Historic, archived document Do not.assume content reflects current scientific knowledge, policies, or practices. ot Circular No. 853 December(1950 - (Washington) D. C. UNITED STATES DEPARTMENT OF AGRICULTURE _Clitocybe Root Rot of Woody Plants in the Southeastern United States ~ Wed Z By ArrHur S. RwoApDs,’ formerly pathologist, Division of Forest Pathology, Bureau of Plant Industry, Soils, and Agricultural Engineering, Agricultural Research Administration, United States Department of Agriculture CONTENTS Page Page Ha ROCUGhION. Ase 1| Relation of incidence of the dis- History, host plants, and eco- ease to timbered lands and MOMUC INTO ORCAMCC= 2 = = a To 2 SOll CONGIMIONS ==- Se 18 SHMUMIO GON Saree hore ere eS 6 | Control measures and treatment Cosa shuMOUst se Bris eee eS 13 of :diseased=trees2- i. 20s 8 19 Tsolations of the fungus_-_~___ __ Poe OU MOM AT yee ee eI ee 22 Inoculation experiments_______- 15-\déiterature- cited 22 oes == 24. Relative susceptibility and resist- ance of host plants in Florida__ 16 INTRODUCTION Clitocybe root rot, caused by the fungus Clitocybe tabescens (KY.) Bres., is a destructive disease of fruit, forest, shade, and ornamental trees, shrubs, and vines, reported to occur from eastern Texas, Okla- homa, Missouri, southern Illinois and Indiana, West Virginia, Vir- ginia, and the District of Columbia southeastward to Florida. Although the causal fungus occurs as far north as New York and Michigan, root rot caused by it has not been reported from the Northern States. The root rot caused by C. tabescens is so similar in its general as- pects to that caused by the closely related root rot fungus Armillaria mellea Fr., that, in the absence of the fruiting bodies or of cultural studies, the diseases caused by these respective fungi may be con- fused readily. Asa result, the diagnosis of such root rots, particularly in sections where both may occur, may be questionable in some cases. Apparently many cases of mushroom root rot, in the absence of 1Much of the work reported here was performed while the writer was em- ployed as pathologist at the Florida Agricultural Experiment Station. 891856—50—_1 1 Ds CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE fruiting bodies or cultural studies, particularly in the more northern States, have been attributed arbitrarily to A. mellea. Since A. mellea occurs in all States where (. tabescens may be found, this is an unsafe procedure. A. me/lea also occurs more or less frequently in parts of northern Florida. Apparently, the higher temperature range in Florida is more favorable for optimum growth of C. tabescens. HISTORY, HOST PLANTS, AND ECONOMIC IMPORTANCE Clitocybe root rot was reported first from Oklahoma in 1901 by Wilcox (33), who stated that it had been destructive to apple, cherry, and peach trees there for a number of years. He estimated that in many cases whole peach and apple orchards had been destroyed by this disease in 2 years. In addition, he found the fungus to be a com- mon parasite and saprophyte on four species of oaks. Unfortunately, Wilcox considered the Oklahoma fungus as a distinct species from the one that had been so well described previously from Ohio by Morgan (74) as Agaricus (Clitocybe) monaldelphus, purely on the basis of sheht differences in morphology and its parasitic habit of growth, and named it C. parasitica. This fungus has been described under various other names in the United States, although Bresadola pointed out in 1900 (2) and again in 1928 (4) that he considered the American plant synonymous with @. tabescens of Europe. It even has been confused with Armillaria mellea, having been erroneously regarded by some mycologists as an exannulate form of that fungus. The writer (719) has reviewed the synonymy of C. tabescens, and more recently (25) has presented the results of a comparative study of the cultural characteristics of these two closely related root rot fungi, clearly showing that they are distinct species. The fact that Wilcox (33) mentioned and illustrated the occurrence of black, shoe- stringlike rhizomorphs in connection with Clitocybe root rot indicates that both this and Armillaria root rot undoubtedly occurred in Okla- homa and that these similar root rots must have been confused by him in the absence of cultural studies. This is confirmed by a recently issued host index of Oklahoma plant diseases (78) in which both C tabescens and A. mellea are listed as occurring on various plants in that State. While Wilcox (33) in 1901 mentioned Clitocybe root rot as occurring on four species of oaks in addition to fruit trees in Okla- homa, Pr eston (7S) in 1945 merely listed C. tabescens and its synonyms as occurring on hickory (Carya alba). bur oak (Quercus macrocarpa) , and blackjack oak (Q. marilandica), in addition to apple, cherry, peach, and grape. Faurot (7) reported observing root rot caused by toadstool-bearing