Fruit infection can occur early but is more common from mid to late season. Often, the first infections appear in cone-shaped areas within the tree beneath mummies or a canker. Factors which determine the time of appearance of bitter rot are the maturity of fruit, temperature and humidity, and the presence of disease in the area. The optimum conditions for disease development include rainfall, relative humidity of 80 to 100 percent, and warm temperatures. At 80 F, lesions can develop and produce spores within 11 days of inoculation.
Epidemics can develop rapidly during prolonged warm, wet weather, and losses can be extensive.
The most severe epidemics usually occur when the early season is warm and wet and primary infection occurs early, providing abundant secondary inoculum. Remove dead wood from the orchard or mulch the brush so that it decays over the period of a year. Inspect trees for apple mummies and remove them from the orchard if possible, since mummies remaining in the trees from the previous season can also serve as a source of inoculum. During mid-season and continuing through preharvest, observe 25 fruit on each sample tree. Nittany' fruits are very susceptible to bitter rot and may provide an early indication of the disease.