inches in a single season following the spring pruning of the previously developed crop of shoots. In the case of water oaks growing in wet bottom-land soil, bunches of mistletoe have been observed to develop in six to eight years into a shrub having a 12579—Bul. 166—10-—3 18 THE MISTLETOE PEST IN THE SOUTHWEST. spread of nearly 3 feet and to be more than an inch in diameter just above the buttressed point of attachment to the slender branch of the host (Pl. II, fig. 1). The rate of growth varies with the host, being much slower in the cases of the mesquite and the osage orange. The location of the host with respect to moist or dry soils naturally also affects the rate of growth of the parasite. LONGEVITY. The parts of mistletoe embedded in the tissues of its host appear to have no fixed limit to continued existence except the death of its host. Specific data in this connection are not at hand for the Fic. 6.—A branch of an Osage orange tree infected with mistletoe, showing deformity and multiplication of branches at the point of infection. American mistletoe, but in the case of European mistletoe Tubeuf states that often sinkers are found extending through sixty to seventy annual rings of growth. There are many cases in Texas where mistletoe has been repeatedly broken from large branches during the past twenty or twenty-five years. In cases where the trunk of the mesquite and the hackberry are notably deformed it is probable that the infection is nearly as old as the host. With regard to the aerial part it would appear that any single bush would scarcely survive more than eight or ten years, chiefly because of the likelihood of its being broken off by storms, or being 166 EFFECT UPON THE HOST. 19 killed by freezing, or otherwise being mechanically destroyed. It is maintained that in extreme cold weather mistletoe has been largely killed out over large areas; for example, in the unprecedented cold wave of February, 1900. Its own bulkiness and brittleness would seem to operate toward the self-destruction of the mistletoe bush. In the case of infection upon the mesquite, spoken of elsewhere, the parasite so injures the branch upon which it sits as to kill the branch and thereby destroy itself. Usually, however, accident to the aerial part of the parasite merely results in stimulating the development of adventitious buds—and thereby in multiplying the number of mistletoe shoots upon a gradually widening area of infection. EFFECT UPON THE HOST. Perhaps in the majority of cases the original point of infection is upon a small, re- mote branch. The habit of the mistletoe is of course to draw sustenance from the branch and in _ increasing quantity as the parasite in- creases in size. The imme- diate result is to starve that portion of the branch lying beyond the point of infec- tion, and while this part may persist for some years alive without noticeable growth, in the end it dies, and the mis- tletoe thus comes to occupy the end of the branch. (See fig. 5.) This habit is particu- Fic. 7.—The trunk of a mesquite tree at Waco, Tex., = deformed by long-standing mistletoe infection. larly well shown in the water oak, where very large clusters 2 to 3 feet in diameter swing fai the end of a long slender branch not more than an inch in diameter next the swollen point of junction of the host and parasite (Pl. II, fig. 1). At the point of attachment also the branch is stimulated to excessive growth, which gives rise to deformi- ties of varying shapes on different host species. In the water oak just mentioned both branch and mistletoe are enlarged like a clumsy piece of welding. Frequently, as in the Osage orange, the branch is stimulated to an excessive formation of shoots, forming a sort of witches’ broom. (See fig. 6 and Pl. II, fig. 2.) peed branches, and not infrequently the main trunks a trees, may be greatly defoanad by the mistletoe. This happens where infection occurred when the tree was young and has persisted to misshape all 166 20 THE MISTLETOE PEST IN THE SOUTHWEST. its subsequent growth. (See fig. 7.) On the other hand, very many trees are infected for years without showing any noteworthy deform- ity, and, as a matter of fact, this item is not one of serious economic importance. Mistletoe not only causes mechanical injury, but it saps the vital- ity of the branch and when sufficiently abundant often the whole tree; and in the case of the hackberry, especially, often results in the death of the entire tree. About Austin considerable numbers of hackberry trees have been and are being destroyed in this way. At Bryan, Tex., this is true of water oaks. It has been supposed that perhaps the mistletoe merely supplants the end of the branch which was starved by its presence, and that while it receives what the branch would