Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/17/2003
Publication Date: 1/15/2004
Citation: Grube, R.C., Ryder, E.J. Identification of lettuce (lactuca sativa l.) germplasm with genetic resistance to drop caused by sclerotinia minor. Journal of the American Society for Horticultural Science. 2004. v. 129(1):70-76. Interpretive Summary: The fungal pathogen Sclerotinia minor causes the disease lettuce drop. The disease is only partially controlled by chemical and cultural practices, and resistant lettuce cultivars do not currently exist. Several cultivated varieties and wild relatives of lettuce were tested for resistance to the pathogen. Resistance was evaluated by several methods. Plants were grown outdoors in an experimental field plot that contained high levels of the soilborne pathogen. Plants were also grown in pots in the greenhouse to which the soilborne pathogen was added, using two different protocols. Although none of the lettuce genotypes were immune lettuce drop, several were more resistant than the most susceptible varieties tested. The resistance in these lines may be used for breeding resistant varieties. Field tests in different years gave similar results, and disease ratings observed in different replicated field tests were consistent. Although ratings obtained in some greenhouse tests were highly correlated with field ratings, this was not the case for all tests. As a result, greenhouse testing may supplement, but can not currently replace, field testing for selection in S. minor resistance breeding programs.
Technical Abstract: 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 inoculation procedures. Many genotypes exhibited partial resistance to S. minor, showing a significant reduction in disease incidence relative to susceptible controls. The similarity of disease ratings observed in different 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 selections tool, field testing remains an essential component of S. minor resistance breeding programs.