oe ne ae ee - paws gh sch RSs eter vettie> hw badly 0 onl ‘ — , = se eh ola th aiaad SRO eng yee DREYER EE: AP ay 3 ¥ , - 5 senate) : AEST Gy eateN A Boece mA ae ¢ tage A ete A ee YORI ot Belg, Stain etn Sen iS 14) Saad | . ~ a é ‘ ‘ Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. UNITED STATES DEPARTMENT OF AGRICULTURE ,; BULLETIN No. 317 | Contribution from the Bureau of Plant Industry WM. A. TAYLOR, Chief Washington, D. C. PROFESSIONAL PAPER January 20, 1916 LARCH MISTLETOE: SOME ECONOMIC CONSIDER- ATIONS OF ITS INJURIOQUS EFFECTS.’ By JAMES R. WEIR, Forest Pathologist, Office of Investigations in Forest Pathology. CONTENTS. Page. Page. MRO CUGHIOM awa Tee eS Ne 1 | The effects of mistletoe on its host_- af Themlanrchemistletoes =. ses ws Ss 3 | Effect of mistletoe burls on the mer- ERIC RRE OTE Si mee ee eee Fe eas See 3) | cchantability of darchvtrees=2 = == DD Physical and climatic features of-the Methods ot controle? s=2=2 sas ease 23 ORCS tale OM ee eee es on He CONCLUSIONS = see ee ee See 24 Fungous enemies of the larch______ 11 INTRODUCTION. During the past four years, in connection with other pathological problems in the forest, the writer has made an extensive survey of the damage to forest growth by some of the mistletoes of coniferous trees. These parasites are very widely distributed in the forest regions of the Northwest, and occur in such abundance in many localities as to assume a very serious aspect in relation to many forest problems. The extent and nature of the injury done vary greatly with the forest type, the topography, and, in some respects, with the chmate. This is well shown in the regions in which investi- gations are now being conducted. In the dense part of many forest regions, as in the vicinity of the great lakes of Idaho, mistletoe does but little damage. However, in the more open stands bordering on the lakes or along the edge of the valleys of the Priest and Pend Oreille Rivers mistletoe occurs so abundantly on the various conifers as to interfere seriously with the development of some of the more valuable timber trees. About the shores of Lake Coeur d’Alene, 1 The writer wishes to express his thanks to Mr. J. F. Pernot, without whose assistance the analysis of the trees would have beer difficult, and to Mr, E. E. Hubert for assistance in the tabulations. 8521°—Bull. 317—_16——1 EE 2, BULLETIN 317, U. S. DEPARTMENT OF AGRICULTURE. along the Spokane River Valley, and extending to the south into the Blue Mountains of Washington and Oregon the mistletoes are very abundant, especially on lodgepole pine (Pinus contorta), yellow pine (Pinus ponderosa), western larch (Larix occidentalis), and Douglas fir (Pseudotsuga taxifolia). In many localities the trees rapidly yield to the suppressing effects of these parasites, causing an open, ragged growth of the crowns, with the production of many brooms. The prevalence of a particular species of mistletoe varies greatly in the same general region. To illustrate: Along the hills fronting on the Pend Oreille River, Idaho, the lodgepole and yellow pine stands are heavily infected. Passing up the Priest River Valley, another mistletoe species appears, working considerable injury to the larch, whereas the same tree, wherever it occurs along the Pend Oreille River, is seldom infected. The yellow pine farther up in the Priest River Valley is not seriously attacked. In the Granite Creek drainage area and beyond the mountains to Sullivan Lake, in the Metaline Falls region of Washington, the larch is again very seriously infected, whereas this mistietoe seldom occurs on the divide between these points. The western hemlock (7Z'suga heterophylla) in the forests of northern Idaho is practically free from mistletoe, as far as the observations have been carried. In a few of the more open valleys several collections of mistletoe have been made from this tree. At many points in Washington and British Columbia where the writer has had an opportunity to collect, the mistletoe on the hemlock seems more abundant. Numerous collections of mistletoes are at hand from many of the forests of southern Montana, and likewise from the northern part of that State and from central Idaho. A trip through Oregon, Washington, and British Columbia during 1913 yielded much information on the occurrence of mistletoes in those localities, so that it will soon be possible, with the additional data now (1915) being collected, to give a fairly detailed statement of the range of these parasites in the principal forest regions of the Northwest. In order to obtain reliable figures on the damage to forest growth by these plants, special studies of a directly practical nature are now being conducted in several of the most important forests of the regions indicated. It is believed that the results from these studies will be applicable to all the forest areas of the Northwest where trees of the same species are found infected by the same mistletoe. At the same time, work of an experimental nature, both in the field and in the laboratory, is adding to our knowledge of these parasites. This work is being continued, as having a practical bearing on the mistletoe problem, and will be reported upon as time and occasion permit. ; eR a LARCH MISTLETOE. 