otherwise receive, it in turn contributes to the tree by its assimilating activity what the displaced branch would have done. This seems never to be the case at any time, and it is certain that when the majority of branches become so supplanted by the mistle- toe, the tree dies, showing the parasite to be always a drain upon its host. One of the curious results of infection has been often observed, especially in the Uvalde region, where bunches of mistletoe on remote branches of the mesquite, becoming large, so injure the branch that it decays beneath the mistletoe, which thus itself is starved and killed and presently falls away, leaving the branch free from infec- tion. This is probably due to decay induced by the starving of the branch beyond the original point of infection. THE POINT OF ATTACK. In the process of establishing itself upon the host, the critical point for the mistletoe seedling is to penetrate the covering of the branch so as to reach the food-supplying cells of the cortex and wood. The sinker can exert pressure and is doubtless able to force an entrance through fissures or through natural openings, such as lenticels, and between bud scales, as shown by Cannon.* The growing tip of the sinker has been shown to secrete an enzyme capable of dissolving the walls of certain cells lying in its path. Whether heavily cutinized walls or the walls of dead cork cells can be dissolved by this secretion has not been fully determined. Cork and bark certainly offer much more resistance to the forward growth of the sinker than cellulose walls do, and it is quite unlikely that a heavy layer of these could ever be penetrated by a mistletoe sinker. When infection begins on the old parts of a tree it must be at some fissure or thin place in the bark. Naturally the younger branches, and especially twigs of last season’s growth, present the most vulner- able points of attack, and as therefore would be expected, infection a@Cannon, W. A. Observations on the Germination of Phoradendron villosum and P. californicum. Bulletin, Torrey Botanical Club, vol. 31, pp. 435-443. 1904. 166 al TREES MOST SUSCEPTIBLE TO INFECTION. yal f begins in the majority of cases on the remote young branches, where the protective covering is thinner and the lenticels (breathing pores) still numerous. Here also the buds offer a point of attack. Cannon states that in the case of a mesquite branch he has seen as many as a half dozen successive buds bearing mistletoe. The hypocotyl! of the seedling pushes its way between bud scales to the more easily penetrated tissue at their base. Cannon also holds that even on the youngest branches the penetration of a sinker is conditioned upon its finding a lenticel or inserting itself between bud scales; but in view of the abundance and varying aspects of infection it seems necessary to ascribe a more active role to the sinker than that of mechanically pushing its way through openings and crevices, and that a less hap- hazard mode of infection prevails. However, no actual proof is at hand that the sinker can secrete an enzyme that will dissotve cutin- ized or cork cell walls. The location of the point of infection upon a tree appears also to have some relation to the perching habits of the birds which eat the berries. This idea is held notably in the case of mocking birds. Large areas of mesquite-covered country to the westward of San Antonio show a predominance of cases where a single plant of mis- tletoe occupies the most conspicuous spot upon the tree, namely,’ near the apex of the farthest spreading top branch. This appears to coincide with the choice of perching places by the bird in its flight from tree to tree. The habits of cedar birds in their repeated short flights and perching during their spring visits in search of berry mast are also such as to afford the largest likelihood of mistletoe seed being deposited on the youngest branches. TREES MOST SUSCEPTIBLE TO MISTLETOE INFECTION. It is a question whether any tree is wholly immune to attacks from mistletoe. Certainly there is reason to believe that mistletoe could be induced to grow upon any living woody plant. But from the actual status of infection in any community where mistletoe grows there are some trees which are practically immune. One of the curious things about the matter is the prevalence of infection upon different species in different localities. Thus, inthe vicinity of Austin, the hackberry and the cedar elm are the trees most frequently and heavily infected. The sycamore, though common in central Texas, both native and cultivated, has not been reported to have mistletoe in any case; whereas in the river bottoms of Arkansas and southeastern Oklahoma it is one of the most commonly infected trees. Broad-leaved elms are practically immune in