The Heritability of Three Parasitic Fitness Attributes ofHelminthosporium maydisrace T

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Abstract
Nine sets of 24 ascoprogeny were isolated from complete coctads obtained from 5 crosses of isolates of H. maydis race T. Conidia of the ascoprogeny were used to quantitatively inoculate RX404 Tms corn [Zea mays] hybrid seedlings. Each experiment included 4 replications of inoculations with the 24 ascoprogeny isolates and the parents. Disease efficiency (DE), the average number of lesions per plant; lesion size (LS), the average lesion area; and sporulation capacity (SC), the average number of conidia produced per square millimeter of lesion was measured. The data obtained from each experiment for each parasitic fitness attribute were analyzed by a 2-way analysis of variance, and genotypic and phenotypic variances were calculated. Fpistasis was estimated by subtracting the progeny mean from the parental mean. NarroW-sense heritability, the ratio of the additive genetic variance to phenotypic variance, was calculated if epistasis was not significant, and broad-sense heritability, the ratio of genotypic variance to phenotypic variance, was calculated if epistasis was significant. The heritability estimates ranged 21-58% for DE, 23-53% for SC, and 0-6% for LS. The fitness attributes of DE and SC are subject to selection. Increased parasitic fitness of DE and SC may result in a reduced effectiveness of nonspecific resistance.

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