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Ecology and Epidemiology

Initiation, Development, Dispersal, and Survival of Cleistothecia of Uncinula necator in New York Vineyards. David M. Gadoury, Research associate, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; Roger C. Pearson, associate professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 78:1413-1421. Accepted for publication 27 May 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1413.

Cleistothecia of Uncinula necator began development as undifferentiated spherical groups of pseudoparenchymatous cells attached to the mildew colony by two parent hyphae of compatible isolates of the heterothallic fungus. As diameter of the ascocarp approached 30 μm, cells of the outer ascocarp wall produced anchorage hyphae that became intertwined in the surrounding mildew colony but did not anastomose or form appressoria. Anchorage hyphae were distinct from appendages, which appeared as the ascocarp grew to 75 μm in diameter, and which were directed upward and away from the mildew colony. Asci formed once the ascocarp had reached 75 μm in diameter, and, at 20 C, ascospores formed within 7 days of the first appearance of asci. The final event in development was the necrosis of the anchorage and parent hyphal connections to the mildew colony, followed by the formation of a basal concavity in the ascocarp wall. The mature concavo-convex ascocarps were dispersed in vineyards during rains and were deposited on the bark of the vine and on the soil surface. Ampelomyces quisqualis, a mycoparasite, had infested 23–61% of the mildewed leaves collected in September 1986 and 10–100% of the leaves collected in September 1987. In five of eight vineyards studied, leaf litter decomposition was complete by the time of bud break in 1986 and 1987. Of the cleistothecia that remained on leaves, canes, and berry cluster stems in spring, approximately 95–100% had died during winter without releasing ascospores. Only dead cleistothecia were recovered from the upper 1 cm of vineyard soils in spring. However, large numbers of cleistothecia were recovered from suspensions prepared from the exfoliating bark of several cultivars, and viability in populations of cleistothecia on bark ranged from approximately 50 to 90%. Temperature, day length, humidity, leaf age, and host resistance did not affect the initiation of cleistothecia, which appeared to require only the pairing of compatible mating types. However, the rate of growth and maturation of cleistothecia was affected by temperature and host resistance. No growth occurred at 4 or 32 C, whereas at 10 C cleistothecia increased in diameter but did not advance beyond the stage of early ascus development. Mature cleistothecia were produced on tissue culture plants and on detached leaves within 25–36 days of inoculation at 16–25 C. Cleistothecia grew and matured more rapidly on susceptible cultivars than on resistant cultivars. The date of first appearance of cleistothecia in vineyards and the date of first dispersal of cleistothecia were a function of the incidence and severity of powdery mildew. In the more severely diseased vineyards, cleistothecia formed and dispersed earlier and in greater numbers than in the less severely diseased vineyards. Dispersal of mature cleistothecia generally preceded parasitism of mildew colonies by A. quisqualis. Cleistothecia parasitized by A. quisqualis were common on leaves, but no parasitized cleistothecia were found on bark. Cleistothecia of U. necator appear to be adapted to dispersal to and overwintering on the bark of grapevines. Dispersal of cleistothecia from the tissues where they form to the bark of the vine places the cleistothecia immediately adjacent to emerging shoots in spring, insures that destruction of shed organs by detritivores does not result in a reduction in primary inoculum, and removes the dispersed cleistothecia from infection by A. quisqualis.