the Austin region, but at Muskogee, Okla., and northward such elms are as thoroughly beset with mistletoe as the cedar elm is at Austin. At San Antonio 166 22 THE MISTLETOE PEST IN THE SOUTHWEST. and southwestward the mesquite is the chief mistletoe host. At Bryan, which lies farther within the humid belt, the water oak is the most infected species. At Marble Falls, about equally distant but westward from Austin, the live oak and mesquite are reported as the main hosts. So in the osage orange belt of northeastern Texas— notably from Greenville to Paris—that species (Torylon pomiferum) is more infected than others. In the vicinity of Calvert, Tex., the blackjack oak (Quercus marilandica) is abundantly infected, but other species not notably so. In reality, the species most infected in all these instances are either the most abundant or else the most promi- nent in certain situations; for example, along water courses or about farmyards and in towns. Even so, it is not clear why, for instance, the blackjack oak, which is the prevailing host in certain districts, should be comparatively immune in another district where mistletoe is even more common. Extending the range of observation, it is found that the dominant host for Phoradendron flavescens and its different forms varies with the geographical location, as follows: In the South, the water oak and other red or black oaks of wet soils, gums, elms, and sycamores; in the Santa Clara Valley, California, especially on deciduous oaks (the prevailing form here being Phora- dendron flavescens villosum); in southern California Phoradendron flavescens macrophyllum on the poplar, willow, ash, and others; at Tucson, Ariz., Phoradendron flavescens macrophyllum on the poplar (cottonwood), ash, and black locust, and Phoradendron flavescens vil- losum on oaks and the hackberry. LIST OF HOSTS OF THE MISTLETOE IN TEXAS. Pecan (Hicoria pecan (Marsh.) Britton). Hickory (Hicoria species). (At Columbus, Tex.) Post oak (Quercus minor (Marsh.) Sargent). Bur oak (Quercus macrocarpa Michx.). Texan oak (Quercus texana Buckl.). Water oak (Quercus nigra L.). Willow oak (Quercus phellos L.). Blackjack (Quercus marilandica Muench.). Live oak (Quercus virginiana Mill.): (Marble Falls and Comanche, Tex.; not noted at Austin). , Cedar elm ( Ulmus crassifolia Nutt.). White elm or American elm (Ulmus americana L.). Hackberry or Sugarberry (Celtis mississippiensis Bosc., and varieties). Osage orange (Toxylon pomiferum Raf.). Paper mulberry (Papyrus papyrifera (L.) Kuntze). Sassafras (Sassafras sassafras (L.) Karsten). Sweet gum (Liquidambar styraciflua L.). Apple (cultivated varieties). Pear (cultivated varieties). Cherry (Prunus species, wild). 166 » FREEDOM OF INTERCHANGE OF HOSTS. 33 Thorn (Crataegus species). Roemer’s acacia (Acacia roemeriana Scheele). Mesquite (Prosopsis juliflora glandulosa (Torr.) Sargent). Water locust (Gleditsia aquatica Marsh.). Honey locust (Gleditsia triacanthos L.). Prickly ash (Xanthorylum clava-herculis L.). China ( Melia azedarach L.). Wild China (Sapindus marginatus Wiild.). Black gum (Nyssa sylvatica Marsh.). Persimmon (Diospyros virginiana L.). Water ash (Fraxinus caroliniana Miller). Berlandier ash (Fraxinus berlandieriana A. de C.). To this list must be added the interesting case of a climbing vine (Tecoma radicans (L.) Juss.) as a host plant observed at Bryan, Tex. No doubt this list might be very much extended by a careful survey throughout the State. FREEDOM OF INTERCHANGE OF HOSTS. The question arises as to whether parasitism in the mistletoe is in any considerable degree exclusive, i. e., whether by continued growth on a given host species it becomes less capable of infecting a different species. A survey of the field outside of the Phoradendron flavescens circle shows that this sort of thing is possible, at least within certain limits. Thus there is a group of species brought together under the generic name Arceuthobium, all of them being parasitic exclusively upon coniferous trees, and some of them upon one species exclusively. More to the point is the case of the European mistletoe (Viscum album) and its circle of related forms. Tubeuf/? distinguishes three forms: (1) That infecting broad-leaved trees, Laubholz mistel; (2) one which infects fir trees (Abies pectinata and A. cephalonica), Tannen mistel; and (3) the form parasitic on pines (Pinus sylvestris and P. laricio), Féhren mistel. Tubeuf maintains that none of these three forms is capable of infecting hosts of the others, and has sus- tained his position by abundant observations in the forest and by inoculation experiments. It should be noted here that in the case just cited the form which is parasitic upon broad-leaved trees has numerous hosts—twenty-two host species are listed for a single park forest—and that a good deal of freedom of interchange among hosts is possible. It seems likely, therefore, that the central Texas form of mistletoe may be more or less easily established upon all of the hosts (at least in any given district) by seed carried from the mistletoe growing upon any one of aC. Tubeuf. Die Mistel (Viscum album) Pflanzenpathologische Wandtafeln no. 1 (text), 1906, and more recently in Naturwissenschaftliche Zeitschrift fiir Land und Forstwirtschaft, no. 5, vol. 7, 1907. 