THE UNIVERSITY OF ILLINOIS LIBRARY A6RICDLTURAL LIBRARY UNIVERSITY OF ILLINOIS Agricultural Experiment Station BULLETIN No. 213 TWO ILLINOIS RHUBARB DISEASES BY FRANK LINCOLN STEVENS URBANA, ILLINOIS, JANUARY, 1919 CONTENTS OF BULLETIN No. 213 PAGE RHUBARB AKTHRACNOSE 299 General Character 299 The Fungus 300 Isolation 301 Culture Characters 301 Inoculations 305 Germination of Conidia 305 Taxonomy 305 RHUBARB LEAF SPOT 308 The Fungus 310 Isolation 312 Culture Characters 312 Taxonomy 312 TWO ILLINOIS RHUBARB DISEASES By FRANK LINCOLN STEVENS, PROFESSOR OF PLANT PATHOLOGY, UNIVERSITY OF ILLINOIS 1 RHUBARB ANTHRACNOSE (Due to Collet otriclium erumpens) Attention was first called to this disease on market rhubarb in the stores of Champaign during the spring of 1918. It has since been repeatedly seen in the grocery stores and has been collected in the field in numerous localities, in both cases in such abundance as to show that it constitutes a real factor in rhubarb culture. GENERAL CHARACTER The disease, which is of fungous origin, consists of a soft rot of the petioles. As yet no natural field infection of other parts of the plant has been seen. The decayed spots usually are soft, watery, and translucent, oval in outline, with the long axis lengthwise of the petiole (Fig. 1). When they attain a length of somewhat more than a centimeter, the black, sporulating bodies (acervuli), smaller than fly specks, appear abundantly in the centers of the spots (Fig. 2). In advanced stages the whole petiole is covered with acervuli and is entirely soft and rotten (Fig. 3). In the market usually only the milder cases of disease are found ; petioles with numerous small rotten spots which have escaped the notice of the grower when preparing his product for sale. In the field, often all old petioles lying dead on the ground are thickly cov- ered with acervuli, and older petioles slightly wilted are heavily infected. In general this disease appears much more abundantly on old plant parts than on fresh, vigorous parts, tho many vigorous, salable petioles are attacked, as is clearly evident from the abundance of the disease in the market as well as in the field. This disease has been collected at Champaign and Urbana, Cham- paign county ; at Kankakee and Bourboiinaise, Kankakee county ; and near Anna, Union county; and is apparently of quite general oc- currence. The injury done is threefold. Even a small amount of the dis- ease in a bunch of market rhubarb practically destroys the possibility of selling the bunch to a careful purchaser. In the field many petioles Assisted in the laboratory and in illustrating by Nora E. Dalbey. 299 300 BULLETIN No. 213 [January, otherwise salable must be discarded on account of the rot. The pre- mature death of infected petioles and consequently of the leaf blades lessens the general strength of the plant. THE FUNGUS The rotten spots are uniformly occupied by a species of Colletot- richum, and in early stages of decay by this fungus alone. Young, watery, translucent spots which have not yet developed acervuli show the diseased tissue quite fully occupied by the mycelium. This mycelium is hyaline and of somewhat characteristic appearance in that FIG. 1. PETIOLES OF EHUBARB SHOWING VARIOUS STAGES OF DECAY EESULTING FROM COLLETOTRICHUM the older parts are rather coarse (4 p.), conspicuously septate, and constricted at the septa (Fig. 4). Younger threads of mycelium are smaller, tapering to 2 /u at the extremities. The acervuli. begin subcuticularly as an aggregate of hyphae, Fig. 5 (1), which soon ruptures the cuticle. When mature they are circular or oval and of an average diameter of about 200 p.. Soon after rupturing the cuticle, one or more setae appear, and in the mature acervulus the setae are numerous, usually from fifteen to 1919] Two ILLINOIS EHUBARB DISEASES 301 Fig. 2. A single spot, enlarged. The minute pustules are the acer- vuli twenty or more Fig. 5 (2, 3). Spores begin to form as soon as the cuticle ruptures, and the mature, undisturbed acervulus is covered with a mound of spores, often so deep as to cover even the tips of the setae. The setae are black, somewhat pale at the tip, usually 3- or 4-septate, upward of 150 p. long, and about 7 p. thick at base, and with an acute tip, Fig 6 (1, 2). The conidia are falcate (Fig. 7), quite uni- form in size, measuring 22 to 29 /* by 3.5 p., hyaline and continuous, and acute at each end. The conidiophores are shorter than the conidia, simple and continuous. Isolation The fungus was readily isolated by sev- eral of the usual methods: 1. By dilution platings of spores. In many cases the plates were entirely pure, with hundreds of colonies of the Colletot- richum. 