PARTIAL CENTRAL OF GRAPE POWDERY MILDEW BY THE MYCOPARASITE AMPELOMYCES-QUISQUALIS

Ampelomyces quisqualis normally infects senescent colonies of Uncinula necator in late summer. Our objective was to introduce the mycoparasite at the start of an epidemic, and thereby reduce the rate of disease increase. Prior to establishing field trials, isolates of A. quisqualis were evaluated for pathogenicity, virulence, and host range in greenhouse and laboratory assays. Infection of powdery mildew colonies only occurred when plants were kept wet and resulted in sporulation of A. quisqualis within 10 days. Two isolates of A. quisqualis (G5 and G273) were evaluated for pathogenicity and virulence against 18 monoconidial isolates of U. necator on grape seedlings and showed little evidence of pathogenic specialization. Three isolates (G273, SF419, and SF423) were equally pathogenic to Sphaerotheca fuliginea on cucurbit, S. macularis on strawberry, and U. necator on grape seedlings. All A. quisqualis isolates appear to have a broad host range and cause significant damage to powdery mildew colonies. Pycnidia of A. quisqualis G273 were produced on cotton wicks saturated with malt extract agar or wheat bran malt extract agar. Wicks were suspended above grapevines of Vitis vinifera 'Riesling' and Vitis interspecific hybrid Aurore. Conidia were dispersed during rain to infect powdery mildew colonies while leaf surfaces were wet. Conidia were released for 3 months in 1990 from a single deployment of wicks. Higher numbers of conidia were released during the entire growing season in 1992 and 1993 due to replenishment of colonized wicks at monthly intervals. Wicks released conidia for 1 to 2 months in 1992 and 1993 before becoming depleted. Powdery mildew development was reduced on Riesling vines in 1990 following deployment of colonized wicks at 15 cm of shoot growth. Disease severity but not incidence was reduced on Aurore vines in 1992 under A. quisqualis-colonized wicks. High rainfall in 1992 provided ample opportunities for dispersal of inoculum of the mycoparasite and the wet conditions were conducive to parasitism. Disease development was late and much reduced in 1993, which was a drier season than 1992. Consequently, no differences were observed in A. quisqualis-treated and untreated plots in the same vineyard that year.