166 24 THE MISTLETOE PEST IN THE SOUTHWEST. them, and that the immunity which some trees seem to have is due to the mechanical hindrance exercised by the bark or cork, or the simple epidermis, to penetration by the primary sinker, or possibly to the density of shade, as in the case of the China tree (Melia. azedarach).’ This relative immunity of species becomes, however, a matter of practical importance as indicating one of the qualities (resistance to mistletoe) that will influence the choice of trees for planting for shade or wind-break or ornament. Further, it should be noted that the mistletoe varies considerably in habit on different hosts, and especially where the hosts occupy notably different situations as to soil and climate, being in this | respect plastic, ike the majority of plants capable of existing in widely different soil and climatic conditions. Probably the qualities acquired in any given situation are not such as to forbid a gradual or even ready tolerance for other conditions. LOCATION OF INFECTED TREES. Speaking generally, mistletoe-infected trees arethose which occupy moist soils, namely, in river bottoms and along creeks and ravines leading to the uplands. In the South generally, certain swamp- inhabiting trees (gums and water oaks) are notably mistletoe hosts. There is no doubt a reason for this in the more or less constant demand which the mistletoe makes on its host for water. In the central Texas mistletoe belt this preference of mistletoe for trees of moist situations is obvious, but 1t is by no means limited to trees of such locations. Quite on the contrary, the most noteworthy and destructive instances of infection are on isolated trees or clumps of trees in dooryards or parks and along streets and highways. There is a notable preference for isolated trees. Even in timbered bottom lands the heavily infected trees are those which stand out conspicu- ously in or about a field or clearing. (PI. I, figs. 1 and 2; PI. II, fig. 1.) In close stands of timber the average tree seems much less likely to be infected than the same species in the open. Those which are infected in heavy stands are the trees which overtop their neighbors. Perhaps the need of mistletoe for abundant sunlight explains in part its absence in close forests and the predominance of it in exposed trees. This fact is of the greatest consequence in the present con- sideration, for while the loss to forest trees in a close stand is negligible, the concentrated attack of mistletoe on isolated trees is a blow on a vital spot, for the maintenance of adequate shade trees is a difficult matter, quite apart from the réle played by mistletoe. b One correspondent states that mistletoe has been observed to grow upon China trees in southern Texas. 166 ‘i METHODS OF COMBATING: THE MISTLETOE. 29 A further item of significance in this connection relates to the extension of mistletoe-infected areas. There exist quite obviously local centers of infection. These are most easily seen in cases where a large area is occupied by an open stand of mainly one species, such as the mesquite at San Antonio and the blackjack oak at Calvert. In traversing such timber lands one notes here an area of some square rods or acres in which almost every tree is infected, followed by a long stretch of timber land practically free from mistletoe. METHODS OF COMBATING THE MISTLETOE. An understanding of the habits of mistletoe, especially with respect to the mode of spreading the infection, will suggest that the surest way to prevent further spread of infection to new hosts would be to prevent any berries from ripening. This would imply, of course, the virtual extinction of mistletoe from any given community, and it may be questioned whether in spite of its harmfulness such an extreme measure is desired. As stated in the beginning, mistletoe has in many cases, if not generally, a traditional hold upon the regard of people which goes far toward condoning its harmfulness. It is reported that a certain prominent citizen in improving his residence property caused the branches to be pruned from one of his trees and mistletoe to be substituted therefor. (By what means and with what success is not stated.) Possibly one of the chief causes of the present conditions lies in the fact