HETEROTHALLISM AND PATHOGENIC SPECIALIZATION IN UNCINULA-NECATOR

A collection of 35 isolates of Uncinula necator was established. Each isolate originated from a single chain of conidia. The collection included isolates from 10 species of Vitis and isolates from Vitis interspecific hybrids. Isolates were paired in all possible combinations on tissue culture plants of the Vitis interspecific hybrid cultivar Chancellor and were incubated for 60 days at 20 C. Initially, U. necator appeared to be composed of two mating types that comprised a bipolar heterothallic system. Ascocarps were produced abundantly within 14 days in approximately one-half of the pairings. However, three isolates consistently produced low numbers of fertile ascocarps 45-60 days after inoculation in pairings that initially appeared to be incompatible; one of these isolates produced sterile ascocarps after 45 days when grown alone. The time between inoculation and sporulation (latent period) of 35 isolates of U. necator from Vitis was 5-6 days at 20 C on in vitro Chancellor grapevines. Sixteen of the 35 isolates from Vitis spp. did not infect in vitro plants of Parthenocissus quinquefolia, and the latent period among the remaining 19 isolates ranged from 10 to 26 days. Three of 35 isolates from Vitis sporulated in 5-18 days on in vitro plants of Parthenocissus tricuspidata. None of five isolates from P. quinquefolia and only one of six isolates from P. tricuspidata sporulated on in vitro Chancellor grapevines. Isolates from Vitis did not differ significantly in the rate of colony expansion on seedlings of V. vinifera L. but differed in rate of colony expansion on seedlings of V. labruscana. The rate of colony expansion of isolates from P. quinquefolia and P. tricuspidata was greatly reduced on seedlings of V. vinifera and V. labruscana as compared to growth on seedlings of either Parthenocissus sp. Growth of isolates from P. quinquefolia was reduced on seedlings of P. tricuspidata. However, on seedlings of P. tricuspidata, one isolate from the Vitis interspecific hybrid cultivar Rosette grew more rapidly than it did on seedlings of V. vinifera or V. labruscana. Pathogenic specialization was not evident when Vitis isolates were grown on in vitro grapevines of a mildew-susceptible cultivar (Chancellor) or on seedlings of a mildew-susceptible Vitis sp. (V. vinifera) but could be identified on seedlings of a mildew-resistant Vitis sp. (V. labruscana). Isolates collected from Vitis spp., P. tricuspidata, and P. quinquefolia are likely to be best adapted to grow upon their respective hosts. However, certain isolates of U. necator can colonize and sporulate upon both Vitis and Parthenocissus spp.