Greenbug, Schizaphis graminum (Rondani), has been a key pest of sorghum since the late 1960ís and has exhibited biotype development since at least the early 1960ís. Biotypes have the ability to overcome host plant resistance, which is the most cost effective mechanism of greenbug control in sorghum. We examined the relationship between virulence (the ability of a biotype to cause plant damage) of five greenbug biotypes on sorghum using one subjective and 6 objective damage measures. Sorghum seedlings of four genotypes were infested with ~30 greenbugs and grown in environmental chambers until the susceptible check was rated at 7 on a 9 point damage scale. Initial and final plant height and whole plant fresh and dry weight were recorded. Regression slopes of dry weight on wet weight were all positive, and were shallower for sorghum biotypes than for the other biotypes. Additionally, stepwise regression revealed two predictors of damage rating: biomass (b=0.467, p=0.001) and plant growth (b=-0.361, p < 0.001). Interaction effects between sorghum genotypes and greenbug biotypes were also significant (biomass: p=0.033, growth: p < 0.001). These results show: a) sorghum biotypes have higher virulence on sorghum, b) the subjective rating scale represents important objective damage measures, and c) interaction effects between biotypes and sorghum genotypes exist for the prediction parameters. Understanding virulence through plant damage affords us a practical application, maximizing yield potential, as well as a theoretical application, determining what plant parameters may influence insect biotype development.
Species 1: Homoptera Aphididae Schizaphis graminum (greenbug)
Keywords: plant resistance, host races
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