that in tolerating the growth because of its biological interest and because of its traditional setting the parasite has insidiously multiplied upon trees until it has become positively a pest. Those who have tried ineffectually for a decade or more to rid valuable shade trees of mistletoe will doubtless be ready. to waive considerations of sentiment and tradition in favor of a radical movement looking to the extermination of the parasite. The immediate problem is to deal with trees which are already in- fected. Here, again, a knowledge of the habits of mistletoe will show that where the infection is upon small branches careful pruning of these a few inches below the point where the parasite is attached to the branch will effectually get rid of it. The chief difficulty in this case is simply that of getting up high enough in the tree to do the pruning properly. This difficulty would be materially lessened by employing the common device in which pruning shears are attached to the end of a pole. The shears are operated by means of a heavy cord attached to the free lever arm. No indiscriminate lopping of branches or breaking them off with ragged fractures should be tolerated. Where, however, the infection is in the larger branches or on the trunk in which the cortical roots of mistletoe have 166 °6 THE MISTLETOE PEST IN THE SOUTHWEST. ramified for years, and where innumerable sinkers penetrate the wood, the task of riddance is altogether more formidable. The embedded parts of the parasite are just as tenacious of life as are the rootstocks of Johnson grass, so that scraping or breaking off the external growth results, as in Johnson grass,in the sprouting up of more plants. Mis- tletoe, however, can be kept well under control by cutting off these successive crops of sprouts, and where this is done every year or two the trees are kept more sightly and the damaging effects of the mis- tletoe reduced to a negligible quantity. This method of treatment is the one most employed by those who devote any care to their trees; and it is here recommended as the most advisable procedure, sient in cases where a skillful tree specialist is in charge. The mistletoe plant is so brittle that it may easily be broken off, and by means of a hook attached to a long pole there is little hee culty in reaching any branches too large to be advisedly cut off. i\ q NY _* ti Fic. 2.—A CEDAR ELM TREE ON A VACANT LOT IN AUSTIN, TEX., SHOWING ITS WINTER CONDITION. All the foliage is mistletoe. ; Bul. 166, Bureau of Plant Industry, U. S. Dept. of Agriculture. PLATE II. Fic. 1.—A WATER OAK TREE IN A CREEK-BOTTOM FIELD NEAR BRYAN, TEX. Numerous bunchesof mistletoe, 2 to 3 feet in diameter, areshown at the ends of the slender branches. Fig. 2.—A DEFORMED BRANCH OF A HACKBERRY TREE WHICH Has BEEN INFECTED BY MISTLETOE FOR TEN TO TWELVE YEARS. The dwarfing of the branch beyond the place of infection isshown. The original mistletoe plant has been destroyed, leaving a decayed spot. The young shoots of mistletoe seen are from adventitious buds. OO gy INDEX. Page Abies spp., Europe, mistletoe infection...... fs ERE ee ee ae Acacia roemeriana, susceptibility to mistletoe infection..................----- 23 Roemer’s, susceptibility to mistletoe infection.-................---.---- 23 Apple trees, susceptibility to mistletoe infection .................-.-.-------- 22 Arceuthobium pusillum, scanty development.......-.......---------------- 10 Arizona, trees most susceptible to mistletoe infection.................--.------ 22. Ash, Berlandier, susceptibility to mistletoe infection. ........-.......------- 23 prickly, susceptibility to mistletoe infection............---......------- 23 water, susceptibility to mistletoe infection...................--.------- aa Berry, mistletoe, development coincident with flowering season of following COE nt fe ee eR ee eee 11 Panetta ia, MCE Oe MMLC oe sk I e+ ne 10-19 Peer Ss eee are aS Oe LE WWE. Oe ns ee es - 2 = - 5 11 Birds, cedar, distributers of mistletoe seed.t...............-------------- 11-12, 21 ee OS: CE ee ee ee 11-12, 21 mocking, distributers of mistletoe seed....................-.--------- 12. 21 perching habits, relation to mistletoe infection.................-...--- 21 Blackjack oak. See Oak, blackjack. Pen, Charmcet Go mioLirio’ secdling-. - 0c... 25--<-2.2--.-.------- 15 scales, mistletoe invasion .- eae Ys RT NN DE e, 0Or ee 21 Buds, adventitious, craiapleation eae pruning or scraping......-.-..--.-- 16,17, 32 Je a Sa Bea ee eee 21 Burlap, wrapping for branches after pruning mistletoe .-......-.-.......-.---- 27,33 Cactus, mistletoe infection in Chile.........................-. Pe os peti Be 10 California, trees most susceptible to mistletoe infection, list................... 22 Carbolineum, use in destroying mistletoe. ....................