fungi to be of very common occurrence and a fatal disease of orchar d trees in some scetions of southern Missouri in 1902. Duggar (5, p. 477) stated that he had observed the Clitocybe root rot funeus to occur abundantly during favorable seasons at Columbia, Mo., on roots of hickory and other “deciduous trees but failed to observe its occurrence in orchards, despite special effort to find it. The writer (79), who found this fungus to be the cause of a previously undetermined root rot of grapevines, as well as of orchard trees, in southern Missouri, * Italic numerals in parentheses refer to Literature Cited, p. 24. CLITOCYBE ROOT ROT OF WOODY PLANTS 3 observed it fruiting in great profusion from the roots of dead trees, stumps, and living “trees in an area of forest, dominantly oak, a few miles from Neosho in the Ozark section. He also found it fr muting on the roots of a declining silver maple (Acer saccharinum) street tree at Neosho. Walker (22) and Hewitt and Hayhurst (10, p. 424) reported that root rot of fruit trees was widespread in Arkansas and caused serious damage in some localities. Walker (52) stated that “besides the apple, it affects probably all the commonly cultivated fruit trees, the grape, as well as a number of forest trees,” and that the disease is caused by toadstool fungi, two forms concerned being Clitocybe para- siticaand Armillaria mellea. A much later report (7, p. 48) from that State mentioned that both these fungi had been under observation for a number of years and had been found commonly parasitizing several types of plants, including privet hedges and apple and peach trees, as well as grapevines. Stevens and Hall (29) reported the death of trees in apple orchards in Hayward County, N. C., and stated that “investigation showed the presence of a fungus, which, to all appearances, was Clitocybe parasitica.” Fromme (9), in a report on the black root rot disease of apple, caused by Xylaria mali, stated that “the symptoms allow this disease to be easily distinguished from the so-called white root rot (believed to be caused by. Clitocybe monadelpha) which occurs in some parts of Virginia occasionally, causing an appreciable loss of apple trees.” It has been suggested by Dr. J. 'S. Cooley, of the United States Department of Agriculture, that Clitocybe possibly was blamed for some root rots of fruit trees that may have been due to Corticiwm galactinum before this fungus was adequately described. He consid- ers this particularly true in the northern limits of Clitocybe, as in Virginia. Authentic records of the occurrence of Clitocybe root rot in Florida have been found by the writer to date back to 1902, when it was recorded by Dr. H. Harold Hume, of the Florida Agricultural Experi- ment Station, on peach and other trees at Lake City, and records of the sudden dying of guava trees from what undoubtedly was this dis- ease have been found dating back to 1885. It was reported by Hole (12) in 1905, as troublesome in peach orchards on newly cleared ham- mock land at F ulton, east of Jacksonville, near the mouth of the St. Johns River. Fawcett (S, p. 66), in 1911, mentioned this as one of the peach diseases being studied by O. F. Burger. The destructive- ness of this disease in a commercial peach planting on newly cleared pine-oak land near Blanton, Fla., was reported by the writer (2/7) and also by Thornton (30) in 1940. Accor ding to the estimate of the latter, root rot had attacked 38 percent of the "1,400 trees by the time they had attained an age of 31% years and had killed 192 trees. In the earlier records of Clitocybe root rot the disease attracted at- tention chiefly as an important cause of loss in orchard trees and, later, grapevines. Even in subsequent years, with the excepticn of Flor ida, only the most meager observations have been made in regard to its occurrence on forest, shade, and ornamental trees and shrubs. Ex- tensive studies of this disease have been conducted by the writer in Florida over a period of 20 years. Some results were published in the Florida Agricultural Experiment Station Annual Reports and in 4. CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE various bulletins. These studies have shown that Clitocybe root rot is of frequent and widespread occurrence in that State, attacking forest and fruit trees, tung-oil trees, and a large variety of ornamental trees, shrubs, and vines (20), including many exotic ones. It has been recorded as attacking 210 species of plants belonging to 137 genera FigurE 1.