The Cottony Leak of Cucumbers Caused by Pythium Aphanidermatum1
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THE COTTONY LEAK OF CUCUMBERS CAUSED BY PYTHIUM APHANIDERMATUM1 By CHARLES DRECHSLER Associate Pathologist, Office of Cotton, Truck, and Forage Crop Disease Investiga- lions, Bureau of Plant Industry, United States Department of Agriculture INTRODUCTION tened in places. The tissue in the interior was found very watery and of a While several species of Pythium, peculiar texture, greatly softened, and notably Pythium debaryanum Hesse, so lacking in mechanical firmness as to have been found destructive to a wide be divided very readily with blunt variety of phanerogams in the seedling instruments. Where not occupied by stage, and inimical to the best develop- secondary bacterial invaders, the juices ment of some of these hosts in later draining copiously from the incisions stages, the association of the genus were only slightly turbid. The mate- with decay of commercial vegetable rial gave off a peculiar odor rather products, representing parts of plants inadequately described by the term approaching maturity, has not been "marshy"—not pleasant, but having frequently recorded. Perhaps the most little in common with the putrid smells generally known instance is represented characteristic of the decay of vegeta- by the "leak" of potato (Solanûm bles due to bacteria. ' tuberosum L.) tubers, apparently en- Since the original discovery of the countered by De Bary (I?)2 in Germany trouble no additional material has been more than four decades ago, and more submitted to the writer, and from in- recently made the subject of special quiry it would appear that the type of study ¿n the United States by Hawkins deterioration in question is not fre- {8). A soft rot of sweet pepper {Cap- quently encountered on the Washing- sicum annum L., var. grossum) has been ton market, or at l.east not in quantity. recorded by Lehman {9) from North However, early in July, 1924, J. I. Carolina as being due similarly to Lauritzen found several carload lots in Pythium debaryanum, the decay always both the Pittsburgh and the Buffalo beginning at the blossom end, and markets, of which not inconsiderable affecting fruits not more than 6 or 8 portions were affected in exactly the inches from the ground. The same manner described in the preceding par- fungus is mentioned in the list of fungi agraph. Almost simultaneously G. B. thriving on fruit in Belgium by Él. and Ramsey observed the same decay with Em. Marchai {11), who observed it on its characteristic display of cottony a pear {Pyrus communis L.) lying on mycelium in a carload lot of cucumbers damp ground. on the Chicago market, the shipment in this instance having originated in MATERIAL EXAMINED North Carolina. It is highly probable that in the case of the cucumbers This paper deals with a disease of grown in the Southeastern States the cucumber {Cucumis sativus L.) fruit destruction from this trouble will gen- which the writer first observed in erally be found greatest in the markets specimens submitted to him June 8, of our more remote northern cities, 1922, by the food products inspector of since, other things being equal, the the Bureau of Agricultural Economics quantity of cucumbers affected evi- at Washington, D. C, as being repre- dently increases with the length of sentative of a type of decay found time the shipment is in transit. responsible for considerable damage to Microscopic examination of the spec- a carlot shipped from St. George, S. C, imens obtained on the Washington June 2, 1922. Each fruit was entirely market revealed the fresh cottony encased in a luxuriant cottony mycelial growth as a mass of mycelium composed weft, matted down here and there as a of nonseptate hyphae. Where the wet membranous layer, at first sight weft had been matted down as a wet thus suggesting being wrapped in membranous layer closely adhering to absorbent cotton that had become mois- the substratum, innumerable thou- i Received for publication, Aug. 20, 1924; issued August, 1925. 2 Reference is made by number (italic) to "Literature cited," p. 1042. Journal of Agricultural Research, Vol. XXX, No. 11 Washington, D. C. June 1,1925 Key No. G-427 52243—25f 4 (1035) 1036 Journal of Agricultural Research Vol. XXX, No. 11 sands of oogonia with antheridia and SOME MORPHOLOGICAL FEATURES oospores, were found in all stages of development, the entire apparatus Zoosporangia of the fungus from being readily recognizable as character- cucumber fruits are readily obtained istic of the genus Pythium. The by putting pieces of invaded cucumber softened tissue was everywhere occu- tissue (watermelon or squash tissue pied by branching mycelium, the ele- occupied by the parasite serve equally ments of which showed little evidence well), or thin slices from the surface of definite orientation (fig. 1). At the of Lima-bean agar cultures, into a points where the hyphae passed through shallow layer of sterile water, which the cell walls they were constricted to should preferably be renewed several approximately half their normal diam- times to wash away soluble staling eter. products and excessive food materials. Pure cultures of the fungus were In the course of 2 to 5 hours an abun- readily obtained by placing pieces of dance of new structures are proliferated diseased tissue on corn-meal agar from the surface and periphery of the plates, and transferring portions of old my celia, consisting of stout axial FIG. 1.