Influence of Grower Activity and Disease Incidence on Concentrations of Airborne Conidia of Botrytis cinerea Among Geranium Stock Plants. M. K. Hausbeck, Former Graduate Research Assistant, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. S. P. Pennypacker, Associate Professor, Department of Plant Pathology, The Pennsylvania State University, University Park 16802. Plant Dis. 75:798-803. Accepted for publication 26 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0798.

The relationship of grower activity, fungicide, and disease incidence to concentrations of airborne conidia of Botrytis cinerea among geranium (Pelargonium × hortorum) stock plants within a commercial greenhouse was studied. Hourly concentrations of conidia of B. cinerea were estimated for selected time periods of the 1986 and 1987 growing seasons using a Burkard recording spore trap. The incidence of stem blight caused by B. cinerea was estimated during 1987. Also, the incidence of stems and necrotic leaves with sporulating lesions was estimated. During both growing seasons, daily conidial concentrations increased even after applications of fungicide to control Botrytis blight. During 1987, increases in daily conidial concentrations coincided with increases in incidences of stem blight and of blighted stems and necrotic leaves with sporulating lesions. Peak conidial concentrations (>50 conidia per cubic meter per hour) were typically associated with grower activity, including irrigation and fertilization with a plastic tube drip system with emitters that prohibited splashing, spraying of pesticides, and harvesting of cuttings. The occurrence of peak conidial concentrations during and immediately after harvesting of cuttings is an important consideration in disease management because new infection courts are made available in the form of wounded stems at the same time that concentrations of airborne conidia are large. Further, cuttings removed from the stock plants are frequently exposed to large concentrations of airborne conidia that may influence disease occurrence during propagation.