-.--.-.------- 27,33 Carbon, power of assimilation of mistletoe seed..................-.---------- i Cedar birds. See Birds, cedar. Cells, tree, dissolving by enzyme secreted by mistletoe...............-- 14, 20, 21, 32 Celtis mississippienses. See Hackberry. Chemicals, use in extermination of mistletoe.........................-------- 27, 33 Cherry, wild, susceptibility to mistletoe infection. ............-.......-.---- 22 Chile, infection of cactus with Phrygilanthus aphyllus....................... 10 China tree, susceptibility to mistletoe infection........................--.--- 23, 24 wild, susceptibility to mistletoe infection...................-.-.--- 23 Chlorophyll pigment in mistletoe embryo and endosperm...............-..-. 13 Climate, conditions, relation to mistletoe infection and development ... 8-9, 15, 19,31 Seeeeame eta WMO 7. 20-8 es. 27, 28. 29, 33 (ED EE Se A eer SC aeee gr 19 Conifers, European, susceptibility to mistletoe infection....................--- 23 susceptibility to Phoradendron bolleanum and P. juniperum...--.--.- 10 Copper sulphate, use as antiseptic wash for tree wounds.....................- 29 Cork cells, invasion by mistletoe problematical.......................- 14, 20, 21, 32 Corrosive sublimate, use as antiseptic wash for tree wounds..................- 28 Crataegus spp., susceptibility to mistletoe infection.....................-.--- 23 Deumnties carded by mistletoe. 2205.25.22 2s. en see ----- 9, 18, 19-20, 29 166 ; 35 86 THE MISTLETOE PEST IN THE SOUTHWEST. Page Diospyros virginiana, susceptibility to mistletoe infection...............-.... 23 Drought resistance, power of mistletoe seed and seedlings................. 13-14, 15 Elm; ‘cedar, protection tromanistletoe infection. 2-5- ces 245455 = ee 29 susceptibility to mistletoe: iniectiom. 2-222. 92e2 ee oe 2 22 OG Elms, broad-leaved, immunity to mistletoe in Texas.-....................-...- ot susceptibility -to mistletoe infectiomasee 52 a. eee eee een eas AO Endocarp, mistletoe seed, water-absorbing power...........--...----.------- 13 Enzyme, mistletoe seedling, power of dissolving cell tissue of ‘host... ..- 14, 20, 21, 32 firitrees; Europe, imistletoe miectiom. 7s. 6 ee eee ee 23 Florists: handling of mistletoe... es ee eee eae ee 31 Flowers, mistletoe, development, blooming season, and means of pollination. . dia Fraxinus spp. See Ash, Berlandier, and Ash, water. Fruit, mistletoe, development coincident with flowering season of following VOAL on Sosy Sala ee Se I eee a eee ne ee 11 Germination, mistletoe seedlings = sae es 14-17 Gleditsia spp. See Locust. Graft; bud, character ot mistletoe seedlingss 35. oee eee ae a eee 15 Grass, Johnson, rootstocks, behavior similar to that of mistletoe roots........-. LAD Growth, mistletoe, rate in different localities and on different trees........... 17-18 Gum, black, susceptibility to mistletoe imieetions: | 9 a eee 23 sweet, susceptibility to’ mistletoe infection 42.245 .2 2 4-= = = eee i Hackberry, delormity ‘by amistletoes- se = 2 ne eee 18 destruction: byemustletoe =.=. 2 oe eee see eee - 20 protection from mistletoe miechione=--- 4-2 s-. ea ee ee 29 susceptibility, to mistletoe infection=>- 225.2224) 2-55-- 4. 10, 16521, 22,29 treatment for protection agamst mistletoe.-----.-2..-22--2---2--— ih Hickory, susceptibility to mistletoe miecetionts=2 esa 5- ae 4 sae ere Sergi) Host of mistletoe, death, cause of death of mistletoe...........-.....-.2.-.-: 20 effect: of infections. aay 5 te ea ee aa es ee ee 19-20 Hosts of mistletoe, freedom ofanterchances:: 2 {ee us = eee ae es ee eee 23-24 in Vexas; liste ieee SON g ee oe ee ee ee ee 22-23 various localities, Viste sos = eee es ee eee 21-23 Infection, mistletoe, application of tree surgery ....--..--------------- 2a. 28-9548 elimination, directions sss. 05 ke ee 2i,.a0 methods ef preventing spread ae See es eee age 25-30 points obattdek. 1: ss rae tate tee eae ea 21 prevention, ‘care/ot trees: ses = a ee eee 29-30 trees susceptible, various locales. Ess etree, oa APM Rens Seat 21-23 Insect pollination of AVase umm jall bois el ee en eee bE Introduetion=tovbulletimis ee ee ae ee eee a al ene Se See 5, eee Oe ara ee: Johnson grass. See Grass, Johnson. Lenticels> tree: bark; mistletociattiack 2-3. 3 = sae ee ee eee PALL Life history, and>* biology of mistletoe. 2. 32. see ee ee eee 10-19 Lime, bird, use.ci pulp-of mustletee bertyees) = = ee ee ee 11 Liquidambar styraciflua, susceptibility to mistletoe infection.............--.-- 22 Locust, honey, susceptibility to mistletoe infection. 2-22-2232 52- 2 =e ee 23 water, susceptibility to mistletoe imfection 2-222 59- 4.22 so eee 23 Melia azedarach, susceptibility to mistletoe infection........- ppibag adhe Oi eo ng Sr 23, 24 Mesquite, area of infection by mistletoe......