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 record ed on the crop prior to deep plowing. A 40 x 102 m area was divided in to four replicate plots of 40 x 24 m with a 2-m space between replicat ions. Each plot was further subdivided into 60 subplots of 4 x 4 m. So il samples were collected and bulked from six random locations within each subplot to a depth of 15 cm before, immediately after, and one le ttuce crop after deep plowing from each subplot. Aliquots of 100 g of soil from each subplot were assayed for S. minor sclerotia by wet siev ing. In two successive lettuce crops following deep plowing, the total number of plants and the number showing lettuce drop symptoms were co unted in each subplot prior to crop harvest. Significant reductions in the mean number of sclerotia and lettuce drop incidence occurred on t he 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 ch anged from a highly aggregated pattern prior to deep plowing to less a ggregated patterns approaching randomness subsequently. While the desi red effect of reducing the number of sclerotia was accomplished with d eep plowing, the altered distribution of sclerotia increased the likel ihood 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 d eep plowing. Deep plowing is therefore unlikely to be a successful dis ease management strategy for lettuce drop in the high inoculum density fields in the Salinas Valley.