Effects of Inoculum Concentration and Host Age on Infection of Geranium by Botrytis cinerea . C. Sirjusingh, Former Graduate Student, University of Guelph, Guelph, ON N1G 2W1, Canada. J. C. Sutton, Professor, Department of Environmental Biology, and M. J. Tsujita, Professor, Department of Horticultural Science, University of Guelph, Guelph, ON N1G 2W1, Canada. Plant Dis. 80:154-59. Accepted for publication 16 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0154.

Conidial concentration and age of host organs were investigated in relation to infection of geranium flowers and leaves by Botrytis cinerea. Infection was assessed indirectly by estimating sporulation incidence of the pathogen in inoculated tissues. Threshold concentrations of conidia for inferred infection were ≤10 spores per ml for petals, 10 to 100 spores per ml for sepals, stamens, pistils, and pedicels, and 10³ to 10⁴ spores per ml for 4- to 6-week-old leaves. Infection efficiency in the leaves increased more than 200-fold when geranium petals were inoculated with conidia and positioned on leaves, compared with direct conidial inoculation. A linear regression model adequately described sporulation incidence and, by inference, infection incidence of sepals, stamens, pistils, and pedicels as a function of log₁₀ conidial concentration. Logistic regression models closely described these relationships in petals and leaves. Sporulation incidence in leaves inoculated with conidia or with petals treated with conidia was high in 1-week-old leaves, declined as leaf age increased to 4 weeks, and increased as leaf age increased from 4 to 10 weeks. Observations have applications in the epidemiology and management of gray mold.