Apple Disease - White Rot
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. This relatively weak fungal pathogen is only problematic when the tree is stressed, such as due to drought, winter injury, insect damage, or fire blight. The fungus also causes fruit rot, which can be considerable, especially in southeastern regions of the United States.
Symptoms
New infections on twigs and limbs start to become evident by early summer, appearing as small circular spots or blisters, sometimes with an orange tint. 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 Diplodia seriata), 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. The scaly, papery bark can be easily peeled from the tree. Tissues beneath the canker surfaces are watery or slimy and brown. Most cankers are not deep, extending at most to the wood.
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 orange spores form on the surface.
Disease Cycle
Similar to 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 onto 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 are usually 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 of the wood tissue.
Disease Management
Management is the same as for black rot of apple. Since stress predisposes the tree to this disease, be sure to take measures to minimize stressors such as water stress, winter injury, disease, and insect damage.
