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 protecting grain yields in winter wheat grown in Virginia under varying levels of intensity of powdery mildew (caused by Blumeria graminis f. sp. tritici) obtained with different fungicide treatments in field experiments in two crop years. Mildew severity was assessed at three to four plant growth stages, and the data were used to calculate mean mildew severity (MMS) and area under the mildew disease progress curve. The susceptible cultivar Saluda had an average MMS of 5.3%. MMS and grain yield for Saluda were significantly negatively correlated in both years, and yield loss averaged 13.4% in untreated plots relative to full-season control plots. Both early- and late-season mildew control were important 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 equivalent to those obtained with full-season control in both years. Untreated plots of the APR cultivars Massey, Redcoat, and Houser had MMS values lower than 1% in both years, and genetic resistance in these cultivars was sufficiently high in most cases to negate any additional benefit from fungicides in reducing mildew development. APR is effective under conditions that favor mildew epidemics, and incorporation of APR into semidwarf wheats with high yield potential should result in cultivars with more durable resistance.