Brown Rot of Stone Fruit
Brown rot, caused by the fungus Monilinia fructicola, is one of the major stone fruit diseases in Pennsylvania. The disease affects peaches, apricots, nectarines, plums, cherries, and most commercially grown Prunus species. The fungus can cause a blossom and twig blight, a canker, a leaf infection, and a fruit rot. Infected fruit will rot on the tree and after being harvested.
Brown rot first affects blossoms, which wilt and turn brown. The infected blossom parts serve as a source of the fungus for future fruit infections. Blossom infections can extend into and eventually girdle a twig, causing a canker to form. These cankers also serve as sources of inoculum. The cankers do not usually extend into the previous year's wood; however, they may girdle the twig, causing it to die.
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 again cause cankers to form. Rotted fruits dry out, become mummified, and either remain attached to the tree or fall to the ground.
The fungus overwinters in mummies formed the previous season, in blighted twigs, and in cankers. Conidia produced on mummies (in the tree) and cankers are the more common inoculum for blossom infections in the spring. An additional source of conidia is produced on apothecia. Apothecia are fruiting structures of the fungus that form on mummies that have fallen on the orchard floor.
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 to blossoms by the wind. 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 disease development. The severity of brown rot increases as the fruit ripens. Wounded fruit are more susceptible to infection. Mature fruit can completely decay in 2 days from the time of infection under favorable weather conditions.
Removing all mummies and blighted twigs after harvest is important in reducing the amount of fungus overwintering in the orchard. Adequate pruning will increase air circulation, allowing faster drying and fewer fruit infections. Fungicide sprays are necessary during bloom and as the fruit ripens. For effective brown rot control, it is important to manage insect pests that serve as vectors and/or provide wounds for new infections to occur.