8 THE LARCH MISTLETOE. This bulletin deals in the main with the immediate practical results of an investigation of the injurious effects of the larch mistletoe on its host in the Blue Mountain region of Oregon and serves to introduce one of a series of studies on the mistletoes of coniferous trees in general. The larch mistletoe (fig. 1), originally named Pazoumof- skya douglasii laricis by Piper? and given as a subspecies of the Douglas-fir mistletoe, has recently been raised by the same writer” to the rank of a full species under the following name and description : Razoumofskya laricis Piper. Pistillate plants olivaceous. clustered, 5-8 cm. long, branched; joints 1.5-2 mm. thick, sharply 4 angled; staminate swollen, yellow, the flowers in short spikes; lobes ovate, acute; fruit oblong, acutish, bluish, 4 mm. long. Common on Larix occidentalis. The investigation was begun in the Whit- man National Forest, Oreg. For some time the general and grad- ual deterioration of the western larch had been reported as occurring throughout the entire Blue Mountain region. The writer was not aware of the great prevalence of the larch mistletoe in this region until his visit there durinoc the “early. spring of 19138, From a2) preliminary survey it I'1g. 1.—Staminate plants of Razoumofskya laricis. Note soon developed that the hypertrophy of the branch. the primary cause of the deterioration of the larch resulted from the suppressing effects of mistletoe. A probable secondary factor on some of the more exposed sites seemed to be certain climatic influences unfavorable to the host but promoting the better development and spread of the parasite. | THE FOREST. The Blue Mountains, in which further studies of the mistletoes are in progress, are well covered with forests. The yellow pine pre- 1 Piper, ©. V. Flora of the State of Washington. U. S. Nat. Mus., Contrib. Nat. Herbarium, v. 11, p. 222. 1906. 2 Piper, C. V., and Beattie, R. K. Flora of Southeastern Washington and Adjacent Idaho, p. 80. Lancaster, Pa., 1914. 4 BULLETIN 317, U. S. DEPARTMENT OF AGRICULTURE. dominates as the principal tree on all the drier slopes and bench lands. This gives an open character to the forest and is of some significance as regards the growth of mistletoe on the larch wherever this tree is associated with the yellow pine. On the lower eleva- tions the yellow pine often occurs in pure parklike stands, with a ground cover quite characteristic of the typical yellow-pine forma- tion, usually indicated by the absence of any great amount of forest litter and by the uniformity and the small number of species of her- baceous and shrubhke plants. On the south slopes and low, dry ridges, where the stand is very open, the yellow pine is quite gener- ally infected with its particular mistletoe, working great injury to the tree. At higher elevations in more moist situations, or even at the same level on protected parts of the typical stand, the yellow pine becomes mixed with larch, Douglas fir, white fir, and lodge- pole pine. The yellow pine gives way to greater percentages of larch, Douglas fir, and lodgepole pine on the north and east slopes. The two last-named species support large quantities of their respec- tive mistletoes wherever the conditions are favorable for the devel- opment of these parasites. The larch predominates in many north- slope stands, especially in the more open situations. Other forest types in which the larch occurs above 6,500 feet or more are of little importance in this connection, since the species of the types at this elevation are not as seriously infected by mistletoe as those on north slopes of 5,000 feet altitude and less. The influence of drainage, slope, and the general moisture condi- tions of the soil on the distribution and vigor of the western larch is well shown in the region studied and is likewise reflected in the prevalence and distribution of its principal parasite. Owing to the general prevailing dryness of the region, the maximum development of the larch is attained in moist draws or in fertile valleys not par- allel with the direction of the prevailing winds. In