External rot is first visible as small, slightly sunken, brown spots that may be surrounded by a red halo. As the decayed area expands, the core becomes rotten and eventually the entire fruit. Photo by K. Peter.
On apple trees it can be observed as a distinct canker on twigs, limbs, and trunks. The fungus produces two types of fruit rot, but 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. This is a relatively weak fungal pathogen and is only problematic when the tree is stressed, such as due to drought, winter injury, insect damage, or fire blight.
New infections on twigs and limbs start to 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 Botryosphareia obtusa), making isolation of the pathogen necessary for correct identification of the causal organism. By spring small, black pycnidia, the spore-containing structures of the fungus, appear on the smooth surface of new cankers. On older cankers, these may be present throughout the year. Cankers exhibit a scaly, papery outer bark that is often orange and that can easily be peeled off of the tree. Tissues beneath the canker surfaces are watery or slimy and brown. Most cankers are not deep, extending at most to the wood.
The fungus causes two fruit rots
Fruit rot infection results in two types of symptoms, depending on the developmental stage of the fruit. One type originates from external infections and the other appears to start internally. External rot is first visible as small, slightly sunken, brown spots that may be surrounded by a red halo. As the decayed area expands, the core becomes rotten and eventually the entire fruit. Red-skinned apple varieties may bleach during the decay process and become a light brown. Because of this characteristic, the disease may be referred to as "white rot."
This external 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, the surface of the spot is not sunken, and pycnidia eventually form. Decayed flesh of Botryosphaeria rot is at first cup- or egg-shaped. The rot is soft, the surface of the spot slightly sunken. Bitter rot causes cone-shaped areas of decay, the surface is sunken, and concentric rings of spores form on the surface.
Apples with no external sign of decay may break down rapidly after harvest or after removal from cold storage. The rot is soft and droplets of a clear sticky liquid may form on the skin. Fruiting bodies of the fungus can be seen under the skin if the apples are left at room temperature. The origin of this internal decay is not known.
Similar to that of black rot. 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 fruiting bodies on dead, woody tissue. During spring and summer rains, spores ooze from these structures and are splashed to other parts of the tree. Dead wood and fire-blighted twigs and branches are especially susceptible to invasion, but living twigs, branches, and trunks may also be attacked. 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. External rot lesions are found most commonly on the sides of fruit exposed to high temperatures. Drought, heat stress, mechanical wounding, and winter injury favor disease development.
Same as for black rot of apple. Since stress predisposes the tree to this disease, be sure to take measures in minimizing stressors, such as water stress, winter injury, disease, and insect damage.