EFFECTIVENESS OF ADULT-PLANT RESISTANCE IN REDUCING GRAIN-YIELD LOSS TO POWDERY MILDEW IN WINTER-WHEAT

We studied the effectiveness of adult-plant resistance (APR) in protec ting grain yields in winter wheat grown in Virginia under varying leve ls of intensity of powdery mildew (caused by Blumeria graminis f. sp. tritici) obtained with different fungicide treatments in field experim ents in two crop years. Mildew severity was assessed at three to four plant growth stages, and the data were used to calculate mean mildew s everity (MMS) and area under the mildew disease progress curve. The su sceptible cultivar Saluda had an average MMS of 5.3%. MMS and grain yi eld for Saluda were significantly negatively correlated in both years, and yield loss averaged 13.4% in untreated plots relative to full-sea son control plots. Both early- and late-season mildew control were imp ortant in protecting grain yield in Saluda. Knox 62 had an average MMS of 2.1%, and disease progress for this cultivar was greater than for other APR cultivars. Grain yields of Knox 62 without fungicides were e quivalent to those obtained with full-season control in both years. Un treated plots of the APR cultivars Massey, Redcoat, and Houser had MMS values lower than 1% in both years, and genetic resistance in these c ultivars was sufficiently high in most cases to negate any additional benefit from fungicides in reducing mildew development. APR is effecti ve under conditions that favor mildew epidemics, and incorporation of APR into semidwarf wheats with high yield potential should result in c ultivars with more durable resistance.