Factors affecting primary infection and spatiotemporal patterns of tomato plants naturally infected with black leaf mold in commercial greenhouses.
To identify the source of primary inocula and epidemics of black leaf mold (BLM) of tomato caused by Pseudocercospora fuligena, we investigated contamination on agricultural equipment and the distribution of plants naturally infected with BLM. Viable conidia were found on equipment in greenhouses that had a large BLM outbreak the previous year. All isolates from the equipment caused BLM lesions after inoculation of tomato leaves. The distribution of diseased plants in commercial greenhouses was analyzed using Morisita's index of dispersion (IB). When the primary diseased plants were found before August, the IB value, calculated using the distribution of diseased plants in total and newly recognized in greenhouses, was over 1.0, indicating an aggregated distribution. However, when the primary diseased plants were first seen after August, the IB value tended not to differ significantly from 1, indicating a random distribution. The statistical analysis in this study suggests that new infections as cluster points around a diseased plant and secondary infections as independent cluster points occurred at the same time when disease development was fast. However, secondary infection was slight when the disease developed slowly. We thus hypothesized that conidia on agricultural equipment serves as primary inoculum, that the primary and secondary infection occur at the same time during large outbreaks and that the primary inoculum has a major role in the disease cycle of BLM over long periods in commercial greenhouses.