-.-.- iL cafe th Reteiete AE tary ain ee AEN ee 25 deformity by: mistletoes22222 asses he eee ee eee 18 infection by mistletoe, description and rate of growth..........-.--- 16, 18 “susceptibility to mistletoeumieetion=:-225. 52 3-62 en ee ee 22,23 Mistel varieties. See Viscum album. 166 INDEX. Set 3 Page Mistletoe, Australian, a nonparasitic tree.......-....---.------- 22 eee eee eee 10 belt, Texas, serious character of infection.........-.-...-....---...- 8, 31 berry, development coincident with flowering season.....-.-..-..--- 11 pulp, use in distribution and protection of seed........-..---.- 12,13 Prepare droid Innes 7S os) Dect... - 11 umerse rani te Wi gy SS oa Sy = 1 et ee See. 2d... 10-19 SUES Terie 02 ake pee oe eee ee ae 31 Mennrietiamne we OM CaRNCt oo sel reey bce Sob. eben eee 19 OU TACT UPS ge 6 Ser ee ae a 20 gee ire cel ge Os en ee 19, 20 dicecious character. . ie ee 11 distribution and haronial occurrence, ee canis aaianee t 7-10, “A, 24, 32 eI UINT RINNE tee hata ee a azn sb ee ete te 19-20 embryo, development and adherence to host plant..............-.. 14, 20 Patlopeatint pecinaty Waiter. goa Fee ke ee 23 See also Viscum album. execrimitiens. Wee. ot choniesis. (2270 5d Sis iS. RS i es flowers, development, blooming season, and means of pollination. - int eet, Oc Beret » ON Season 2 ne = So eb ee SS 11 growth, rate, different localities and on different trees..............- 17-18 haptear, notiner limit im United States: .:-2-:.-.-.2.2---.-aame-..-- 8 Spumante ee ee Re oe ea ade. 2 31 hosts, freedom of interchange....-.- MAZE oe asd ot et eee 23-24 ee Ra ett eet in a. . 22-28 ete eee Mier MAIO he sen See Sk ee cS ee eee eee ee 24-25 Mipeian ap niicaion ol ites SUNBEry 222525222056. 2. 2.---.... 28-29,33 oii AOROG MUMGAUOTADIE 2.2 205.2 2.525 is ee 9, 24 Metmsdds of provemtme spread. =. 22 2 ffs 25.22 tele. 25-30 peevetiinl eaten tees ys yee Se bb. ek eee 29-30 trees most susceptible, various localities................ 21-22 infections, old, possibility of elimimation.............-..-----.---.. 27 SS Ee See Re ee On ee - 18-19 origin of name and classification by botanists............-:...-..-- 7 SLU ee OS OP Bee ae Sh ce 5 ae ee 20-21 pruning or scraping favorable to reproduction from cortical roots.... 16,17 resistance, importance in choice of trees for planting.............. 24, 32 roots, ees eS EINE ye nS or 2 a ee eg ae 18 seed and seedlings, power of drought resistamce.. -.. 2.22. -2....: 13-14, 15 ditriboison by-birds and’ otherwise. 2... <2. s-. 2... See. 11-12 endocarp, water-absorbing power........--...--..-......-...- 13 powEte. asewmutiatine Carbon: 2.250250 .2es2 eae oS es 13 permerie ameaGy e822: tee oe Se a Sak 12-14 Mie MMNNEE DOWOT? 2. <0 722 twas es oak EA eS 13 seedling, enzyme, power of dissolving cell tissue of host... .. 14, 20, 21, 32 eernunaien and establishments. .5..< 2/55/2200... 5 02. oe 14-17 sinker, development and adjustment to growth of host.. 14, 15, 32 mnie ae Die rales) le 2 Pe ee ek ae. 15 South American species, an absolute parasite on cactus............ 10 Mocking birds. See Birds, mocking. Mulberry, paper, susceptibility to mistletoe infection.................2...... 22 Nuytsia, Australian mistletoe, a nonparasitic tree. .........................- 10 Oak, blackjack, susceptibility to mistletoe infection.......................... 22,: 166 38 THE MISTLETOE PEST IN THE SOUTHWEST. Page Oak, swater-desizuction by mistletoe: e:sola 5 eee eee ee eee ee 20 mistletoe amtection, deseripiigny: 4:22 a5 eee eee ee eee 16, 17,19 Oaks, susceptibility to mistletoe mfection. -_--.........--1..... 16, 17, 19, 20, 22, 25 Orange, Osage, susceptibility to mistletoe infection.......................- 18, 19, 22 Osage orange. See Orange, Osage. Paimt, asphalt; for.tree wounds 5.225. 3 s seca te nae eye et ee eee 27, 28, 29, 33 use: in destroyane muspletoeae = ee eee eee Dinas Papyrus papyrifera, susceptibility to mistletoe infection....................- 22 Parasitism, progressive development in mistletoe family.._................... 10 Pear trees, susceptibility to mistletoe-miectiom._ - 3.4920 aye ee 22 Pecan trees, susceptibility to mistletoe iniection. _---3...5. ee se eee 22 Persimmon trees, susceptibility to mistletoe infection.....................--- 23 Phoradendron flavescens varieties, dominant hosts in various localities... ...- 22 generic name of American mistletoe. .......- Bt Sk Sh ad ee ee a juniperum, scanty .development.so2h0- =a eae ee 10 : species and varieties in arid Southwest. _22--..11.24.0.--... =: 9-10 Phrygilanthus aphyllus, South American mistletoe, an absolute parasite on CaCtUS.. 25. 62 heb d oes BS 2 BSS a ee ee ee 10 Pines, Hurope; mistletoe infections S=-o40s 2 Sa. ae eet eee eRe RMT ee 23 Pinus spp.; Murope,. mistletoe smiee tonite eee eae 23 Plates, description :: 425. 