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Heterotic Effects of Resistance in Maize to Helminthosporium maydis Race O. S. M. Lim, Assistant Professor, Department of Plant Pathology, University of Illinois, Urbana 61801; Phytopathology 65:1117-1120. Accepted for publication 28 April 1975. DOI: 10.1094/Phyto-65-1117.

Heterotic effects of resistance to Helminthosporium maydis race O in maize were estimated from a set of diallel crosses involving 10 parental inbreds [i.e., P(P+1)/2 entries]. Disease evaluations, based upon disease rating on a scale of 0-100 and lesion length, were made at mid-silk following artificial inoculation. Variations attributed to average heterosis, by line, and specific heterosis, between lines, were highly significant. Resistance was partially dominant. Resistant inbreds exhibited resistant-line effects, with inbred Pa884p exhibiting the most resistant effect for both disease evaluations. Differences existed between average heterosis effects of the disease evaluations. Based on disease rating, resistant inbreds contributed less heterotic effects for resistance to single crosses than did susceptible inbreds. Susceptible inbred Mo19 contributed more heterotic effects for resistance to single crosses than any other parental inbreds. Data on lesion length, however, showed that resistant inbreds Pa884p and Va43 contributed greater effects of average heterosis for resistance to single crosses than other inbreds. The heterotic effect for lesion length was smaller in magnitude than that for disease rating. Disease evaluation, generally, indicated that progeny from crosses between resistant inbreds are more resistant to race O than are those involving intermediately resistant or susceptible inbreds.

Additional keywords: Zea mays, southern corn leaf blight, disease resistance.