By Fanny Iriarte
Extension Plant Pathologist
Iowa State University
Gray mold caused serious losses to some greenhouse growers of raspberries in Iowa last year. Gray mold or Botrytis blight is caused by the fungus Botrytis cinerea, which has a wide host range with over 200 reported hosts among ornamentals plants, vegetables and fruits. In addition, Botrytis can cause different kinds of plant diseases, attacking seedlings, flowers, fruits, stems and foliage. It is also a major cause of postharvest rot in fruits. The disease can occur in the greenhouse as well as in the field when weather is humid and cool.
In raspberry and other small fruits, the first visible signs of the pathogen in late winter are small black spots (overwintering fungal structures called “sclerotia”) on infected canes or plant residue. When conditions are right for the pathogen (70 F and over 90 percent relative humidity), the fungus produces masses of spores in a characteristic grape-cluster arrangement.
These spores can be easily carried by air currents or splashing water. Botrytis infects raspberry canes when the canes are flowering, occasionally causing flowers to turn dark and die. Interestingly, the fungus may infect raspberry flowers without displaying obvious symptoms and then go dormant until fruit begins to mature.
Botrytis rot is common near the stalk end, although closed flowers are not usually attacked but may be infected once they open. As berries ripe grayish brown spots appear and eventually the entire fruit may be affected. The dusty gray spots give the disease its name gray mold. Any kind of physical damage, such as wind and insects, predisposes fruit to infection especially in humid environments. If left unharvested, the infected berries become mummified and remain attached to the plant where more fungal spores are produced and spread to adjacent berries. The more mature the berries get, greater the chance to get infected.
Gray mold fruit rot is favored by cool, wet conditions especially rainfall or overhead irrigation prior to and during the harvest period. The length of time surface water is present and temperature influence infection progress. Therefore, the best way to manage this disease is by removing all infected plant material, including mummified berries, from the greenhouse, which must be discarded or burned.
Avoiding overhead irrigation and spacing the plant to improve air circulation will greatly help reduce the disease. In addition, avoiding excess nitrogen fertilization and efficient weed control will help maintain the environment less favorable to the disease.
Protectant fungicide sprays can be applied at seven to 14-day intervals from bloom to petal fall or harvest. Some effective fungicides are Captan, Cabrio, Abound and Pristine; these must be alternated to avoid the fungus from becoming resistant to the fungicide. Follow fungicide label recommendations and precautionary information. For more information on commercial small fruit and grape spray guidelines, refer to the 2009 Midwest Tree Fruit Spray Guide, PM 1375, available from your nearest Iowa State University Extension county office or online at https://www.extension.iastate.edu/store/ListItems.aspx?Keyword=1375.