Designation of a new greenbug, Biotype K, injurious to resistant sorghum

A sorghum [Sorghum bicolor (L.) Moench] line, 'PI 550610', resistant to Biotype I greenbug [Schizaphis graminum (Rondani)] was susceptible to greenbugs collected in Haskell County, KS, in 1992. This isolate is designated Biotype K. It may be of considerable economic importance because of its virulence to PI 550610, which is being used widely in breeding programs to develop sorghum hybrids resistant to Biotype I greenbugs. The other sources of resistance to Biotype I, 'PI 266965', 'PI 550607', and 'Cargill 607E', show ed at least some resistance to Biotype K. PI 266965, derived from Sorghum halepense L., maintained the highest level of resistance to Biotype K. However, this tetraploid is not currently useable in breeding programs. In addition, 12 sorghum lines resistant to Biotypes C and E were susceptible to Biotype K. Thus, PI 550607 and 'Cargill 607E' are the only known sources tested that resist Biotype K, but the level of resistance in these genotypes needs further evaluation. The multigenic greenbug resistance of hybrids such as Cargill 607E may be more durable in the field than resistance based on a single gene. Although Biotype K was discovered before the existence of selection pressure from hybrids resistant to Biotype I, the potential for Biotype K to replace Biotype I would be enhanced by the use of hybrids derived from PI 550610 which are resistant to Biotype I but susceptible to Biotype K. Greenbug resistance to Biotype I in the small grains [wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), and rye (Secale cereale L.] remained effective against Biotype K. Since the small grains are resistant to both biotypes their use in the field probably would not effect the status of Biotype K relative to Biotype I.