Evaluation of sorghum genotype mixtures in controlling sorghum leaf blight

Abstract

Field and greenhouse experiments were conducted to evaluate the effect of genotype mixtures on the development of sorghum leaf blight caused by Exserohilum turcicum. For the field experiments, three locations of Central Mexico were chosen with different disease potential. Seed of resistant and susceptible sorghum hybrids were mixed in 1:0, 3:1, 1:1, 1:3, and 0:1 ratios. A randomized complete block was used with 8 x 8 m plots in eight replications. Maize was planted around each plot to reduce interplot interference. Diseased area of the leaf was measured for the top 5-6 leaves. Resistant and susceptible plants were evaluated separately. Disease severity on the resistant hybrid was not affected by the mixtures in any location. In contrast, there were significant differences in the amount of diseased tissue on the susceptible hybrid (P <.01) in two locations where high disease incidence was observed. There, addition of as little as 25% resistant plants to the susceptible plant population significantly reduced disease development (P <.05). In the location with low disease incidence, the susceptible hybrid had a nonsignificant tendency to suffer less disease in the mixtures. The susceptible plants had a nonsignificant tendency towards higher yield in the mixtures in all the locations. The variability in pathogenicity of the fungus was investigated in greenhouse experiments. Five isolates from different parts of the world were inoculated onto eight sorghum inbred lines with different degrees of susceptibility to E. turcicum. The inbred lines were ranked according to their disease severity scores. Analysis of variance was done with the rankings. An isolate from Henderson ranked the sorghum lines tested in significantly different way than the other isolates. The components of resistance to E. turcicum were studied in the two hybrids used in the field experiments. The hybrids significantly differed in their values for infection frequency, incubation period, rate of lesion growth, and spore production per unit area. A mathematical model was used to predict how much disease would develop in mixtures. The model underestimated the values for disease development observed in the 1:1 mixture. However, it was found useful to qualitatively predict disease development in mixtures before mixing.