such situations the tree is usually quite free from mistletoe, and uninfected trees often attain a diameter, breast high, of 60 inches or more. A full crown composed of the original branches is retained until late in life, the tree showing few defects except an occasional root-rot or a dead top occasioned by agents other than mistletoe. These situations are more favorable to the development of the host than to the mistletoe occasionally found upon it and must be considered the best sites for growing larch in these regions. On the drier slopes and benches, where the larch is associated more with yellow pine, the influence of the site on the vigor of the mistletoe is at once expressed by its oreater abundance and its effects on its host, causing smaller diame- ters and thinner crowns on the infected trees. Occasionally trees in exactly similar situations for some reason escape the ravages of the mistletoe and attain a size of considerable proportions. The full Ee ee a e >. a ¥ LARCH MISTLETOE. 5 crown and degree of vigor shown by these trees late in life prove conclusively that the ragged, suppressed condition of their neighbors is not due wholly to unfavorable climatic or soil conditions, but to the effect of the mistletoe upon them. On some north slopes where the larch is crowded by lodgepole pine and white fir it becomes suppressed for a time very early in life, as indicated by the zone of suppression in the older trees. Those trees _ finally escaping by their more rapid growth from the influence of their neighbors usually become infected by mistletoe when the crown spreads out to the light and air above. The opportunities for the mistletoe to attack suppressed trees with crowns overtopped by other species not subject to its ravages are not as great as when the trees are standing more in the open. This is due in part to the other species protecting the larch from seed falling on it, and in part to the fact that permanent tissue incapable of being penetrated by the primary sinker is more rapidly developed in the case of suppressed individuals. New growth is of short duration and fewer vulnerable points of easy infection exist. If the infection of the suppressed trees does occur and the infection succeeds for a time, the mistletoe plant may itself become suppressed, partly from a poor nutrient rela- tionship with its host and partly through lack of light, and eventually may die without producing new infections higher up. The signs of old infections are frequently noted in the area of the zone of sup- pression in trees that have afterwards escaped from the crowding of their neighbors. If such trees again become infected later in life, they may attain a fair merchantable size before the influence of the parasite is made manifest. PHYSICAL AND CLIMATIC FEATURES OF THE FOREST REGION. The later geological history of the Blue Mountains, in which the Whitman National Forest is located, is one of a great basaltic uplift surrounding but not submerging the older granitic formations. The several high and low laterally arranged ridges are composed in the main of granitic rocks, forming a type of soil upon which the yellow pine usually becomes the climax species. Other soil characters in- duced by local variations of climate, slope, and type of ground cover influence the distribution of the forest trees of this region to a marked degree, and indirectly that of the mistletoe. Summarizing the chief climatic characteristics of the region, com- piled from the reports of the United States Weather Bureau, they are (1) scanty rainfall, (2) wide range of temperatures, (3) low absolute humidity, (4) rapid evaporation, and (5) an abundance of sunshine. The influence of such climatic conditions may be con- sidered in general as unfavorable in a few localities to the best 6 BULLETIN 317, U. S. DEPARTMENT OF AGRICULTURE. development of the larch, but decidedly favorable to the mistletoe found upon it. This is at once evident to those familiar with the environmental requirements of host and parasite. The region affords a most instructive study of the advance and predominance of a forest- tree parasite on its host, showing this advance to be in as near an exact proportion as the conditions for its SPR development become more favorable. The problem of the mistletoes in their ecological relationships, re- - gardless of the fact that they are parasitic, is similar to that of all chlorophyllaceous plants: hence, they respond to light, gravity, and Fic. 2.