Diallel Analyses for Tolerance in Winter Wheat to the Barley Yellow Dwarf Virus1

Twelve winter wheat (Triticum aestivum L. em Thell) parents, chosen for their differential reaction to Barley Yellow Dwarf Virus (BYDV), were crossed in a dlallel mating design, reciprocals included, to study the inheritance of tolerance to BYDV under field conditions. Plants in F1 and F2 generations were inoculated with a vector nonspecific isolate of the virus in separate fall and spring tests by infestations induced with greenhouse reared viruliferous Rhopalosiphum padi L. F1 and F2 controls were also grown. Visual disease severity ratings and grain yield of infected plants (fall or spring) expressed as a percent the entry control mean (YLD%C) were used measures of tolerance. Significant mean squares (P = 0.01) were found in all cases for general combining ability (GCA) and for specific combining ability (SCA), and in five of eight cases for reciprocal effect. GCA effects appeared most important in progeny tolerance to BYDV. GCA effects for tolerance and mean parental response to BYDV infection were good indicators of parental value, particularly if the parent was very tolerant or very susceptible. There was no association of GCA effect for yield in the control diallels with GCA effect for YLD%C in infected diallels. SCA effects significantly different from zero were infrequent, indicating less importance as compared to GCA. The number of significant reciprocal effects in the various tests were frequently attributable to random chance. Failure of significant reciprocal effects to occur consistently with crosses involving a particular female parent, or to occur consistently between tests in F1 and F2 generations, indicated lesser importance of reciprocal effect on progeny response to BYDV.