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

SHANER, G., and R. E. FINNEY. 1977. The effect of nitrogen fertilization on the expression of slow-mildewing resistance in Knox wheat. Phytopathology 67: 1051-1056. Powdery mildew development on the slow-mildewing conditions disease severity increased greatly on Vermillion wheat cultivar Knox was compared to that on the susceptible but increased little on Knox. The area under the disease cultivar Vermillion over a period of 4 yr in the field at progress curve had a lower error variance than statistics Lafayette, Indiana. Cultivars received three levels of nitrogen associated with the logit transformation of severity data and fertilizer to determine if high levels of N affected the hence was a superior measurement of slow-mildewing. Slowexpression of slow-mildewing in Knox wheat. Knox's mildewing remains effective under the highest rates of resistance was evident under conditions favoring moderate to nitrogen fertilization likely to be applied to wheat. In severe disease on Vermillion. Under low nitrogen fertility or breeding for slow-mildewing, high rates of N provide optimal unfavorable weather there was little difference in level of conditions for recognition of this resistance. mildew on the two cultivars; under more favorable Additional key words: Erysiphe graminis, Triticum aestivum, epidemiology, general resistance, breeding for disease resistance, nitrogen fertilization effects. Wheat Triticum aestivum L. em. Thell. 'Knox' has cultivars with shorter and stiffer straw become available resistance to Erysiphe graminis f. sp. tritici that has been and because soil nitrogen level is an important factor in described as slow-mildewing to distinguish it from the mildew development (1, 4, 5, 6, 7, 8, 14). An additional resistance conferred by Pin genes (9). Because Knox has objective of this study was to find a way to measure slowmaintained its slow-mildewing character for many years mildewing suitable for use in a wheat breeding program. over a wide geographical area, this resistance appears to be race nonspecific and to have value for wheat breeding MATERIALS AND METHODS programs (3, 9, 10). Slow-mildewing in Knox is characterized by fewer successful infections and lower Wheat cultivars Knox (C. I. 12798) and Vermillion (C. spore production per colony, both of which restrict the I. 13080) were grown in four-row plots, each 2.4 m long rate of disease development in the field (10, 11). with 30 cm between rows. The plots were sown in Unfortunately, slow-mildewing is only a partial September on a fine sandy loam soil on the Purdue resistance, it is not expressed in seedlings, and it behaves Agronomy Farm, West Lafayette, Indiana. Potassium genetically as a quantitative trait (12). Moreover, Knox's and phosphorus were applied uniformly to the plot area slow-mildewing is a relative trait which can only be each fall in amounts adequate for high yield. Nitrogen (N) assessed by comparing mildew development on Knox and cultivar treatments were arranged in a split-plot with mildew development on a susceptible cultivar grown design with N levels as main plots and cultivars as under the same conditions. Because the rate of mildew subplots. Main plots were separated by four rows of the development depends on environment as well as host wheat cultivar Arthur, highly resistant to E. graminis. genotype, we would expect the measured level of slowShortly after emergence, a nitrogen topdress was applied mildewing in Knox to vary among years and locations, as follows: High-N (N3) plots received N as ammonium Therefore, we measured the slow-mildewing in Knox over nitrate at 100 kg/ha in 1971 and 1972 and 67 kg/ ha in a range of conditions favorable or unfavorable for the 1973 and 1974; intermediate-N (N2) plots received half disease to see if it would be expressed consistently. that amount each year; and low-N (NI) plots received Mildew development on Knox was compared with that none. In the spring, plots were topdressed as follows: N3, on the susceptible wheat cultivar Vermillion for 4 yr. Each 66 kg/ha in 1971 and 1972 and 90 kg/ ha in 1973 and 1974; year, three levels of nitrogen fertilizer were compared N2, half the amount applied to N3; and NI, none. About because farmers tend to use higher levels of nitrogen as mid-April wheat seedlings infected with an E. graminis culture collected in the field at Lafayette, Indiana in 1970 Copyright © 1977 The American Phytopathological Society, 3340 were placed between rows in each plot as a source of Pilot Knob Road, St. Paul, MN 55121. All rights reserved. inoculum. Beginning in early May and at weekly intervals