—Recorded distribution of Clitocybe root rot in Florida, the dots in- dicating localities where from 1 to more than 50 collections have been made. and 59 famihes. Of these, members of the Casuarinaceae, Rosaceae, Leguminosae, Rutaceae, and Myrtaceae are represented with outstand- ing frequency. Conifers are attacked as well as broadleaf trees, and even bananas (23) and palms. The known distribution of Clitocybe root rot in Florida is shown in figure 1. CLITOCYBE ROOT ROT OF WOODY PLANTS dD Plakidas reperted a crown girdle of sand pear (/5) and tung trees (76) in Louisiana, with which ©. tabescens was later determined to be associated, along with Dothiorella (Botryosphaeria) and sometimes other fungi. C litocybe root rot was reported subsequently by Hines (a5). Bain (2), Large (13), and the writer (24) as causing losses of trees in tung tree orchards in one or more of the following States: Louisiana, Mississippi, and Florida. In Opelika, Ala., in July 1941, the writer observed two dead Lom- bardy poplars (Populus nigra var. italica), a nearly dead silver maple (Acer saccharinum), and a dying pomegranate (Punica gr anatum) all attacked by Clitocybe root rot. On the grounds of the experiment station at Auburn, Ala., where a number of peach trees had died a short time before from a trouble thought to be waterlogging of the clay soil, a peach tree (Prunus persica) and a wilting American plum (Prunus americana) also were found dying from “this root rot. 0. tabescens was isolated from the roots of each of these trees. Under- wood and Earle (37) in 1896 and Earle and Austin (6) in 1900 re- ported a heavy mortality of grapes on the Alabama station grounds from a root rot of undetermined cause characterized by a “whitish mycelial growth under the bark. Earle and Austin (6) stated that, of the 651 vines alive or planted in 1894, 483, or 75 percent, were dead by 1900. The economic importance of Clitocybe root rot varies greatly, ac- cording to the prevalence of the fungus in local areas, the age and extent of the planting, its esthetic or commercial value, and the sus- ceptibility of the host species to the disease. Losses on numerous properties examined in Florida have varied from an occasional tree or shrub to serious losses continuing over a period of several years; the loss of scattered trees or groups of trees in commercial plantings of citrus, peach, and tung; or the loss of a majority of the trees in windbreak or roadside plantings of the highly susceptible Australian pine * or beefwood (Casuarina spp.). ‘The occasional loss of a plant in a yard rarely causes much concern unless it is a particularly prized specimen. However, on properties where dying of plants continues over a period of years, losses may be extensive. When such losses occur on a large number of properties in Florida alone, it is evident that the ageregate losses in ornamental plantings become of consider- able importance. Occasional properties in various parts of Florida have been found to be veritable hotbeds of infection for the Chtocybe fungus, with trees and shrubs thereon dying at frequent intervals over a period of several years. Properties at points in Florida where this fungus has been unusually destructive invariably have been ham- mock, pine-oak, or scrub oak types of land on which various species of oaks grew abundantly prior to clearing. The writer has reported Clitocybe root rot in Florida on bananas (23), citrus (27), Australian pines (28), tung tree (24), and miscella- neous forest trees, fruit trees and ornamental trees, shrubs, and vines (C5) 2 Ata, Florida nursery a 34-acre planting of closely set stock plants of both the India-rubber fig (/%cuws elastica) and the variety variegata was being grown under a lath shed for commercial propaga- * Although these trees are often called beefwood, the name Australian pine, by which they are generally known in Florida will be used in this circular. They are hardwoods, however, not pines. 6 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE tion. Clitocybe root rot appeared on some of these plants. So destruc- tive was the disease and so rapid was its spread that culture of the plants was discontinued within 5 years after the first apperance of the disease (26). In this case the unusually rapid spread and destructive- ness of root rot was attributed to maintaining a high moisture content of the soil by daily use of an overhead sprinkling system to keep the moss layers wet. SYMPTOMS The symptoms of Clitecybe root rot vary considerably with the kind and size of the plants attacked and the rapidity with which girdhiing occurs. Root-rot symptoms are not apparent until the fungus has invaded the roet system or root crown, or both, sufficiently to interfere with the life processes of the host plant. The dying of small trees or shrubs may follow rapidly, while that of large ones 1s likely to proceed much more slowly, with large trees often showing symptoms of decline for a few years before death. On conifers, such as arborvitae and pines, a slight yellowing of the foliage may occur occasionally, but as a rule, at least on small trees, the foliage on the first limbs to be affected by the progress of the invading “fungus simply turns brown and dies and the decline of the tree progresses