—Cross section of a part of a trunk of a larch tree, showing the regeneration of branches from the same whorl to the fourth generation. (Tape in feet graduated in tenths. ) chemical influences, and in a far less degree to the influences of tem- perature and moisture. How, then, do the ecological requirements of the larch mistletoe hold with the climate of the region described, over which the parasite is widely distributed? The great variation in temperature, occasionally abnormally high, and the late, early, and winterkilling frosts of some sections, although seriously injuring the host, produce but little effect on the parasite. The uniform dryness of the air at all seasons of the year throughout the region does not greatly influence the mistletoe plant, which is essentially xerophytic. On the other hand, the large percentage of sunshiny days and the ab- sence of clinging fogs are directly favorable to the parasite, as it is rially aided by the insecure rooting of the LARCH MISTLETOE. 7 positively phototropic. The possible influence of the low absolute humidity and rapid evaporation on the entrance of host, reproduc- tion, etc., is counteracted by the parasite by means of special struc- tures enabling it to withstand long periods of drought. Probably no factor of the region so greatly aids the destructive effects of the mistletoe on the larch as the high, strong winds so prevalent in these mountains. The velocity of the winds is sometimes very great. During 1913 hundreds of reserved yellow pines on the sales area of the Whitman National Forest were up- rooted. The wind in this case was mate- trees on the surface of a hard stratum of rocks and gravel, together with a cer- tain amount of decay in the brace roots. This is a condition often found in cases of this kind. Strong winds probably do greater injury to the larch than to any other conifer. An examination of the branching or crown of a mature or middle-aged healthy larch will show that in most cases, especially in windy regions, the tree has been able to reach the standard size only through the production of several generations of branches replacing those broken off by the wind and by other causes (fig. 2). The loss of branches through crowding or natural pruning is not here considered. Trees standing under open conditions from the beginning will show this interesting Fic. 3.—A larch tree of greatly phenomenon of regeneration. Increasing = faves’ visor ae ene age, within a certain limit, on the part of brooms and the accumulation the main trunk doe’ not interfere with the — ° {*" he eae tee ce anatomical and physiological connections _ the branches, also the witehes’- bfeolde branches: Consequently, branches °r°™s * the Upper Portion : : : of the crown, are dying. forming at any age sufficiently high on the trunk to escape the influence of suppression should and would remain intact, barring all deteriorating influences, during the natural life of the tree. Trees with wood exhibiting a natural brittleness, which is always very pronounced at the bases of branches, suffer greatly from break- age by the wind. The western larch is especially subject to this form of injury. The brittleness of its branches at their point of attachment with the main trunk is so pronounced that it is not un- 8 BULLETIN 317, U.S. DEPARTMENT OF AGRICULTURE. common to find them lopped off by the wind. This is especially true of tall stems that have come up in close canopy and afterwards become more or less isolated. In the case of the larch the ill effects of the wind are greatly augmented by the heavy loads of long, trail- ing lichens (Alectoria fremontii Tuck. and Usnea longissima Ach.) supported by the branches (fig. 3). During rainy periods these lichens, through the absorption of large quantities of water, increase the weight of the branch by several pounds and, hanging downward in a saturated condition, offer a greater resistance to the wind. The amount of damage to the larch in many locations from this cause alone is much greater than is ordi- narily supposed. In the study in the Whitman _ National Forest it devel- oped that the injury to the larch by mistletoe (aside from the gradual effects of suppression by brooming up the branches and reduc- ing the assimilatory sur- face) was in a large meas- ure due to the pruning by the wind of the many branches which, being heavily loaded with witches’-brooms, caused an increased weight to be ex- I'ic. 4.