White Rot of Apple
The white rot fungus, Botryosphaeria dothidea, often referred to as "Bot rot" or Botryosphaeria rot, occurs most commonly on apple trees, but also attacks crabapple, pear, grape, and chestnut trees. On apple trees, it can be observed as a distinct canker on twigs, limbs, and trunks; leaf infections do not occur. Losses from fruit rot can be considerable, especially in southeastern regions of the United States. Drought stress and winter injury have been associated with an increase in infection and canker expansion sizes in northern states.
New infections on twigs and limbs become evident by early summer, appearing as small, circular spots or blisters. As the lesions expand, the area becomes slightly depressed. Cankers stop enlarging in late fall and can be indistinguishable from black rot canker (caused by Botryosphaeria obtusa), making isolation of the pathogen necessary for correct identification of the fungus. By spring, small, black pycnidia--the spore-containing structures of the fungus--appear on the smooth surface of new cankers. On older cankers, these might be present throughout the year. Cankers exhibit a scaly, papery outer bark that often is orange. Tissues beneath the canker surfaces are watery or slimy and brown. Most cankers are not deep, extending at most to the wood.
Fruit rot first appears as small, slightly sunken, brown spots that can be surrounded by a red halo. As the decayed area expands, the core and eventually the entire fruit become rotten. Red-skinned apple varieties can bleach during the decay process and become a light brown. Because of this characteristic, the disease sometimes is referred to as "white rot."
Bot rot of fruit can be confused with both black rot and bitter rot. The decayed apple flesh of black rot is firm and somewhat leathery, that of Botryosphaeria rot is soft. Bitter rot causes a cone-shaped area of decay and concentric rings of spores on the surface.
The fungus grows best under warm conditions, with the optimum temperature for infection about 86°F. Conversely, for black rot infection, the optimum temperature is about 68°F.
White rot overwinters in dead bark, twigs, and cankers within the tree. Fire-blighted branches and dead wood are colonized rapidly and are an important source of inoculum. Living twigs, branches, and trunks might also be attacked. During spring and summer rains, spores ooze from these structures and are splashed onto other parts of the tree. Fruit infections can occur at any time from the bloom period to harvest. Infections in young apples usually are not evident until the apples are nearly mature. Drought, heat stress, mechanical wounding, and winter injury favor disease development.
Same as for black rot of apple.