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Expression of Susceptibility to Fusarium Head Blight and Grain Mold in A₁ and A₂ Cytoplasms of Sorghum bicolor

Panicle diseases are among the major constraints to sorghum (Sorghum bicolor) production in the northern Great Plains; host plant resistance is the primary management option. However, essentially all commercial sorghum hybrids contain A₁ cytoplasm, which raises the concern about increased disease risk as a result of cytoplasmic genetic uniformity. To determine the influence of cytoplasmic background on the expression of susceptibility to panicle diseases, F₁ hybrids with four nuclear genotypes in each of two cytoplasms (A₁ and A₂) were planted in three environmentally diverse geographic locations in Nebraska. Fusarium head blight ranged in incidence from 13 to 100% across locations. Grain mold, caused primarily by species of Alternaria, Fusarium, and Cladosporium, ranged in incidence from 5 to 100% across locations. There was a significant effect of nuclear genotype on the incidence and severity of both head blight and grain mold across the three locations. Cytoplasm had no effect on head blight incidence or severity, or on grain mold severity. Cytoplasm had a significant effect on grain mold incidence, with A₁ exhibiting slightly lower incidence than A₂ (64 versus 70%). Although the cytoplasm effect for grain mold incidence was statistically significant, most of the variation in grain mold incidence was attributable to nuclear genotype. The slight increase in grain mold incidence attributable to A₂ cytoplasm should be overcome easily by selection of nuclear genotypes with grain mold resistance. The use of A₂ cytoplasm to incorporate genetic diversity into grain sorghum hybrids should not increase the risk of head blight or grain mold in commercial grain production.