—A larch, showing the original crown entirely erted at their bases. ‘These renored bs tevoming, The secondary erowsis2° brooms are often formed tities of “black moss” (Alectoria fremontii Tuck.), far out on the branch and which grows over and mats the foliar spurs. become densely matted with dead leaves and lichens (fig. 3). This increase in weight often amounts to several pounds more than that of a normal branch of the same age and size (Table II) and is further increased by the absorp- tion of water during rainy weather. In the winter the broom fur- nishes a collecting place for snow. It is very evident how the re- sultant of the two forces, viz, the wind in a lateral and the weight of the broom-laden branch in a vertical direction, may bring about the removal of all the main original branches (fig. 4). It is not uncommon to find large heaps of branches heavily loaded with brooms under the infected tree. Up to this point the breakage of normal wood uninfected by mistletoe roots has alone been considered. tee dire eet LARCH MISTLETOE. 9 The infected wood of the branch, either at its base or other por- tions, where not too greatly enlarged by the stimulating effects of the parasite, requires a much smaller force to break it at the point of infection than is the case in normal branches of like age and thick- ness. The penetration and embedding of the vertical root system of the parasite in the wood of the host add nothing to the strength of the infected tissue, but diminish its normal strength when the force, as in the case of the wind, is applied at right angles to the grain of the infected branch. Since numerous infections occur at the bases of branches, the point of greatest stress, much injury to the tree results. The meristematic tissue in the cambium layer at the point lig. 5.—Cross section of the trunk of a larch tree, showing a typical basal branch burl. Note that the dead wood is attacked by borers which do not encroach upon the living sapwood. where the branch breaks usually produces secondary branches (see fig. 2). These in turn may become infected and are lopped off, so that eventually great burls are produced at this point on the trunk (fig. 5), seriously reducing the merchantable material. The dead wood thus exposed is a place of entrance for insects and fungi. Since it requires years for the secondary branches to attain a size and assimilatory surface capable of supporting the present bulk of the trunk, the vigor of the tree is gradually reduced. The younger portion of the crown above, being continually encroached upon by the parasite, is not able to supply the deficiency in food materials, and the tree, merely existing for a time, finally becomes a prey to various 8521°—Bull, 317—16——2 10 BULLETIN 3li, U. S. DEPARTMENT OF AGRICULTURE. deteriorating agents and eventually dies before reaching its maxi- mum development (fig. 6). The radial dimensions of the last annual ring of trees in the final stages of mistletoe suppression (fig. 7) Fic. 6.—A larch tree in the last stages of mistletoe suppression. A few of the witches’-brooms contain living branches. The tree was making no percepti- ble increment and was far be- low the normal size for the region. It was necessary to clear away from the base of the tree the heap of fallen witches’-brooms before it could be cut. Fig. 7.—Iwo larch trees barely living, as evi- denced by dissection of the bole. Note the very large witches’-brooms and numerous dead branches. were often so fine and narrow that they could be counted only with the aid of a compound microscope. In some of the worst cases the tree was able to produce but a single layer of tracheids in a year. In so far as climate influences the prevalence and destructive effects of the larch mistletoe, that of the Blue Moun- tain region is most favorable. It might be here added that when a particular tree species has succeeded in establish- ing itself outside of what may be con- sidered its optimum range and at the came time is followed up by a most destructive parasite which responds favorably to the habitat, the rapid deterioration of the species must necessarily follow, at least in the more unfavorable sites. ») LARCH MISTLETOR. 11 FUNGOUS ENEMIES OF THE LARCH. The larch on the tract examined was not attacked to any extent by fungi. The fungi collected were not present in sufficient quantity nor were their effects sufficiently evident to be considered the prime factor in the universal deterioration of the tree. The dead wood and bark of the mistletoe burls were usually infested by the larve of A/elanophila drummondi Iirby (figs. 5 and 8) and occasionally were followed by a fungus causing a black stain. Two burls were found infected Fic. 8.—Cross section of the trunk of a larch tree, showing characteristic fan-shaped burl tissues resulting from an original infection when the tree was 7 years old. The tree was 145 years old when cut. Note the presence of borers. (Tape in feet grad- uated in twelfths.) 