Fruit infections begin as small brown spots, and under wet and humid conditions, ash-gray to brown tufts of fungus develop over the surface of the infected area. Photo by S. Bardsley.
The disease can also infect apple fruit late in the season, especially if the orchard is in proximity to stone fruit with a high incidence of brown rot. It is one of the major stone fruit diseases in Pennsylvania.
Brown rot first affects blossoms, which wilt and turn brown. Blossom infections may also extend into twigs, causing necrosis and eventual girdling. The infected blossom parts serve as a source of fungus spores for future fruit infections. Fruit decay occurs as the fruit ripens. The infections begin as small brown spots, and the entire fruit can rot within a few hours under favorable conditions. Under wet and humid conditions, ash-gray to brown tufts of fungus develop over the surface of the infected area. If favorable weather conditions persist, the infection can spread from the fruit into small twigs and cause a canker. The canker may girdle the twig, causing it to die. Rotted fruits dry out and become mummified.
The fungus overwinters in mummies formed the previous season. The mummies persist in the trees or on the ground over winter. The first fungus spores are formed about the time the blossoms begin to open. Upon wetting, the spores are forcibly ejected into the air to be blown by the wind to blossoms. Infected blossoms serve as a source of the fungus for future fruit infections.
Environmental conditions are important for the development of the disease.
Warm, wet, or humid weather is very favorable for the development of the disease. The severity of brown rot increases as the fruit ripens. Wounded fruit is more susceptible to infection. Mature fruit can completely decay in two days from the time of infection under favorable weather conditions.
Removing all rotted fruit after harvest helps to reduce the amount of fungus overwintering in orchards. Adequate pruning will increase air circulation, allowing faster drying and fewer fruit infections. Apply fungicide sprays during bloom and as fruit ripens. To reduce the risk of resistance, alternate fungicides by FRAC group.