Germination of Ascospores and Infection ofVitisbyUncinula necator

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Abstract
Ascospores of Unicula 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, asocarps 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 of 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 ascopores that germinated and formed appressoria increased as temperature as 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 asocarp dehiscence and ascospore discharge and has no deleterious effect upon ascospore germination or infection of Vitis by ascospores.