Effect of Wetness Duration and Temperature on Infection of Geranium by Botrytis cinerea . C. Sirjusingh, Former Graduate Student, University of Guelph, Guelph, ON NIG 2W1, Canada. J. C. Sutton, Professor, Department of Environmental Biology, University of Guelph, Guelph, ON NIG 2W1, Canada. Plant Dis. 80:160-165. Accepted for publication 16 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0160.

Duration and temperature of postinoculation wetness periods were investigated in relation to infection of flowers and leaves of geranium by Botrytis cinerea. Infection was assessed indirectly by estimating sporulation incidence of the pathogen in inoculated tissues. Sporulation incidence in whole flowers inoculated with conidia increased sharply when wetness duration at 15, 21, 25, and 30°C was increased from 8 to 24 h, 4 to 12 h, 4 to 12 h, and 4 to 6 h, respectively. The pathogen did not sporulate after shorter wetness periods at these temperatures or after 24 h wetness at 5 and 10°C. No sporulation was observed in sepals, petals, and stamens when the wet period at 25°C was ≤4 h, or in pistils and pedicels when wetness lasted ≤6 h, but sporulation increased to 100, 100, 100, 73, and 60% in the respective organs as the wet period was increased to 24, 12, 12, 24, and 24 h. Following conidial inoculations of the foliage, B. cinerea sporulated in 1-week-old leaves only when the postinoculation wetness period at 15, 21, 25, and 30°C was ≥6, ≥4, ≥6, and ≥6 h, respectively, and in 10-week-old leaves when wetness at the respective temperatures lasted ≥6, ≥6, ≥8, and ≥8 h. Sporulation was observed in 4-week-old leaves after ≥12, ≥8, and ≥6 h wetness at 15, 21, and 25°C, respectively. The pathogen did not sporulate in leaves of any age group when postinoculation wet periods were 0 to 24 h at 5 or 10°C, or in 4-week-old leaves at 30°C. Botrytis cinerea sporulated more frequently in 1- and 10-week-old leaves than in 4-week-old leaves when wetness lasted for 8 to 24 h at 25°C. The pathogen infected leaves more efficiently when conidia were applied to geranium petals that after 24 h were positioned on the leaves, than when conidia were applied directly to the leaves. Logistic regression models were developed that adequately described effects of postinoculation wetness duration and temperature on sporulation incidence and, by inference, infection incidence, in flowers and leaves