Evaluation of Onion (Allium cepa L.) Lines and Hybrids Possessing Bs1 for Resistance against Multiple Isolates of Botrytis squamosa

Onion (Allium cepa L.) lines resistant to Botrytis leaf blight (BLB) were produced by backcrossing the Bs1 BLB resistance of Allium roylei Stearn into cultivated onion. This study evaluated important issues for the commercial use of BLB-resistant onions: the relative degree of BLB control possible when Bs1 is homozygous rather than heterozygous, the control achieved against multiple pathogen isolates, and the potential for negative traits associated with transfer of Bs1. Homozygous BLB-resistant onions tested in inoculated growth chamber screens with five distinct Botrytis squamosa Walker isolates and in naturally infested regional field trials showed significantly lower BLB symptoms than susceptible commercial hybrid controls. F-1 hybrids heterozygous for the Bs1 gene showed levels of BLB symptoms between those of the homozygous resistant and susceptible controls, confirming previous reports of partial dominance. The difference in BLB control in plants heterozygous for Bs1 compared with homozygous plants was more noticeable under heavier disease pressure. Hybrids heterozygous for Bs1 had similar level of BLB control in most of the field trials as their parental homozygous BLB-resistant lines, indicating that either heterozygous or homozygous BLB resistant hybrids could be used commercially for BLB control. There were no differences in the degree of pathogenicity across the five B. squamosa isolates, which were genetically different based on polymorphism for DNA markers, or among the naturally occurring pathogens present at the six different field locations used. No obvious association was noted between presence of the Bs1 resistance gene with unfavorable characteristics such as reduced bulb size or yield.