3 with Trametes pint (Brot.) Fr. (figs. 5 and 9), but here, as in a number of cther cases where fungi had entered at the burl, the hard- ness and pitchy condition of the wood counteracted the advance of the fungus, and it had not spread much beyond the burl tissue. It is safe to state, from long field observations in other regions, that mistletoe burls furnish admirable starting points for fungi; but since the burl in its early stages is very pitchy (fig. 10) and the dead wood becomes pronounced only after the tree is greatly injured by the mistletoe itself, the effect of the fungi is to destroy later the merchantability of the tree, and the mistletoe may not be the original cause of its deterioration. 12 — BULLETIN 317, U. S. DEPARTMENT OF AGRICULTURE. THE EFFECTS OF MISTLETOE ON ITS HOST. A preliminary survey by the strip method made at the foot of a north slope and partly on the level resulted in the accumulation of the data given in Table I, showing conclusively that the larch in this region is heavily infected with mistletoe. No attempt was made to ascertain the age of the trees given here, so as to show the degree of suppression. A good idea may be obtained, however, of the nature of the infection, distribution, and quantity of mistletoe present on the trees. In general, the height of the trees here recorded is somewhat less than that of normal or uninfected trees in the same region. Fic. 9.—Cross section of the trunk of a larch tree, showing a large burl with white cellulose pits caused by Trametes pini. Note the small amount of living wood and that the dry wood is attacked by fungi and insects. (Tape in feet graduated in tenths. ) The youngest specimen found infected was less than 5 years old, which means, of course, that such early infection will not allow a very high state of merchantability to be attained, even if the young tree is not killed prematurely. Usually very young growth first be- comes infected somewhere on the trunk where the bark is not yet protected by cork (fig. 11). The infection of very young seedlings causes them to assume various abnormal shapes and positions, espe- cially when the mistletoe is confined to one side of the stem. Burl Se. LARCH MISTLETOR. 13 tissues begin to form usually within a comparatively short time, from one to two years in young plants. If the infection occurs on the stem near the base of a branch, the cortical root system advances into the bast tissues of the branch, initiating a burl or witches’-broom at that point (fig. 1). The extension of the cortical roots upward along the main trunk is sometimes sufficiently rapid to keep within the 4 to 5 year old portion of the stem, although the larch mistletoe seldom Fic. 10.—Cross section of a trunk of a larch tree with a large burl, showing its struc- ture and a large pitch pocket. spreads very far from the point of original infection. The parasite, however, usually travels more rapidly along the young shoots which develop in number at the place of first infection. The spread of the parasite outwardly along the branch or upwardly along the leader may be hastened by the dissemination of seeds from the older infec- tions. In this manner the last year’s growth is often infected, and even the terminal bud. The branches of the parasite eventually fall away, leaving scars easily discernible on the older parts of the voung trees (fig. 1). 14 BULLETIN 317, U. S. DEPARTMENT OF AGRICULTURE. TaBLe I.—Eztent and nature of mistletoe infection in 36 larch. trees in the Whitman National Forest. Number of burls. Number of witches’-brooms. Gen- Gen- eral z ee Height | l eral | mer- Tree No. diam- of At base | At base Total | Fallen | Y289F chant- Bier tree. ¥ a | of SEE tes One oF frond poe aie | unk. | pranches. branches.) 247° | tree tree cs ae x a aa aa a Inches Feet ba ape eee 7 5 1 4 5 13 18 1 Waal 20 0) Gee | chee 1Ge ene Cee W. 68 1 4 6 10 16 12 do esse oor 1 Sper te ee 9 75 0 0 0 0 0 0 | Good..| Good pi! iad Sein eee 10 50 0 ) 0 18 18 1.) Bar Do; fle Re og eed ae 11 85 0 2 2 12 14 1S da Do. Qiiiphce 2) meets he 12 64 0 0 4 15 19 2.4 do. Do. LORE ee es 12 66 0 0 3 16 19 0: 1=.2do: edo. j Fs beaece® Saotene so 13 95 0 0) 6 12 18 2 do...| Fair. Y eee AN ee ae 14 7 0 0 0 0 | 0 0 | Good..| Good Die aes nets 14 122 0 0 0 0 0 0 doi |e20: Neca hee 15 98 1 0 4 8 | 12 2 | Fair.. | Fair. | vols seo eee 15 84 0 3 4 15 19 3 de ape- Le es 15 83 0 0 0 0 0 0 | Good..| Good. TR eee 16 88 1 2 4 20 24 3 | Poor.-.| Poor. ] res OS ee 17 85 0 4 5 15 20 17 do==|1"-D0. DAGt ee weer Aly, 88 2 8 4 20 24 3 dos Do. DOr rh es iets a 17 90 0 1 2 11 13 1 |...do...| Good TOR ere 18 99 1 0 3 8 11 fol agrees Do Die el a 18 103 ) 0 0 0 0 0 | Good.. Do SOG Fortes Fee 18 80 il 1 3 5 | 8 3 | Fair... Do DRE cio hae ae 19 99 0 0 0 0 0) 0 | Good.. Do. DA}: dacenanelt oa tee 20 103 1 2 4 18 | 21 Ae) Shain tia DAs NE eee 20 120 0 3 3 20 23 6 do:=21)= Da: QB rae ayer we 20 109 3 4 3 21 23 4 reeves |e hee Siemon 22 107 1 1 3 10 13 0 do...