2. By differential growth, by direct planting of bits of infected tissue upon poured corn-meal agar plates. When this method was employed, the Colletot- richum outgrew its contaminators, and could thus be easily isolated. 3. By direct planting of spores from drops of sterile water upon poured agar plates. The colonies so secured on corn- meal agar did not produce normal acer- vuli, but they did eventually produce characteristic conidia, and w r hen trans- ferred to sterile rhubarb gave abundant normal acervuli. Culture Characters On corn-meal agar plates the myce- lium showed a tendency to darken in some cases, producing many filaments almost black, and giving to all the col- onies a dark appearance. Analysis shows that the darkening of the colonies is due to an excessive production of ap- Fig. 3. Advanced stages of de- cay of the rhubarb petioles 302 BULLETIN No. 213 [January, pressoria 1 and of structures which are morphologically appressoria. Often these are produced in large numbers in clumps which appeal- as black sclerotia-like bodies, several millimeters in diameter. This was particularly true where two colonies approached each other in the plate (Fig. 8). The structure of the component parts of such a spot is represented in Fig. 6 (3). A general view of several col- onies on corn-meal agar is shown in Fig. 8. On this medium the acervuli are produced somewhat sparsely, after an interval of about a week. FIG. 4. CHARACTERISTIC BRANCHING AND SEPTATION OF THE COLLETOTRICHUM MYCELIUM When sterile (autoclaved) rhubarb was laid upon poured plates of corn-meal agar and then inoculated with the fungus, acervuli formed much more quickly than upon corn-meal agar alone, and spores were produced in much greater quantity. Thus abundant mature acervuli were present upon the bits of rhubarb at the end of three days. Bits of rhubarb petioles were cut aseptically, and placed on poured corn-meal agar plates. Upon these growth was as upon the auto- claved rhubarb but decidedly more vigorous. Corn-meal agar was poured into plates, then inoculated. When the colonies attained a diameter of about a centimeter, bits of raw, aseptic rhubarb were laid on the agar about a centimeter from the edge of the colony, in order to see whether the growth rate in this sector of the colony would be altered by the rhubarb. No such effect was noticeable, but it was found that tho the fungus made acervuli 'Hasselbring, H. Bot. Gaz. 42, 135, 1906; and Halsted, B.D., N.J. Agr. Exp. Sta. Ept., 1892, 303. 1919] Two ILLINOIS RHUBARB DISEASES 303 but very sparsely on the agar it did make them abundantly upon the bits of raw rhubarb upon the agar. On various autoclaved plugs the fungus grew well : e.g., on Bumex crispus, with typical acervuli but growth rather slow; on Rumex acetosella, similarly but growth more vigorous ; on Polygonum erectum growth was scant and aeervuli small; on Polygonum lapatliifolium growth was similar to that on Polygonum erectum but slightly more vigorous; on Tropoeolum stems, growth was very rapid and acervuli FIG. 5. (1) A VERY YOUNG ACERVULUS; AN AGGREGATE OP MYCELIUM BELOW THE CUTICLE WITH INDICATIONS OF BEGINNINGS OF SETAE. (2) TYPICAL GROUP OF ACERVULI. (3) A TYPICAL ACERVULUS numerous, showing these steins to be more favorable than any other medium. On cabbage plugs the whole culture was less dark than on rhu- barb plugs, the floccose aerial mycelium was white, and there was almost entire absence of appressoria, and of sclerotia, or other dark structures except those of the acervuli, which were very abundant and normal. 