GENETIC VARIATION FOR REACTION TO Cephalosporium gramineum IN FOUR CROSSES OF WINTER WHEAT

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Abstract
The magnitude of genetic variation for reaction to C. gramineum and the relationship among indices measuring host reaction to C. gramineum were studied in 4 winter wheat crosses. PI 278212 and MT 7579 (resistance sources) were each crossed with ''Marias'' and ''Winalta'' (adapted tester lines). Seventy random F2-derived F3 progenies per cross were planted in nonreplicated rows and were inoculated with C. gramineum. Visual disease scores were recorded on a scale of 0 to 5 (0 = no white heads and 5 = 100% white heads). The F2-derived F4 progenies were planted in a replicated yield trial with control and adjacent inoculated plots the following season. C. gramineum caused significant yield loss in all crosses and in all parents except PI 278212. Comparisons among cross means showed that mean symptom expression approached the resistant parent in all crosses. Crosses from MT 7579 had greater yield in both control and inoculated conditions but had greater yield reduction than crosses from PI 278212. Parents performed similarly in cross combinations, as there was no significant resistance source .times. tester line interaction. F3 disease score was not a reliable predictor of F4 inoculated yield or yield reduction. Estimates of genetic variance components and heritabilities pooled over crosses were largest relative to their standard errors for inoculated yield and smallest for yield reduction. Phenotypic and genotypic correlations were of the same sign and nearly the same magnitude with inoculated yield being positively correlated with control yield and negatively correlated with yield reduction.