| Good Bee ee ee 23 100 2 4 8 8 16 9 | Poor..} Fair. Gs. Ceres eee 23 | 110 3 6 5 16 21 71-22 d0222|2eor: 32 eee 23 | 109 4 2 1 10 ial 1 do Do. =O): Bat eee a eae 24 | 110 1 2 3 15 18 4 | Fair...| Fair. Fr (eh pa ee a5 111 2 3 4 10 14 4 | Poor..} Poor poi (eae Sapees sees] 28 126 0 0 0 0 0 0 | Good..| Good Deh ea aeee aS 28 110 1 2 1 10 11 2 | Fair... Do Pee ye Bers 28 119 3 1 2 20 22 2 | Poor..| Poor. BE se eee 29 | 123 3 4 6 10 16 2 do... Do. A eee 29 | 125 0 14 5 20 25 | 6 do Do. 0 | 3 2 0 | Fair...) Fair. Owing to the slower growth of the branches in length as compared to the stem, the cortical roots of the mistletoe are enabled to extend into the older part of the 2-year-old internodes. After a time the branch is suppressed and the terminal bud becomes infectéd, resulting in a terminal broom. The cortical roots likewise penetrate the foliar spurs, causing them to become greatly enlarged, with the result that few leaves are produced (fig. 12). It is remarkable how rapidly in some instances the bur] tissues become differentiated. A slight swell- ing is first noticeable; then the bark begins to lose its fresh appear- ance, becomes rough around the edges of the infected tissues, and finally separates altogether from the normal bark (fig. 11). The vertical roots of the parasite continue to live for many years, elon- gating with the same rapidity with which the annual increment of the host is laid down. The hypertrophied tissues resulting from these early infections on the stem spread out in fanlike shapes when viewed in cross section (figs. 8 and 10). Original infections on branches not only cause a local hypertrophy of the immediately ort vw eae RTE GPS = tion, they may not seeds must be LARCH MISTLETOE. : teas bs) infected area (fig. 18), but large brooms are almost invariably produced. In mistletoe regions no trees of any age are safe from infection. A great many trees surrounded by other species not attacked by the i 3) mifamtiitien ae ilu aaa Re TS Fic. 11.—The main stem of a young larch, showing two separate infections, one at the whorl of branches and the other on the internode. ‘ Both infections are of the same. age, as indicated by the large primary sinkers, which terminate at the same annual ring. Note the rough bark on the swellings, the beginning of typical trunk burls. The branches of the mistletoe have fallen, but the sinkers are still living and will remain alive for an indefinite period, stimulating the host tissues to a greater devel- opment. The central areas of the burl soon die, leaving an open wound. same mistletoe escape early infection and grow to a fair size, with normal, healthy crowns. Such uninfected trees are always conspicu- ous among their more heavily infected neighbors. These trees are eventually attacked, but owing to the ad- vantage of a some- what isolated posi- become badly in- fected, since the brought from a dis- tance greater than the natural expelling force of the seed capsule is capable of ; ti iG Oubn: Fig. 12.—A larch twig, showing the abnormal size of the . exerting. naou foliar spurs when stimulated by the parasite. 'These spurs edly this force is are nearly four times the size of the normal spurs on the aided by the wind. ~ ee The final result is the infection of the terminal twigs, and in most cases those of the lower branches. The infection gradually spreads upward; the branches either become broomed 16 BULLETIN $17, U. $. DEPARTMENT OF AGRICULTURE. and are broken off, followed in many cases by a secondary crown (figs. 3, 4, 6, and 7), too late to supply the deficiency in food materials; or the vigor of the present bulk growth and the vitality of the tree are reduced by a general infection throughout the entire crown. The latter type of mistletoe injury frequently occurs. The tree seems to become infected at many different parts of the crown at once, and while the branches are not broken by the formation of large brocms, the vitality of the host gradually sinks under the drain on its resources of so widespread an infection. When young trees are infected there is such an excessive broom development by the time they have reached pole size that the original crown has practically Fic. 13.