Factors Influencing Airborne Conidial Concentrations of Alternaria panax in Cultivated American Ginseng Gardens

Leaf blight, caused by Alternaria panax, is the most common disease of cultivated ginseng and is an annual threat. To determine the influence of weather parameters on airborne conidial concentrations (ACCs), 3- and 4-year-old commercial American ginseng (Panax quinquefolium) gardens were monitored from mid-May to September for two growing seasons. Hourly concentrations of airborne A. panax conidia were enumerated using a Burkard volumetric spore sampler. The hourly averages of air temperature, rainfall, leaf wetness, and relative humidity were also collected. Fungicides were not applied. The incidence of leaf blight was assessed in predetermined areas of the monitored gardens. Disease pressure from A. panax was high in 2005 and 2006 and resulted in crop defoliation. Each year, ACCs were detected beginning in late May and continued through the growing season. Daily conidial concentrations followed a diurnal pattern and were greatest during periods of rapidly decreasing relative humidity. Relative humidity was negatively correlated to hourly ACCs. Each year, hourly ACCs were negatively correlated to leaf wetness and rainfall. A positive correlation was detected for hourly ACCs following 16 h or more of rainfall. A significant correlation was observed between hourly ACCs and temperature.