Comparative Analyses of Lettuce Drop Epidemics Caused by Sclerotinia minor and S. sclerotiorum

Temporal and spatial patterns of lettuce (Lactuca sativa) drop caused by Sclerotinia minor and S. sclerotiorum were determined in lettuce fields in the Salinas, Santa Maria, and San Joaquin Valleys in California during 1995 to 1998. Of the 25 commercial fields assessed, 14 had predominantly S. minor, 9 had predominantly S. sclerotiorum, and 2 had varying levels of both species. Sclerotinia infections were classified based on symptoms: those caused directly by eruptive germination of sclerotia (type I) and those caused by the airborne ascospores (type II). The precise location of diseased and healthy plants was mapped and lettuce drop progress was determined at different crop growth stages. Spatial patterns of disease incidence were analyzed using 1-by-1-, 2-by-2-, 3-by-3-, and 4-by-4-m quadrat sizes. Regardless of the analytical method employed, disease incidence with type I infection showed an aggregated pattern in a majority of the fields evaluated and random patterns in fields where incidence was low. In all fields with type I infection, disease progress followed the monomolecular model, typical of soilborne diseases. For fields with aggregated distribution, spatial dependence was observed up to 10 m and was either isotropic or random in direction, suggesting the potential influence of tillage operations on inoculum distribution and disease incidence. Lettuce drop incidence in fields with type II infection was erratic in time and peaked within a very short time. However, disease incidence showed an aggregated pattern in all fields evaluated. Spatial dependence of quadrats generally was detected in two adjacent directions, suggesting a directional gradient perhaps caused by wind direction during ascospore dissemination. Increasing quadrat sizes usually increased the degree of aggregation of lettuce drop, but not the distribution pattern itself. These results demonstrate that the source of inoculum and the type of infections they cause are most likely to determine spatial patterns of lettuce drop in the field.