—A common type of original infection on a larch branch, showing the beginning of branch witches’-brooms. disappeared. Bushy secondary branches grow out from the stumps of the old ones, and the lopping process may be continued to a third or fourth generation of branches (fig. 2). The width of the second- ary crowns becomes less and less, until practically nothing remains but the stubs of the former branches, bearing a few straggling green twigs (figs. 3,6, and 7). By this means the assimilatory surface of the tree is gradually reduced. During the period between the fall of the primary and the appearance of the secondary branches, the tree is robbed of a great amount of food material necessary to main- tain its vigor at its present stage, and it begins to show signs of LARCH MISTLETOR. LY, developing a “spike top” (figs. 4,6,and7). All heavily infected trees by the time they have reached the age of 200 years, if they succeed in living to that age, have developed a “spike top” (Table IT). Occasionally infected trees attain a considerable size, due to the fact that the original infections occurred chiefly at the bases of the branches and did not spread. The attendant broom formation oc- curring on the branches next to the trunk allows the retention of the branches for a longer time than if brooms were developed farther out on the branch. The merchantability of the tree is greatly reduced, however, by the formation of a series of basal branch burls, causing streaks of pitchy wood to extend along the trunk from one burl to another. The spread of the parasite in the direction of the prevailing winds was very interestingly shown in a number of cases. One case in particular was noted in which a series of trees of nearly the same age standing in a row extending in the general direction of the more con- stant winds indicated that the infection had gradually traveled from the first and most seriously infected specimen to those least infected at the other end of the row. These trees had apparently originated under the protection of an old windfall. Since there were no in- fected trees immediately to the right or left, it is fairly evident that the wind was a factor in seed distribution and also determined the direction of distribution. In order to appreciate thoroughly the significance of the effects of mistletoe on the larch, a study should be made of figures 3, 4, 6, and 7, representing different stages of sup- _ pression and various types of infection. On the drier slopes, from 80 to 90 per cent of the larch of all ages has been found infected. On the more favorable sites, the percentage of infection was very low and therefore did not interfere seriously with the best development of the species. (See trees Nos. 40 and 41, © Pable IL) After the preliminary survey, and in order to answer definitely the question whether or not mistletoe is as great an enemy to the host as outward appearance seems to indicate and to obtain, as near as possible, comparable figures on mistletoe injury, 45 infected and uninfected trees were cut and such measurements taken as were thought necessary for the needs of the problem in hand. These data, along with many other observations having a bearing on the subject, have been arranged in Table II, whereby it is possible to follow out the main factors operative in the deterioration or suppression of the trees studied and by means of which fairly conclusive comparisons may be drawn. 18 BULLETIN 317, U. 8. DEPARTMENT OF AGRICULTURE. TABLE JI.—Comparison of 45 lurch trees in the Whitman National Forest with respect to mistletoe infection. [Abbreviations: In columns 14 and 40, increasing degree of infection is shown by the number of cross marks; in columns 26, 31, and 32, B=borers, F=fungi, S=sound, St—stain; in column 36, D=dominant, I= intermediate, S=suppressed.] Branches on Average weight | Average diame- Braue purl: burls. oi— ter oi branch. ea SSS Highest |Branch | Length =e Tree point of | witch- of Su tree. | NO- yar | pae infec- es’- | spike Branch ee Normal, 3 Num- £ Liy-| Origi- | tion. {brooms.| top. | Normal | witch- | ? = mae me ber. |neisnt.| ig: | nally. branches.| es’- bf “| part of ght. brooms. Eee. whorl. 1 2 3 4 5 6 7 8 9 10 11 12 13 ans: Feet Feet Feet. | Pounds. ees Inches. | Inches. SOUP AGS | SLES eee ce ate See se eee eo | aan ee | ee ee |e an nen 63 3) D7 Seo Sette ee 2 Sao ae Se ee oe ee ee Se ee ee eae (ae aS a ea FN ee es 47.0 (8 Se ne 2.0 3.0 0.5 0.8 V4 265. [8552.2)ot. eee |e ce. Se le es es ae a a ee eee EXO) WUC a tere en | es Pe eee rem eoe a2 Dips soo Ss| Scene eee cee See eee ee eee O4 | Avo ales ce Sfece eS S LS a ee SE ee ee es | a eee ee er 96. | Ore a len eee le se ee asl aces Selocsc = Ses |/Seee co ee ee eee [Soca oe ee oe | eee eee 96 | 43._. 2 20.0 8 10 57.0 6: ees 3.0 4.5 1.0 1.5 1005| ie 3 35.3 10 18 80.0 Gi See aos ae eee 4.5 1 ae Poe aha 105 | 8 .- 1 23.0 3 4 77.0 ll aes ae 5.25 3.2 1.3 1.5 LOGR|P1O== 1 23.0 2 2 59.0 2A la a See 6 3.2 1.0 -75 POT Ee ee eee ee ee el EL is Se a Se ee ee eee DEUS A SC ca sou aclemcel ls ccs mapec cle noe| sees