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Optimum Disease Potential for Evaluating Resistance to Stenocarpella maydis Ear Rot in Corn Hybrids. B. C. FLETT, Senior Plant Pathologist, Grain Crops Institute, Summer Grain Centre, Private Bag XI251, Potchefstroom, 2520, Republic of South Africa. N. W. McLAREN, Principal Plant Pathologist, Grain Crops Institute, Summer Grain Centre, Private Bag XI251, Potchefstroom, 2520, Republic of South Africa. Plant Dis. 78:587-589. Accepted lor publication 23 September 1993. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0587.

Field trials were carried out at four localities in South Africa to evaluate 35 corn cultivars for resistance to ear rot caused by Stenocarpella maydis. Trials at each locality were split to include inoculated plants and plants infected by natural inoculum. Differences in hybrid disease resistance reactions were recorded, but ranking of genotypes over trial sites was poorly correlated. Regression analyses (Y = AXb) were used to determine the relationship between disease potential of a trial site (X) and observed disease incidence (Y) within a genotype. Disease potential was quantified as the mean disease incidence over all hybrids in a trial. Genotypes could be divided into three categories: 1) linearly related to disease potential, 2) high susceptibility despite a low disease potential, and 3) various degrees of resistance despite increasing disease potentials. This served to explain the absence of constant rankings of hybrids and the often conflicting results when genotypes screened at different localities were compared for disease resistance. Confidence limits fitted to regression lines showed that screening of hybrids within a disease potential of 0.6% to 50.6% was acceptable for determining differences between highly resistant and susceptible hybrids, but the range for distinguishing moderately resistant (intermediate) hybrids from resistant and susceptible hybrids was limited to 4.4 and 26.3% and 1.9 and 40.9%, respectively. Beyond these points, disease reactions converged, suggesting that at very high or very low potentials, the use of cultivars in disease control is limited.