Influence of Environment and Inoculum Density on the Incidence of Brown Rot Blossom Blight of Sour Cherry

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Abstract
When potted Montmorency sour cherry trees were inoculated in full bloom with 5,000 conidia/ml of Monilinia fructicola, placed into mist chambers at 8, 12, 16, or 20.degree. C for 0-24 hr, then incubated for 7 additional days at 20.degree. C and 60-70% relative humidity (RH), blossom blight incidence was proportional to the temperature and duration of the wetting period. Disease incidence was nil without wetting and minimal with 3 hr of wetting, regardless of temperature, but was 5, 7, 28, and 72% with 5 hr wetting at 8, 12, 16, and 20.degree. C, respectively. Disease incidence increased to 21, 78, 90, and 98% after 10 hr of wetting at the same respective temperatures, and reached a level of 85-97% after 24-hr wetting periods, regardless of temperature. The influence of time and temperature on the rate of conidium germination in vitro closely paralleled the influence of wetting duration and temperature on the rate of increase of disease incidence. When potted Montmorency trees were inoculated with either 50, 500, or 5,000 conidia/ml of M. fructicola, placed into mist chambers for 8-24 hr at the abovementioned temperatures, and subsequently incubated at 20.degree. C and 85-90% RH, blossom blight incidence was influenced by inoculum concentration in addition to temperature and wetting duration. Production of conidia of M. fructicola was assessed for 3 yr on mummified Montmorency fruits collected regularly from orchard trees between bud break and harvest, yielding only 2-18% as many conidia during the sample periods before petal fall as during the peak sampling period before harvest. These data suggest that under the New York [USA] conditions, mummified fruits are often a more important inoculum source for initiating fruit rot than blossom blight.

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