more or less rapidly. Small broadleaf trees and shrubs frequently are killed so rapidly that they may not develop any particular symptoms until they begin to wither and die. Large trees and shrubs usually show more or less yellowing and defoliation, accompanied by the rapid wilting and dying of individual limbs or trunk divisions. Finally, the ‘whole plant dies. In other cases the wilting develops more or less simul- taneously through the whole crown. When large trees or shrubs are not killed during the year in which they become “partially @ girdled, the folhlage dev eloped the following year tends to show reduction in S1Ze, sparseness, and marked chlorosis, which usually increases in intensity toward the top. In the case of Austrahan pine trees, the first symptom is a slight yellowing of the foliage branches,* which generally develops first on the lower limbs of the crown on the side where the roots are first attacked. This yellowing soon becomes more conspicuous in extent and is accompanied by shedding of the affected foliage branches. As the disease progresses the crown acquires a thin, sickly appearance (fig. 2) and an unusually heavy ltter of shed foliage branches accu- mulates under the tree. Attacked trees generally become about three- fourths defoliated, and often completely so on the terminal parts of the crowns, before dying. However, in very young trees, where gird- ling proceeds quite rapidly, the foliage branches over the entire crown may turn brown and shrivel within a brief period without any prelimi- nary yellowing .and shedding. In older trees, where girdling may * Tn these trees the leaves are reduced to whorls of mere scalelike teeth at inter- vals around the dehiscent foliage branches, which may appear to the layman as needlelike leaves. CLITOCYBE ROOT ROT OF WOODY Fiegure 2.—Part of 10-year-old scalybark Australian pine windbreak, showing tree three-fourths defoliated from girdling by Clitocybe root rot, in contrast with healthy trees on either side. 8 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE require 2 or 3 years from the time the first symptoms become apparent, new foliage branches may develop following the shedding of a large proportion, but the new ones usually attain only about half the normal length for the species. By the time a pronounced yellowing or defoliation or other decline of the tops of attacked trees is apparent, the mycelial growth of the fungus usually has progressed upward on the base sufficiently so that its presence can be verified by making a cut in the bark down to the wood; though sometimes it may have developed only up one side of the tree. The upward progress of the mycelium at the base of the attacked tree frequently is indicated by a slightly sunken lesion. Such FIGURE 3.—Base of a large guava tree killed by Clitocybe root rot. Lesions extend up the trunk divisions; dead bark cracked loose on one at right, and shed on two at left. basal lesions are most conspicuous on trees and shrubs with thin bark and do not show on thick-barked trees. They frequently develop first on one side before encircling the trunk. By the time the crown begins to become thin from defoliation, the cutting test of the bark ustially shows the base of the tree immediately above the ground to be completely girdled. This girdling is often accompanied by longi- tudinal cr acking of the bark as it dies, and sometimes also by a pro- nounced cracking, demarking the extent of the lesions (fig. 3). The basal lesions ustially extend upward from a few inches to about a foot and rarely as high as 2 feet above the ground before the attacked tree dies, the height fr equently being g oreater on one side than on the other. CLITOCYBE ROOT ROT OF WOODY PLANTS 9 The mycelial sheets that develop under the bark are coextensive with the lesions (fig. 4). In certain species of Australian pine trees the basal girdling by root rot frequently stimulates a pronounced hypertrophy of the trunks im- mediately above the girdled parts. Stone-fruit trees, such as peach, plum, and Carolina laurelcherry, commonly develop more or less gum formation in the cambial region, which may be so copious as to exude through cracks in the bark. Figure 4.—Large guava tree dying from Clitocybe root rot. Dead bark has been cut away on basal lesion to show extent to which mycelium has spread up trunk. When the bark of roots attacked by Clitecybe root rot is peeled off, the mycelium of the fungus is seen to have developed between the bark and the wood (fig. 5) and also thr ough the inner layers of the bark. This varies from ne filmy wefts to leathery sheets that are white when freshly developed but become cream- to chamois-colored with age. ‘The mycelial sheets often show a more or less radiating, fan- shaped type of development, but this feature is less conspicuous than usually occurs with Armillaria mellea and is less apparent in old, compact mycelial sheets. When especially favorable soil moisture conditions induce an unusually luxuriant growth, the advancing mar- 891856—50——_2 10 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE FictreE 5.