Fruit Disease Focus - October, 1997

Black Rot and White Rot of Apples

Black Rot, Botryosphaeria obtusa

I. Introduction: Black rot is a fungal disease that can cause serious losses in apple orchards, especially in warm, humid areas. Three forms of the disease can occur: a fruit rot, a leaf spot known as frogeye leaf spot, and a limb canker. Infected fruit become unmarketable, severe leaf spotting can result in defoliation which weakens the tree, and limb cankers can girdle and eventually kill entire branches.

II. Symptoms: Leaf symptoms first occur early in the spring when the leaves are unfolding. They appear as small, purple specks on the upper surface of the leaves that enlarge into circular lesions 1/8 to 1/4 inch (3-6 mm) in diameter. The margin of the lesions remains purple, while the center turns tan to brown. In a few weeks, secondary enlargement of these leaf spots occurs. At this time, the lesions assume a characteristic "frog-eye" appearance (photo 2-30). As they age, a series of concentric rings develops around the original infection point. Occasionally, small black pycnidia (asexual fungus fruiting body) can be found in the center of the lesion. Heavily infected leaves become chlorotic and defoliation occurs. If defoliation occurs on a yearly basis, tree vigor is greatly reduced and the tree becomes stressed.

Fruit infection, of which sepal infection is the most common form, can occur early in the season. These infections result in blossom-end rot later in the season. Early fruit infection usually appears at the calyx end of the fruit. These lesions begin as reddish spots which later turn purple and are bordered by a red ring. Infected areas on mature fruit become black, are irregular in shape, and are occasionally surrounded by a red halo. As the rotted area enlarges, a series of concentric bands of uniform width form which alternate in color from black to brown. The flesh of the rotted area remains firm and leathery. Black pycnidia are often seen on the surface of the infected fruit. Eventually, a dry mummy is produced that may remain attached to the tree.

Lesions resulting in canker formation usually are associated with a wound in the bark. In the early stages, the bark is slightly sunken and reddish-brown in color. Some cankers remain small and may die out by the end of the year, while others enlarge from year to year. Some cankers are observed to be merely a superficial roughening of the bark (photo 2-31). In other cases, the canker can kill the bark to the wood and the area becomes cracked (photo 2-32). By the end of the second year, fruiting bodies of the fungus can be observed in the cankered area. Limbs can be completely girdled by this time. The black rot fungus often can be found on wood previously killed by fire blight or damaged by cold temperatures.

III. Disease Cycle: The fungus overwinters in cankers, especially in those initiated by fire blight, in dead bark, and in mummified fruit. These overwintering structures provide an important source of disease inoculum. The black rot fungus covers a wide geographical range and can infect many hosts other than apple. The role these hosts play in the spread and development of the disease is not known. In the spring, spores are released during rainfall. The amount and duration of rainfall, as well as temperature, are the main factors influencing spore release, germination, and infection. Conidia are primarily waterborne and continue to be produced during wet periods throughout the summer. Ascospores are primarily airborne and are most common during the petal fall period. Ascospores and conidia germinate after four hours of wetting over a temperature range of 61 to 90 F (16-32 C). Below temperatures of 61 F (16 C), longer wetting periods are needed for infection to occur. The optimum temperature for leaf infection is 80 F (27 C). At this temperature, four and a half hours are necessary for infection. Leaf infection will not occur, however, at 46 F (80 C) even when leaves have been wet for 48 hours. For fruit infection to occur, temperatures between 68 and 75 F (20-24 C) with at least a nine hour wetting period are required. During rain, conidia ooze out by the thousands and are disseminated by splashing rain, wind, and insects. Spores attach themselves to the plant, germinate in a film of moisture within five to six hours and penetrate the leaf surface through stomata. Early season infection of fruit also occurs through stomata. Later in the season, infection of fruit occurs through cracks in the cuticle or via wounds and possibly lenticels. Often, harvest injuries may become infected and the fruit may decay during or after storage. Throughout the growing season, infections occur through wounds in the bark or on killed wood.

IV. Monitoring: Monitor each tree for cankers (photos 2-31, 2-32, 2-35). Cankers are a source of inoculum which can initiate leaf, fruit, and wood infections. Old fire blight cankers, winter-injured wood, and dead prunings left in the tree often serve as sources of inoculum. Remove cankered wood from the orchard or mulch the brush so that it decays over the period of a year. Inspect trees for apple mummies and remove them from the orchard if possible, since mummies remaining in the trees from the previous season can also serve as a source of inoculum.

Observe 25 fruit and leaves on each sample tree from mid-season through the preharvest period. Be observant for the presence of frogeye leaf spots (photo 2-30) caused by the black rot fungus. Leaf spots may indicate an inoculum source for fruit rot infections. The most susceptible varieties to leaf spots are 'Rome', 'Jonathan', 'Stayman', and 'McIntosh'. Sports of 'Golden Delicious' and 'Delicious' are not as susceptible. If leaf spots are observed it may be possible to locate mummies, dead wood, or cankers (photos 2-31, 2-32, 2-35) higher in the tree; these should be removed if possible. Fruit is most commonly infected at an injury, but infection can occur without the fruit being injured. Although there are some differences in fruit susceptibility among the varieties, all are susceptible. Record the location of trees with an abundance of dead wood and cankers (photos 2-31, 2-32, 2-35) so these can be scheduled for intensive pruning during the dormant period.

