VIEW ARTICLE

Ecology and Epidemiology

Germination of Ascospores and Infection of Vitis by Uncinula necator. David M. Gadoury, Research associate, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; Roger C. Pearson, professor, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456. Phytopathology 80:1198-1203. Accepted for publication 25 June 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1198.

Ascospores of Uncinula necator were released from cleistothecia periodically from November (leaf fall) to May (bud break) by fracturing the ascocarp wall and allowing the spores to be discharged onto glass slides. Water potential of ascospore cytoplasm decreased continuously during this period. Ascospores did not germinate before November. Thereafter (until April), ascospores germinated only in the presence of free water but frequently burst in water, presumably because of high pressure potentials within the spore. After April, ascocarps dehisced naturally when wet, the released ascospores rarely burst in water, and ascospores germinated in water and at a vapor pressure deficit (VPD) of 1453 Pa at 25 C. However, germination at 25 C was reduced from 92 to 97% in water or 0 Pa VPD, to 40.4% at 600 Pa, 30.6% at 760 Pa, and 16.9% at 1453 Pa VPD. Similar effects of free water and humidity upon appressorium formation were recorded. The percentage of ascospores that germinated and formed appressoria increased as temperature was increased from 10 to 23 C. At 10, 15, 20, 22, and 25 C, ascospores infected 23, 57, 94, 92, and 89%, respectively, of inoculated in vitro plants of the Vitis interspecific hybrid cultivar Chancellor. Ascospores did not form appressoria or infect tissue culture plants at 5 C, or at 31 and 36 C. Ascospores incubated at 20?25 C germinated within 4 hr, formed lobate appressoria within 12 hr, and occasionally formed multiple germ tubes after 24 hr at 0 Pa VPD. U. necator is generally considered to be a xerophyte whose anamorph is adversely affected by free water. However, free water is required for ascocarp dehiscence and ascospore discharge and has no deleterious effect upon ascospore germination or infection of Vitis by ascospores.