—Roots of Bourbon Dombeya bush dying from Clitocybe root rot. Bark has been peeled off to show luxuriant growth of mycelial sheets developed dur- ing hot, rainy weather. Note thick, waxy margin and perforate character of older parts. gins of the mycelial sheets may be considerably thickened (fig. 5). In addition to mycelial sheets of considerable extent, there may also develop flattened, narrow to broad, thalluslike structures of the same color and either entire in outline or with numerous threadlike branch- ing from the margins. These structures, which are essentially rhizo- CLITOCY BE) ROOT ROT OF WOODY - PLANTS i morphie (rootlike) in character, are commonly produced in cultures of the fungus. An additional feature of the mycelial sheets is the frequent occurrence of a peculiar perforate character, clearly apparent (figs. 5 and 6). This appears to be a character of diagnostic value for C. tabescens and one which apparently is not found in A. medlea. Freshly dug reots that show mycelial sheets under the bark are char- acterized by a proncunced mushroom or fungus odor. Fictre 6.—Part of mycelial sheet developed between bark and wood of root of sand pine killed by Clitocybe root rot, showing perforate character and sug- gestion of radial growth. 5. The black, rounded or flattened cortical or subterranean shoestring- like rhizomorphs so frequently accompanying the closely related root rot fungus 4. medlea have not been observed to occur in the case Gili tabescens. Wowever, a peculiar development of blackish, hardened, sometimes frilly, mycelial extrusions, similar to those reported for A. mellea, frequently occur on the bark of roots attacked by C. ta- bescens. These structures originate as outgrowths of the mycelial sheets or xylostromata developed between the wood and the bark, becoming extruded through longitudinal fissures in the bark (fig. 7). An additional symptom of Clitocybe root rot is the occasional de- velopment of clusters of the mushroom fruiting bodies of the fungus 12 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE Ficure 7.—Roct of seedling sweet orange tree attacked by Clitocybe root rot, showing blackish xylostroma extrusions developed through longitudinal fis- sures in the bark. Natural siZe. FIGURE 8.—Clitocybe tabescens fruiting at base of Brazilian Butia palm attacked by root rot. Photo by Dr. George F. Weber. at the bases of trees or shrubs that have been infected by it for some time (fig. 8). The presence of one or more clusters of these growing from the roots or bases is an infallible sign that the root systems have been more or less extensively invaded by the fungus, even though symptoms of decline may not be apparent. CLITOCYBE ROOT ROT OF WOODY PLANTS 1) CAUSAL FUNGUS Clitocybe root rot is caused by Clitocybe tabescens (Fr.) Bres., one of the gill fungi. The fruiting bodies of this fungus consist of a few to many individuals, with the stems developing from a common base (fig. 9, 4). When fully developed, the caps are convex to flattened or centrally depressed with age, whitish to light tan or honey-colored, smooth, or adorned with tufts of fibrils near the center, and from 2 to 31% inches in diameter, with whitish gills underneath (fig. 9, 2). The united Clitocybe, as this fungus 1s commonly called, resembles the closely related honey agaric (Ar millaria mellea) in habit of growth, color, texture, and gener ral appearance, but is distinguished pr incipally by the absence of an annulus on the stem, dissimilar spores, and in being more slender from the beginning. It is considered superior to A. mellea in flavor and edibility. The latter fungus 1s of cosmopolitan occurrence and causes a very similar root rot. The production of fruiting bodies by the Clitocybe root rot fungus depends largely upon the progress of the disease and the seasonal conditions. They develop with greatest frequency during the fall, though in Florida they occur most F commonly from the middle of Sep- tember to the middle of October, but may develop also in other months if favorable soil moisture conditions prevail. As a rule, only one or two clusters develop during the year and frequently none develop. They usually attain their full development within a few days from the time the young “buttons” appear and decay rapidly in warm, rainy weather. However, if dry weather follows their development, the clusters may dry up and turn dark brown to blackish but remain recognizable for several weeks to one familiar with the fungus. The presence of either fresh or old clusters of the fruiting bodies of the fungus frequently makes it possible to locate diseased trees, especially citrus, even before the tops begin to decline