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

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Abstract
Powdery mildew [Erysiphe graminis] development on the slow-mildewing wheat [Triticum aestivum] cultivar Knox was compared to that on the susceptible cultivar Vermillion over a period of 4 yr in the field at Lafayette, Indiana [USA]. Cultivars received 3 levels of N 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 N fertility or unfavorable weather there was little difference in level of mildew on the 2 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 N fertilization likely to be applied to wheat. In breeding for slow-mildewing, high rates of N provide optimal conditions for recognition of this resistance.

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