304 BULLETIN No. 213 [January, On rice there was an unusually large production of appressoria and sclerotia, giving the culture a striking black character (Fig. 9). In tubes on autoclaved rhubarb plugs, growth was vigorous, the sur- face of the plugs becoming closely covered with acervuli which pro- duced spores about a week after inoculation. In the older portions FIG. 6. (1) DETAIL OF SETAE AND CONIDIOPHORES. (2) A SINGLE SETUM. (3) COMPONENT PARTS OF THE SCLEROTIA-LIKE BODIES PRODUCED IN AGAR PLATES the plugs were covered by a thin growth of aerial mycelium. On corn-meal agar in slant tubes the surface of the slant soon became covered with black sclerotia-like structures, and a ring of these struc- tures was formed wherever the fungus touched the glass. 1919] Two ILLINOIS EHUBARB DISEASES 305 Inoculations When spores from diseased tissue, or fragments of diseased tissue, or mycelium from a pure culture were inoculated into rhubarb petioles, the typical rot rapidly followed. Tho the disease was not found in the field upon leaf blades, laying the spores upon leaf blades in a moist chamber in the laboratory resulted, in about seven days, in large, rotten, leaf spots bearing numer- ous typical acervuli. The disease also often appeared thru natural infection on leaf blades in the laboratory. Fig. 7. Spores of Colletot- richum Germination of Conidia Germination in water in hanging drop gave the usual characters of the anthrac- nose fungi; i.e., the spores usually became two-celled, developed a germ tube from one or both cells, and often from the germ tubes one or more appressoria (Fig. 10). TAXONOMY The fungus is a typical Colletotrichum. Never were setae absent from mature acervuli, nor is there any basal tubercular structure which would throw it into the genus Volutella. Colletotrichum as FIG. 8. TYPICAL COLONIES ox CORX-MEAL AGAR LEFT: FOUR COLONIES EIGHT: MANY COLOXIES A dark line due to sclerotia-like bodies is evident in all cases when one colony meets another. 306 BULLETIN No. 213 [January, given by Saccardo 1 contains 113 form species. Of these, however, only 33 have falcate spores. There are 83 species of Volutella listed by Saccardo. The falcate-spored Colletotrichums are given in the accompanying table, together with such of the closely related genus Volu- tella as have falcate spores. From this tabulation have been omitted such forms as are described as having spores "straight or slightly curved," "straight or curved," or similarly designated, on the as- sumption that such are not typically falcate, a character which were it present would be sufficiently obvious to be mentioned. Accepting the descriptions as correct, Nos. 1 to 27 inclusive and Nos. 29, 38, and 43 do not agree in spore size with the fungus under consideration. Nos. 44 to 47 inclusive need not be considered. Nos. 28, 30 to 37, and 39 to 42 are so close in spore measure- ments to the species causing the rhubarb rot, that this might indeed, on that basis alone, be considered as co-specific with any one of them ; nor is there any evidence from measurements of setae, except in the case of No. 37, that would bar such an assump- tion. Close inspection of the specific descriptions eliminates Nos. 40 and 41, which are clearly tubercular. There then remains a group consisting of ten species which from the descriptions are indistinguishable from each other and from the Fig. 9. Growth of Col- letotrichum on auto- claved rice FIG. 10. GERMINATING CONIDIA AND APPRESSORIA 1 Saceardo. Sylloge Fungorum. 1910] Two ILLINOIS EHUBARB DISEASES 307 FALCATE-SFORED FORMS OF COLLETOTRICHUM AND VOLUTELLA Arranged in order of maximum spore length (/*) No. Saccardian Name Spore Setae Host Vol. No. Length X Breadth Length X Thickness 1 2 3 4 3234 11 3676 10 6851 V. chalybea C. sanguisorbae C. sphaeriaeforme 7-8X4-5 9-10X2-2.5 10 X3 1 115-140X2.3 80-10.0X3-6 "fimo" Sanguisorbia Menispermum 4 5 6 7 8 4 3237 14 4026 10 6846 14 4019 4 3227 V. buxi C. yuccae C. pisi C. solitarium V. comata 10-11X3^ 13X3.5 11-13X3-4 12-14X2.5-3 12-14 X 100-120X4 60X6 60- 90X6 65- 75X2-3 Buxus Yucca Pisum Solidago Robinia 9 10 18 5034 16 3885 C. liliacearum C. piri 12-17X2.5-3.5 13-18X3.5-5.5 70X5-5.5 Hemerocallus Pirus 11 12 13 3 3843 4 3263 18 5014 C. volutella V. therryana C. theobromicolum 14-18X2.5-3.5 15-18X3 15-18X4-5 300X5-6 150X1 Ulmaria