or the presence of the disease 1s suspected. It should be borne in mind, however, that by no means every mush- roomlike or toadstoollike fungus that. may occur occasionally at the bases of trees is the particular species causing Clitocybe root. rot. Aside from differences in character, other fungi are unlikely to be growing in clusters with several individual fruiting bodies arising trom a common base at the foot of the tree and certainly will lack organic connection with the bark of the trunk or roots. ISOLATIONS OF EFHE FUNGUS Clitocybe tabescens has been isolated consistently from a diverse array of trees, shrubs, and vines attacked by mushroom root rot in various parts of Florida from 1924 to 1944. The only other organisms developing in these isolations were the ubiquitous bacteria, molds, and soil-inhabiting fungi that may be expected to occur as contamina- tions. In most cases a large percentage of pure cultures of C’. tabescens resulted, while in others a large percentage of contaminations oc- curred. The root rot fungus develops very slowly, usually requiring from 1 to 2 weeks, and occasionally longer, for growth to begin. As a result, if any contaminations are present, they usually develop to the exclusion of the desired fungus. 14 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE vf B FIGURE 9.—A, Cluster of mushrooms of Clitocybe tabescens that developed on oak root left in cleared land, showing caespitose character. 6, Under side of the cluster of mushrooms of C. tabescens shown in A, illustrating decurrent gills and lack of an annulus. CLITOCYBE ROOT ROT OF WOODY PLANTS 15 After repeatedly isolating C. tabescens over a period of several years and carrying most of the isolates to the fr uiting stage, there ap- peared to be no point in making further isolations, except in the case of inoculated trees or new host ‘plants, though isolation of the fungus is always of value, especially when there is a possibility that the closely related root rot fungus Armillaria mellea may be involved. O. tabescens has been isolated from root material of 156 plants, com- prising 90 species, from various parts of Florida. In the writer’s extensive cultural work with mushroom root rot in Florida, only 2 instances, both at Gainesville, have occurred in which A. mellea was isolated and in both it was suspected when roots were collected that this fungus rather than C. tabescens was involved. The writer has also isolated C. tabescens from infected grapevines in Missouri (79) and from five different kinds of trees in Alabama, as reported earlier in this circular. During the course of his work on Chitocybe root rot in Florida he compared isolates of this fungus from a number of sources outside of Florida with Florida isolates, and they were the same. Isolates from sand pear and tung-oil trees in Louisiana were received from Dr. A. G. Plakidas of the Louisiana Agricultural Experiment Station. Isolates from a Japanese cherry (Prunus sp.) at Washington, D. C., and from a sporophore that de- veloped at the base of an eastern redcedar (Juniperus virginiana) at Arlington, Va., were received from Mr. Ross W. Davidson, of the Division of Forest Pathology. INOCULATION EXPERIMENTS Various attempts to produce artificial infection with Clitocybe tabescens have been made by the writer over a number of years, be- ginning in 1931. The earher attempts, which involved placing agar slants from large test-tube cultures against roots, both injured and uninjured, of native and exotic trees growing in woods and vacant lots, of potted woody plants in the greenhouse, and of Australian pine and citrus trees in buckets in a lath house were all unsuccessful. The ineffectiveness of this method was probably due to the short life of the ioculum in this unprotected state and to the relatively long time required for the fungus to infect the host. It was not until after Tengths of roots naturally infected by the Clitocybe root rot fungus were placed underground in contact with uninjured roots that infection took place, and even then several months elapsed before noticeable lesions developed. However, the writer’s Inoculation experiments were carried on under very droughty conditions. Successful artificial infections of horsetail and scalybark Australian pines (Casuarina equisetifolia and C. lepidophloia) finally were obtained, both through the use of pieces of naturally infected roots and pure cultures of the fungus grown on lengths of oak stems placed in contact with uninjured roots. In both cases the fungus was reisolated and carried to fruiting. Successful inoculation ex- periments with this fungus also have been reported by Plakidas (77), who was able to infect small Pineapple pear trees by placing blocks of wood, on which pure cultures of the fungus were grown, in con- tact with fr eshly wounded roots. 