Relationship between leaf freckles and wilt severity and yield losses in closely related maize hybrids

The relationship between severity of leaf freckles and wilt, caused by Clavibacter michiganense subsp. nebraskense, and the percentage of grain yield loss was examined in a set of 42 closely related maize hybrids. Forty-two sister inbred lines, derived from a modified backcrossing program that used the inbred A632 as the recurrent parent, were crossed to A619. The resulting hybrids were evaluated over 2 yr in a split-plot field experiment with hybrids as whole plots and inoculated vs. uninoculated treatments as split plots. The hybrids varied widely in reaction to leaf freckles and wilt and in yield loss sustained from the disease. The percentages of yield loss were significantly correlated with disease severities in both years and in the combined analysis. Several hybrids had high disease severity but sustained insignificant yield loss compared with susceptible hybrids, indicating possible leaf freckles and wilt tolerance, However, when a more rigorous test of tolerance that used studentized residuals from the loss-severity regression was applied to the data, tolerance appeared to be an unstable character. Resistance to leaf freckles and wilt was not related to poor grain yield in the absence of disease