Etiology, epidemiology, and management of Sclerotinia rot of carrot caused by Sclerotinia sclerotiorum (Lib.) de Bary

The etiology and epidemiology of Sclerotinia rot of carrot was investigated and relevant information was used to develop models for predicting the occurrence of 'S. sclerotiorum' and the start of epidemics. The mechanical reduction of canopy width by lateral clipping was evaluated as a cultural approach for disease management. In addition, the potential of 'S. sclerotiorum' occurring on infested storage boxes was evaluated as a source of inoculum for initiating new infections on stored carrots. In general, disease developed as sequential pre- and postharvest epidemics during the same cropping season. Disease on carrot leaves appeared after canopy closure, after senescing leaves collapsed on the soil, after ascospores appeared in the crop, and after rain initiated 12 to 24 h day-1 of leaf wetness. Foliar disease incidence at harvest ranged from 0 to 62.2% and incidence of root disease after 6 months in storage ranged from 0 to 37.6%. In 2 plots, higher foliar disease at harvest was associated with higher root disease in storage, but one preharvest epidemic did not result in disease in storage. Apothecia were observed after canopy closure, and after 7 to 11 days with soil matric potential of -0.1 to -0.4 bars and soil temperatures of 14 to 23°C. Ascospores were detected after soil matric potentials of -0.1 to -0.3 bars and air temperatures of 15 to 21°C persisted for 7 to 12 days. Variables of canopy closure, foliar senescence, soil matric potential, and soil temperature were selected to construct risk algorithms and expert systems for predicting the occurrence of apothecia and ascospores, the start of epidemics, and the need for initial application of fungicides. Canopy clipping increased maximum air and soil temperatures and decreased relative humidity within the crop, reduced the number of apothecia by up to 76%, and controlled disease without affecting carrot yield. Transmission of 'S. sclerotiorum' through used wooden storage boxes appeared unimportant. The results of this study contribute towards developing improved management options for Sclerotinia rot that integrate crop and microclimate monitoring, prediction of inoculum and disease, canopy modification, and timing the application of disease control practices.