Sorghum germplasm tolerant to greenbug (Homoptera : Aphididae) feeding damage as measured by reduced chlorophyll loss

The SPAD chlorophyll meter was used to assess tolerance to feeding damage ; by the greenbug, Schizaphis graminum (Rondani). Grain sorghum [Sorghum bico lor (L.) Moench] cultivars considered tolerant by the tolerance index (TI) and measurements of proportional dry weight change (DWT) measurements, also were found to be tolerant in terms of reduced chlorophyll as quantified by the SPAD chlorophyll meter. This result indicated that the SPAD meter coul d be used as a rapid method of assessing plants tolerant to aphids. Damage progression tests, in which SPAD readings taken days after greenbugs were r emoved, confirmed earlier work that chlorophyll loss continued in the absen ce of greenbugs; the rate of chlorophyll loss varied among cultivars studie d. The susceptible sorghum cultivar 'KS 86' lost chlorophyll twice as fast as the tolerant cultivar 'Cargill 607E' did. The proportional chlorophyll l oss, represented by the SPAD Index, at a specified point in time, as well a s the rate of chlorophyll loss (damage progression) could be utilized as a method of assessing tolerance. Sorghum germplasm accessions were screened f or tolerance to greenbug biotype I using the SPAD Index. Out of 51 accessio ns screened, 11 were tolerant enough, as compared to 'NC+ 160' (control), t o be tested further. The pooled test of these accessions showed that the ge rmplasm accession PI 550610 was highly tolerant to biotype I greenbug feedi ng damage; PI 550610 only lost 22.9% as much chlorophyll as the susceptible check. Eight other entries also lost significantly less chlorophyll due to biotype I greenbug feeding damage, than the susceptible check.