Submitted to: HortScience
Publication Type: Abstract Only
Publication Acceptance Date: 2/16/2001
Publication Date: N/A
Interpretive Summary: Sclerotinia minor causes 'lettuce drop' on lettuce and its relatives. Lettuce drop causes complete collapse, decay and death of infected plants, and routinely causes economically significant levels of damage for commercial lettuce growers. Varieties of lettuce with genetic resistance to Sclerotinia do not currently exist. This study describes progress we have made towards breeding resistant lettuce varieties. Several wild relatives of cultivated lettuce have been evaluated for resistance, and some showed reduced infected levels, compared with susceptible lettuce varieties, when inoculated with Sclerotinia. This study also describes investigations we have conducted in order to determine optimal conditions for evaluating lettuce for resistance to Sclerotinia under artificial (i.e. greenhouse) conditions. This work represents a valuable first step towards understanding the number and types of genes conferring fungal resistance in nlettuce, and developing varieties of lettuce that are capable of resisting infection by Sclerotinia minor.
Technical Abstract: Sclerotinia minor causes 'lettuce drop' on lettuce & its relatives (Lactuca spp.), which results in complete collapse, decay & death of infected plants. Breeding & genetic studies of resistance to S. minor have been hindered by sizeable variability in disease incidence (DI) between experiments, resulting from influence of environmental factors on artificial inoculation procedures. To increase the reproducibility of greenhouse screening in the context of this breeding program, we have examined several factors including age of inoculated plants, quantity of inoculum (a mixture of S. minor sclerotia & mycelia), & S. minor isolates. Several candidate resistant/tolerant lines have been identified by artificial inoculation experiments. Select candidate lines have been further evaluated in randomized replicated greenhouse & field experiments, either naturally or artificially inoculated with S. minor. Complete resistance has not been identified in any line, however, several lines show a statistically significant reduction in overall DI and/or increased interval between first observation of symptoms & plant death, as compared with susceptible control cultivars. The most resistant lines, none of which are commercially acceptable varieties, have been intercrossed with susceptible cultivars with desirable agronomic characters. F2 & BC1F2 populations have been evaluated for resistance, results from the breeding program will be presented. We have chosen to focus on one resistant genotype, 'PI251246', a primitive L. sativa, to determine the genetic basis of resistance to S. minor. Recombinant inbred lines (RILs) are being generated from a ('Salinas' x 'PI251246') F2 population to determine the gentic basis for resistance derived from 'PI251246'.