Identification of lettuce (Lactuca sativa L.) germplasm with genetic resistance to drop caused by Sclerotinia minor

Incidence of the disease lettuce drop caused by Sclerotinia minor is often high in California lettuce fields despite the use of cultural and chemical controls. Development of resistant lettuce cultivars has been hindered by the difficulty of evaluating resistance in field tests and the lack of a screening procedure that reliably predicts field performance. Several lettuce genotypes of diverse geographic origin and plant architecture including modern and heirloom cultivars, plant introduction accessions, and breeding lines, were evaluated for resistance to S. minor using several methods. Resistance was evaluated in fields that contained naturally occurring S. minor, in a field that contained both naturally occurring and manually incorporated S. minor inoculum, and in the greenhouse using two types of inocula. Many genotypes exhibited partial resistance to S. minor, with significantly reduced disease incidence relative to susceptible controls. The similarity of disease ratings observed in replicated field tests supports the conclusion that partial resistance is under genetic control. Ratings obtained in some greenhouse tests were highly correlated with field ratings, but this was not true for all tests. Therefore, although greenhouse evaluation with adequate replication and repetition can be used as a selection tool, field testing remains an essential component of S. minor resistance breeding programs.