2223 3. oe en See ee eee 34 Pollination;: naistletoe flowers is. eae eh ee ee eee 11 Pruning, careless, damage to: treess-_ 2:24 22-2 see ee ab eee Se ee 25, 28 mistletoe, favorable to production of adventitious buds.............- 16, 17 for extermination, directions and cautions...............-. 25-27 Prunus sp., susceptibility to mistletoe infection in Texas. .....-...-2-.2--5--- 22 Quercus spp. See Oaks. Rainfall agent im mistletoe seed’ disttibuimons - 42°72. Le ee 12 Robins; distributers:of mistletoe seed jase se See es eee ae ee 1x | Root, mistletoe, lateral growth and infringement on host...-..........---.-- 15-17 seedling, adherence’ to:host plant. 520-2 eee 14, 20 similarity to Boston! Ly yc reOts. 2 eo N- e e 14 Roots, mistletoe, behavior similar to that of Johnson grass rootstocks.......... 17, 26 cortical \deseriphion:/ 222 eens See oe rns ae 15-17, 32 Lomge vit yee = ios) AE a a eh pales ages eee A ee 18 means of feeding: onchost: plante. 2a222 9 kee ae ee 14-15 Sapindus marginatus, susceptibility to mistletoe infection. .........--...---. 23 Sassafras, susceptibility to mistletoe infection... .---.- Se lg gue tn ge ae eae 22 Seed, mistletoe; developments22c2 25522 2 eo ee eee ee ee ee ee i} distribution by birds and otherwise. . -..--..------------- 11-12, 21 drought, resistance ss: tis ee eae is ae eee eee 12, 13-14, 15, 32 endocarp, water-absorbine power it. e, see sane ee 13 power of assimilating carbons Ga 35 2. etree rae eee 13 structure; vitality, amd tmermmnnbione 25a ae ae 12-17 Seedling, mistletoe, a budigraits.cs oe See ee eae 15 drought Tesistamee:: ¢ 2./B5Se Et ee aie ee See 13-14, 15 enzyme, power of dissolving cell tissue of host... -- 14, 20, 21, 32 germination and establishments-s225 32s eS) Aaa 14-17 sinker, development and adjustment to growth of host. 14, 15, 32 Shade trees. See Trees, shade. Sinker, mistletoe seedling, development and adjustment to growth of host. 14, 15, 32 See also Root. 166 INDEX. 39 Page. Soap, laundry, for washing branches after pruning mistletoe, unsatisfactory RIE = 2 ae fe te ee ee oe ee se ----- 7 Sugarberry. See Hackberry. f° SS 31-33 Surgery, tree, as applied to mistletoe infection..........-.-..----.----------- 28-29 Sycamore, immunity to mistletoe infection in Texas. .........-......------- 21 susceptibility to mistletoe infection outside of Texas...........- 21 Tar, coal, for painting wounds aiter pruning mistletoe................. 27,28, 29,33 Tecoma radicans, susceptibility to mistletoe infection.........-...-..--.---- 23 ee 8 Ee Se ge Sa en 8, 25 een wee nies SOMME Se. oP nk eis. -- 8,31 trees susceptible to mistletoe infection, list. ...............-......... 22-23 iar, HusecanliLy 10 Miiniictoe mieciton:.-. -_ 2.2. --:-=--.-.----------- 23 Toxylon pomiferum. See Orange, Osage. Tree, China, susceptibility to mistletoe infection. .......-.-.-- pees Foon 6. Cee 2 23, 24 wild, susceptibility to mistletoe infection...................---- 23 surgery as applied to mistletoe infection -.-..........-....--.------- 28-29, 33 wounds, treatment, directions -.....-.- See aie eee 2 at Bl, 20-29, OD Trees. broad-leaved, Europe, mistletoe infection.. Bs ONE Oy ee ee 23 Pore a) [RoVeUL MHinLICLOe MiCCKON.-.. 22.2.2... 2-2-2. -+--2:--- 29-30 eer eae MNEEENE 2 ree Oe fee ac ace ee -x~se.-3. 25,28 Geupenesed canned Wy mistletoe ..--...----.. 2. -2. 2-62 s2.s +. 9, 18, 19-20, 29 eueeenNN Ie GILMER 222 omen ee es et Bee ces ltt tss--- 9,20 er Seale, ees oe nee 29 infected with mistletoe, location... .-. Pe po OPE Ee ee ae 24-25 isolated, mistletoe infection, causes and seriousness of problem....... 9, 24, 32 shade, destruction by mistletoe................--.--.----- Petes eh 8, 32 in Southern States, difficulty of selection........-............-- 8, 24 stunted, susceptibility to mistletoe infection.......................-... 9 susceptibility to mistletoe infection, various iaeukies Bt: So es 21-23 transplanted, susceptibility to mistletoe infection.-..................-. 9 transplanting, advantage of using small uninjured trees.._.-............ 30 Eo atl a ee ora sel) eee Se ae ee ee 29 See also Apple, Ash, Cherry, Conifers, Elm, Fir, Gum, Hickory, Locust, Mulberry, Oak, Orange, Pear, Pecan, Persimmon, Pines, Sycamore. ae. +. comin relaitve uy Viscum albamt.o¢. = 52.2 - soc 25 22 22. kee 18, 23 Viscum album, identical with European mistletoe.................... 2.2... 7 ee 0 Re eer ee ee 11 EES ene eee Ss Seer Se 62 ee OE oa te oS ee Bee AS 12-14 Washes, antiseptic, for tree wounds, formulas..........................-..-..- 28-29 Water oak. See Oak, water. eS a ee 27, 28-29, 33 Xanthoxylum clava-herculis, susceptibility to mistletoe infection............. 23 Xerophyte, seed of mistletoe, character................-.......---.--- oe 12,14 166