Effects of deep plowing on the distribution and density of Sclerotinia minor sclerotia and lettuce drop incidence

Effects of deep plowing on the density and distribution of Sclerotinia minor sclerotia and disease incidence were evaluated in a field with a history of severe lettuce drop. Incidence of lettuce drop was recorded on the crop prior to deep plowing. A 40 x 102 m area was divided into four replicate plots of 40 x 24 m with a 2-m space between replications. Each plot was further subdivided into 60 subplots of 4 x 4 m. Soil samples were collected and bulked from six random locations within each subplot to a depth of 15 cm before, immediately after, and one lettuce crop after deep plowing from each subplot. Aliquots of 100 g of soil from each subplot were assayed for S. minor sclerotia by wet sieving. In two successive lettuce crops following deep plowing, the total number of plants and the number showing lettuce drop symptoms were counted in each subplot prior to crop harvest. Significant reductions in the mean number of sclerotia and lettuce drop incidence occurred on the crop immediately after deep plowing; however, disease incidence was significantly greater in the second crop. Calculated values of Lloyd's index of patchiness showed that the distribution of sclerotia had changed from a highly aggregated pattern prior to deep plowing to less aggregated patterns approaching randomness subsequently. While the desired effect of reducing the number of sclerotia was accomplished with deep plowing, the altered distribution of sclerotia increased the likelihood of infection of a greater number of lettuce plants. Consequently, a higher lettuce drop incidence was recorded in the succeeding crops. The viability of sclerotia was also significantly higher following deep plowing. Deep plowing is therefore unlikely to be a successful disease management strategy for lettuce drop in the high inoculum density fields in the Salinas Valley.