—Section of cucumber affected with cottony leak, showing tissue occupied by abundance of branching hyphae, and constriction of latter where passing through host cell wall. X 250 mycelium from the margins of the rer elements bearing swollen digitate and suiting growth to tubes of sterile media. short diverticulate branches, these Through the courtesy of J. I. Laur- branches frequently undergoing close itzen and G. B. Ramsey, transfers of successive ramification to yield some- the fungus isolated by them from the what involved complexes corresponding diseased material found in Pittsburgh to the structure discussed by Butler (#) and in Chicago, respectively, were also as "budlike lateral processes." At procured. In general appearance the other times the branches are fewer in cultures thus obtained were practically number and at irregular intervals in indistinguishable from cultures of the open racemose arrangement. In any damping-off fungus, Pythium debary- case, if the entire apparatus is well anum Hesse. A minor but not insig- developed a number of septa varying nificant difference could usually be from one or several to a dozen are made out in watching the development inserted, thus bringing about the de- of the two types of parasites in parallel limitation of a variable number of cultures, as under suitable conditions units, each of which may consist, for the cucumber fungus shows develop- example, oí a digitate branch with its ment of aerial mycelium in quantity by secondary lobulate branches, or of a the end of the second day, whereas in portion of the axial element with per- cultures of the damping-off organism haps one or more diverticulate or such development generally fails to branching laterals. After pronounced ensue until the third day. vacuolization of the protoplasm usual Tlie Cottony Leak of Cucumbers for the sporangia of Pythium, and the development of an evacuation tube from the tip of one of the digitate elements, the contents of each unit escape to form a vesicle in which the that the sporangia of the remaining zoospores are fashioned. The latter forms consist of a simple or branching usually vary from 30 to 40 in num- filament, analogous, for example, to the ber, but individual vesicles develop- sporangium of Aphanomyces among ing as few as half a dozen or as many the Saprolegniaceae, would appear to be as 60 are not rare. Under favorable in need of drastic revision. The spor- conditions zoospore production is ex- angia characteristic of the parasite traordinarily abundant, the amount of causing the cucumber decay discussed material that can conveniently be ac- in this paper are represented, as has commodated in a 10 cm. Petri dish been pointed out, by units resulting giving rise to numbers estimated in from the septation of conspicuously excess of 100,000 in the course of an swollen elements, corresponding to the structures which Ward (15) first figured The organism evidently corresponds and described in his account of a fungus to a fungus apparently first noted in he designated as Pythium grazile De the literature as a variety of Pythium Bary, and which later Butler discov- gracile Schenk by Butler (3), who in ered in all the members of the subgenus India found it parasitic on roots and Aphragmium examined by him. Neither base of stem of ginger (Zingiber offi- of these authors appears to have ob- cinale Rose.) as well as on the roots served the participation of these struc- of castor bean {Ricinus communis L.). tures in the formation of zoospores, Later, Subramaniam (14) investigated Ward supposing them to serve as what he regarded as the same form reservoirs of protoplasm for mycelial more closely and set it off as a new growth or the development of oogonia, species, Pythium butleri. In the mean- while Butler assigned to them a prob- time it had been encountered in the able capacity for surviving unfavorable United States as the cause of a disease conditions. In their studies of what of radishes (Baphanus sativus L.) and presumably were forms identical with sugar beets (Beta vulgaris L.) by Edson the one attacking cucumbers, Edson, (6), who described it as Rheosporangium Subramaniam, and Carpenter illus- aphanidermatum, the type of a new trated and discussed the same type of genus of Saprolegniaceae. The simi- structures as "presporangia," "buds," larity and apparent identity of the and "sporangia," respectively, although American and Indian forms were perhaps without observing them in their pointed out by Carpenter (4), who most luxuriant development.