The 2009 ASHS Annual Conference

Leafminer (Liriomyza spp.) is a major insect pest of many important agricultural crops including lettuce (Lactuca sativa L.). Downy mildew caused by Bremia lactucae is one of the most important diseases of lettuce. Resistant varieties remain the most economical means of insect and disease control. Resistance to leafminers was found in PI 509525, an accession of wild relative Lactuca saligna that is considered a non-host of Bremia. The objective of this study was to determine the genetics of resistance to leafminer stings and downy mildew in PI 509525. PI 509525 was crossed to susceptible cultivars Bibb, Da Ye Wo Sun, Margarita, and Salad Bowl, and progenies were evaluated in the field for leafminer sting density and downy mildew severity in four years. F₁ plants all had low sting density and downy mildew resistance, indicating that the resistances were dominant. Chi-square analyses of F₂ progenies showed that each resistance was conferred by a dominant allele in PI 509525. Monogenic control of resistance was confirmed by the segregation ratios of F₃ families. Identification of the resistance genes will greatly facilitate the integration of resistance to leafminers and downy mildew from the wild species into cultivated lettuce in a breeding program.