The Effect of Nitrogen Fertilization on the Expression of Slow-Mildewing Resistance in Knox Wheat

Powdery mildew development on the slow-mildewing wheat cultivar Knox was compared to that on the susceptible cultivar Vermillion over a period of 4 yr in the field at Lafayette, Indiana. Cultivars received three levels of nitrogen fertilizer to determine if high levels of N affected the expression of slow-mildewing in Knox wheat. Knox's resistance was evident under conditions favoring moderate to severe disease on Vermillion. Under low nitrogen fertility or unfavorable weather there was little difference in level of mildew on the two cultivars; under more favorable conditions disease severity increased greatly on Vermillion but increased little on Knox. The area under the disease progress curve had a lower error variance than statistics associated with the logit transformation of severity data and hence was a superior measurement of slow-mildewing. Slow-mildewing remains effective under the highest rates of nitrogen fertilization likely to be applied to wheat. In breeding for slow-mildewing, high rates of N provide optimal conditions for recognition of this resistance.