Grasses Theobroma 14 15 16 17 18 16 3882 18 5772 10 6852 16 3886 10 6854 C. violae-tricoloris V. tristis C. spinaciae C. elasticae C. gloeosporioides 20X5 11-20X2 14-20X2.5-3 16-20X4.5-5 18-20X5-6 20- 70 X 400X7-8 60- 75X4-4.5 200-250 X Viola Erica Spinacia Ficus Hedera 19 16 4258 V. violae 15-21X3-4 Viola 20 21 22 8 4 3244 16 4262 10 6847 22 7733 V. fusarioides V. allii C. carpophilum C. (impelinum 18-21X2-3.5 18-21X3-4 16-22X2.5-4 17-22X4-5 60- 72X5-7 60-175X6 60-100X5-6 Citrus Allium Astragalus Vitus 24 25 26 27 *8 16 3883 22 7767 11 3679 22 7741 10- 6845 C. rhoinum C. vermicularia C. ailanthi C. fructus (V. fructus} C. brassicae 20-22X4-4.5 20-22X4 22X4-5 17-23X2.5-3.5 19-24 X 150-180 80- 90X4 90-135X5-9 100-4"OOX5-8 Rhus Bromus Ailanthus Pirus Brassica 29 30 31 32 33 34 35 22 7763 22 7744 3 3842 11 3683 22 7732 22 7768 11 3681 C. sublineola C. hedericola C. erumpens C. falcatum C. vitis C. cereale C. omnivoriim 15-25X4-5 17-25X3.5 25X5 25X4 21-25X2.5 18-26X3-4 20-28x3-5 '30- 50X7 140X7 150X4 100-200X4 140-160X6-9 60-120X6-8 Sorghum Hedera Ruscus Saccharum Vitis Grasses Funkia, Aspedis- 36 37 38 39 40 41 42 43 44 22 7753 3 3844 22 7765 18 5025 14 4493 14 4494 18 5036 22 7766 4 3245 C. platani C. lineola C. antarcticum C. fici-elasticae V. acalyphae V . oxyspora C. andro'pogonis C. janczewslcii V . isabcllina 25-28X3-5 25-28X3.5-4 15-30X3-4 20-30X3-4 25-30X2-3 25-30X2-3 30X4 24-34X3-6 200 60X3 30-100X4-6 200X6 150-210X4-7 100-150X4-5 100-150X8 70-150X4 tra, etc. Platinus Andropogon, Umbellifers Poa Ficus Acalypha Beta Andropogon Poa Salix 45 4 3232 V ' . pulchella Branches 40 4 3252 V ' . melaloma Carica 47 4 3226 V . georginae Georgina 'Spuriously 2 to 3 septate. 308 BULLETIN No. 213 species described in this paper. These are designated as follows, f rom their oldest species, as the Colletotrichum erumpens group: No. 31. Colletotrichum erumpens Sacc. 28. C. brassicae Schulz and Sace. 35. C. omnivorum Hals. 32. (7. falcatum Went. 39. G. fici-elasticae Zimm. 33. C. vitis 1st. 42. C. andropogonis Zimm. 30. C. hedcricola Laub. 34. C. cereale Manns 36. C. platani Da Camara In the present condition of knowledge concerning these forms, and until monographic work is completed regarding them, it seems wise to assume that the rhubarb Colletotrichum belongs to this group, and tentatively to refer to it as Colletotriclmm erumpens. RHUBARB LEAF SPOT (Due to Phyllosticta straminella) This disease was first collected at Bourbonnaise, Kankakee county, Illinois, where it was in great abundance in one field. Nearly every leaf in the plantation was affected with many spots, often occupying considerably more than half of the leaf area. A general idea of the FIG. 11. A SINGLE DISEASED SPOT OF THE PHYLLOSTICTA DISEASE ox RHUBARB 1919] Two ILLINOIS RHUBARB DISEASES 309 nature of the effect upon the leaf is given in the front- cover illustration. More detail of a single spot is shown in Fig. 11. Unlike the preced- ing disease this is not preeminently a disease of old leaves but even the c o mp aratively young leaves may be seriously spot- ted. While mainly affecting the leaf blade, it has also been found upon the petioles. Upon the leaf blade the chief character is the irregularly circular dead spot, varying from a few millimeters up to several centimeters in diameter. The dead area is tan color and often dry and cracked or torn (see front cover). The margin of the spot is definite, with a rather sharp limitation between diseased and healthy tissue. Close inspection shows numer- FIG. 12. AN ENLARGED VIEW OF A PORTION OF THE SPOT SHOWN IN FIG. 11 The pycnidia are here clearly shown. FIG. 13. PHYLLOSTICTA. A PYCNIDIUM IN SECTION, SHOWING SUBCUTICULAR DE- VELOPMENT AND SHAPE OF THE PYCNIDIUM The cavity was entirely filled with spores. 