16 CIRCULAR 853, U. S. DEPARTMENT OF AGRICULTURE Mechanical injury is not necessary for infection, since it has been observed repeatedly in field investigations that, ike Armillaria mellea, C. tabescens can penetrate healthy roots readily through uninjured bark. The parasitic nature of this fungus is apparent from the readi- ness with which it attacks the most healthy and vigorous plants and the ease with which it may spread from one tree to another through root contact. RELATIVE SUSCEPTIBIEITY AND: RESISEANCE, OF HOSE PLANTS-IN- FEORIDA Little information is available on the comparative susceptibility and resistance of various plants to Clitocybe root rot, even in Florida where the disease has been studied rather intensively. It is difficult to gather such data on the basis of the occasional dying of miscel- laneous plants on various properties, since one rarely finds suflicient plants on individual properties to make reliable comparisons. Under these circumstances, comparisons of the susceptibility and resistance of plants can be made only by considering them with respect to the general prevalence of the disease in a large number of plantings in various parts of the State. It must be borne in mind, however, that the hkehhood of susceptible plants being attacked varies oreatly, ac- cording to the site on which they are planted. For example, even the extremely susceptible Australian pines may prove fairly free from root rot when planted on low hammock, prairie, flatwoods, coastal beach, or other land that is free from infection, but very subject to it when planted on well-drained, sandy land where oak trees grew prior to clearing or some oak scrub remains. The repeated occurrence of Clitocybe root rot on various species of Australian pine in widely scattered localities in Florida clearly demonstrates that these trees head the list of susceptible plants. These exotics are utilized extensively in Florida as windbreaks for citrus and other subtropical fruits, street and roadside trees, and screen, hedge, and other ornamental plantings. Aside from the susceptibility of some species to cold, their extreme susceptibility to this disease has proved the most serious drawback in many parts of the State to the extensive plantings of these very useful and extremely fast- growing trees. The inroads of Clitocybe root rot frequently result in such “widespr ead mortality of Australian pines that their usefulness or esthetic value is often rapidly impaired, or even virtually destroyed, within a few years. Peach trees also are extremely susceptible to Clitocybe root rot. This disease has proved to constitute an important factor of loss in a number of commercial plantings of tung trees. Citrus trees on rough lemon stock hkewise have proved quite susceptible to Clitocybe root rot, although it has not been found in those on sour orange stock. The common guava, sturdy cypress-pine, turkscap waxmallow. , Amer- ican arborvitae, eucalyptus, azaleas, and other plants have proved very susceptible. The disease also has been found to be the major cause of the dying of roses in Florida. A general idea ‘of the relative fr equency of occurrence of Clitocybe root rot on the more commonly cultivated plants in Florida is given in table 1. The frequency with which it has been recorded on ‘these CLITOCYBE ROOT ROT OF WOODY PLANTS lez TaBLE 1.—Jist of plants in Florida on which Clitocybe root rot has been found 10 or more times Number of Common name Scientific name diseased plants reported Horsetail Australian pine________ Casuarina equisetifolia L_______- 14,500 intra Tub beret os Soa Ls Eacus elastica Roxb = 2 = ne es 2 2, 500 Sealybark Australian pine________ Casuarina lepidophloia F. Muell- 660 J EASYEN CI oe ie a a a a Ng ce te Prunus persica (l.) Batseh__-___ 1 625 MptHIMNe Rte Cheeses cere os a Aleurites fordit Hemsl__________ I 375 Rough lemon (rootstock) ________ Cirusilimon Gz.) Burm fo. 330 Cunningham Australian pine_____ Casuarina cunninghamiana Miq_ 149 Common-euavare. 6. 82 oe T2SUOUU TN GUL ICUG,. Waka erase 4 see 102 Sturdy cypress-pine_____________ Caliiivis robusta Re bres.s5 sae 1100 Coast Australian pine =_-_~2_ =. _ Casuarinag:-stricta: Alt. wea. 86 itunkscap: waxmallow. 0222) 2 2s os: Malvaviscus granditlorus H. B. 58 Ke. AMMeTICAN-arborvitae= 2.3 2. _ Ehuggcoccidentalis loa eae oh 36 Beakpod eucalyptus:-_- == .- 2 = Eucalyptus robusta J. EK. Sm__ __ 30 Nanichicag azaleas: =) eos te Rhododendron indicum (1...) ol Sweet. Clmmese hibiscusss: 2's ss oe Hibiscus rosa-sinensis L________ 25 Swamp Australian pine__________ Casuarina-glauca Sieb= 2-2 22 Brazil peppertrees sos Ga sae Schinus terebinthifolia Raddi-____ 21 SHNOG! OSM es 5 as oe a ee Pyrus pyrifolia (Burm.) Nakai__ 20 Camiphor=trees 2 os Cinnamomum camphora (1..) 20 Nees & Eberm. Ptinke yy? Onlkete reer eats ata ES hex Quercus: \aevise Walt soo. See 19 A FOOD Ge | OY ETNY OLE SS a a ets Mee ree ee Iigustrum amurense Carr_______ 17 PAU SUA MAM