Host x Isolate Interactions in Corn Inbreds Inoculated withCochliobolus carbonumRace 3

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Abstract
Nine corn (Z. mays) inbred lines were examined after inoculation with isolates of C. carbonum race 3 (= Helminthosporium carbonum) differing in levels of virulence. Reaction was measured by determining the effects of the lines on 3 components of parasitic fitness: disease efficiency, lesion length and sporulation capacity. Differences among host genotypes were significant when tested against the 6 isolates. Isolates differed significantly in their degree of virulence on the host lines. Host .times. isolate interactions for the 3 fitness traits were significant suggesting that isolates responded differently to different hosts. Regression analysis was used to determine the stability of resistance. An inbred line was defined as having stable resistance when it had a low mean disease rating, a regression coefficient near zero, and small deviation from regression. The large variation detected in this study suggested that stable resistance in corn lines to C. carboum race 3 would not easily be found.