V. Disease Management: The disease is managed by removal of inoculum sources (dead wood and mummies) and application of fungicides.  Fungicide applications from silver tip through harvest may be necessary where the disease is a recurrent problem.  The differences in varietal susceptibility to fruit rot are small, although Cortland and Empire may be slightly more susceptible.   View a table of fungicide effectiveness.

White Rot, Botryosphaeria dothidea

I. Introduction: The fungus causing white rot is ubiquitous in nature, occurring on a wide variety of woody plants including birch, chestnut, peach, and blueberry. White rot is often referred to as Botryosphaeria rot or Bot rot and is a serious pathogen of apple fruit and wood. Fruit rot infection is most common in areas of the southeastern U.S. where losses of up to 50 percent have been reported. The canker phase of the pathogen can also cause considerable loss in many regions of the south, midwest, and northeast. Drought stress and winter injury have also been associated with an increase in infection and canker expansion.

II. Symptoms: The white rot fungus only infects fruit and wood. New infections on twigs and limbs become evident in early summer and originate around lenticels, appearing as small, circular spots or blisters. As the lesions expand, the area becomes depressed and a watery exudate may appear on the bark around the blisters. In four to eight weeks, black fruiting structures can be observed within cankers. Cankers stop enlarging by late fall and are indistinguishable from black rot cankers (caused by B. obtusa), making isolation of the pathogen necessary for the correct identification of the causal organism. As the cankers progress, they exhibit a scaly, papery outer bark that is orange in color and often sloughs off (photo 2-35). Under favorable conditions for disease development, cankers will fuse and the girdling of large limbs can occur. The sudden appearance of bright yellow foliage on some apple limbs in late May to early June is one of the more striking symptoms of white rot and occurs when a canker associated with a wound girdles the limb.

Symptoms of fruit rot infection can be seen four to six weeks before harvest and depend on the developmental stage of the fruit. Lesions begin 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 rots. Black fruiting structures may be observed on the surface of the rotted fruit in advanced stages. Red-skinned apple cultivars may "bleach" during the decay process and become light brown in color. Because of this characteristic, the disease is sometimes referred to as "white rot." Decayed flesh associated with white rot is very soft and watery under warm conditions (photo 2-36). When fruit rot develops under cool conditions, however, the rotted area is much firmer and is very similar to black rot infection. On certain apple cultivars, even with these criteria, it is difficult to determine which fungus initiated the infection. Rotted fruit usually drop from the tree, but some will mummify and remain attached.

III. Disease Cycle: The disease cycle for white rot is almost identical to that of black rot. The host range of white rot is broad, but the role these other hosts play in the development of the disease in apples is not known. The white rot fungus overwinters in cankered wood, wood that had been previously killed by fire blight, dead bark, and in mummified fruit. Ascospores and conidia are produced on these structures throughout the growing season. The optimum temperature for germination of both spore types is 82 to 90 F (28-32 C), which can occur in as little as 90 minutes at 82 F (28 C). During wet periods, spores ooze out of fruiting structures and are dispersed by rainfall. Infection of wood can occur through lenticels and wounds. It has also been observed that moisture stress (drought) and winter injury facilitate canker development, especially on older limbs.

Infection can occur anytime from the bloom period to harvest.  Early infections can remain latent until the preharvest period or may not appear until fruit are harvested, stored, and then removed from storage.  There are also differences in cultivar susceptibility to white rot infection. For fruit infection to occur, wounding is not necessary; however, when wounds are present, they are colonized rapidly by the fungus. Infection of wounded fruit can occur in as little as two hours at 82 F (28 C); however, 16 hours of a wetting period is necessary for infection to occur at 46 F (8 C).

IV. Monitoring: Monitor each tree for cankers (photo 2-35). Cankers are a source of inoculum which can initiate leaf, fruit, and wood infections. Old fire blight cankers, winter-injured wood, and dead prunings left in the tree often serve as sources of inoculum. Remove cankered wood from the orchard or mulch the brush so that it decays over the period of a year. Inspect trees for apple mummies and remove them from the orchard if possible, since mummies remaining in the trees from the previous season can also serve as a source of inoculum.

During early midseason, observe 25 fruit on each sample tree. Fruit is most commonly infected (photo 2-36) at an injury, but infection can occur without the fruit being injured. Although there are some differences in fruit susceptibility among the varieties, all are susceptible. Where white rot has been a problem, observe 'Golden Delicious' trees for an early indication of the disease. Record the location of trees with an abundance of dead wood and cankers (photos 2-35) so these can be scheduled for intensive pruning during the dormant period.

V. Disease Management: The disease is managed by removal of inoculum sources (dead wood and mummies) and application of fungicides.  Prompt removal of fire blighted twigs also helps reduce inoculum since the fungus rapidly colonizes and reproduces on dead woody tissues.  Fungicide applications from bloom through harvest may be necessary where the disease is a recurrent problem.  The differences in varietal susceptibility to fruit rot are small, although Golden Delicious, Empire, and Jersey Mac may be slightly more susceptible.  Irrigation during hot, dry weather may help prevent occurrence of twig and branch infections.  View a table of fungicide effectiveness.

Prepared by A. R. Biggs (modified from original text by J. W. Travis, J. L. Rytter, and A. R. Biggs in the Mid-Altantic Orchard Monitoring Guide)