310 BULLETIN No. 213 Fig. 14. Phyllosticta, Pycnidia viewed from above ous very minute dark pustules (pycnidia) (Figs. 11 and 12). On petioles and stems the spots are oval 1 en gthwise of the petiole, dis- tinctly sunken, and quite dry. S om e times a spot extends for nearly the whole length of a peti- ole, occupying p e r haps only one side or part of one side of it. In addition to collections at Fisr. 15. Phyllosticta spores from a leaf spot Fig. 16. Phyllosticta small type spores, Bourbonnaise, others have also been made at Urbana, Kankakee, and Champaign. Specimens collected at Ladoga, Indiana, in 1916, by Dr. P. J. Anderson and at Racine, Wisconsin, by M. W. Gardener, bear the same fungus. When prevalent to the extent shown in the cover illus- tration, and to that degree on many leaves, as is the case in some fields, the drain on the vigor of the plants must be large. THE FUNGUS Microscopic examination invariably revealed the presence of a pycnidial fungus of the Phoma or Phyllosticta type. Fig. 12, an en- FIG. 17. PORTION OF PETRI DISH SHOWING PHYLLOSTICTA COLONIES This plating was made direct from rhubarb leaves. 1919] Two ILLINOIS RHUBARB DISEASES 311 FIG. 18. A SINGLE COLONY OF THE PHYLLOSTICTA ON CORN-MEAL AGAR FIG. 19. PORTION OP PETRI DISH SHOWING SEVERAL COLONIES OF THE PHYLLOSTICTA larged view of a single spot, shows the numerous pycnidia. These are approximately circular, tan colored, about 120 to 150 /x in diameter, with an ostiole 20 to 30 p. wide (Fig. 14). Under appropriate con- ditions the spores issue in cirrhi or long agglutinated coils. From the leaf spot the spores are rather large for a Phyllosticta ( 10 to 19 p by 4 to 5/x), and oblong (Fig. 15) and straight or very slightly curved. Specimens from the petiole and from some cultures to be described later gave a much smaller type of spore, as small as 4 to 5 //, by 1.5 to 5 /x (Fig. 16). 312 BULLETIN No. 213 [January, Isolation The fungus was isolated on corn-meal agar in all of the ways noted on page 301, and with the same general results ; i.e., the fungus grew readily and normally in culture (Fig. 17). Photographs of plates and colonies are given in Figs. 17, 18, and 19. On plates on corn-meal agar, the fungus grows rapidly and pro- duces pycnidia quickly. When thickly sown, sporiferous pycnidia are produced in thirty-six hours ; when more thinly sown, pycnidia are not so quickly produced. The colonies eventually become quite dark in the central region. The pycnidia arise uniformly by the simple meri- stogenous method. There is in corn-meal agar a very remarkable change in spore size from that exhibited by the same fungus when in its natural habitat. On this medium the spores are very rarely as long as 7 p. (Fig. 16). In very old cultures spores of somewhat larger size are found, but none as large as on the original leaf spots. On various autoclaved vegetable plugs the fungus grew well and formed numerous pycnidia normally. As with the Colletotrichum, Tropceolum stems were the most favorable medium. On cabbage the growth was exceptionally white. On rice the whole culture became black and was nearly like that of Colletotrichum (cf. Fig. 9). The conidia germinated readily in hanging drops of water, nearly every spore germinating. There were formed no appressoria such as were invariably formed under similar conditions by the Colletotri- chum. TAXONOMY Regarding this as a Phyllosticta and its spore measurements to be the extremes noted above; i.e., 4 to 19 /x by 1.5 to 5 /x, there are 829 species 1 listed by Saccardo from which, on a basis of spore size, it would be indistinguishable. Accepting the narrower limits of spore measurement obtained from the large spored forms from foliar mate- rial ; i.e., 10 to 19 /x by 4 to 5 /*, there are 48 species listed by Saccardo from which it could not be distinguished by spore measurements. One only of these, however, occurs on any of the Polygonaceae. Ten other species of Phyllosticta are listed by Saccardo on Polygonaceae, but none of these has spores as large as the large-spored form described in this paper. All of these, with the exception of one, have maximum lengths of 12 /x, while in most of them the maximum length is below 8 11. The one large-spored species, P. straminella Bres., described on Rumex acestosa, is given as having spores 12 to 20 p. long and is suf- ficiently close in morphology to the form under consideration to be regarded as the same species. lr riiese facts are taken from a tabulation of the species of Phyllosticta listed by Saceardo, prepared for future publication by Mrs. E. Young True. UNIVERSITY OF ILLINOIS-URBAN*