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 (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. 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.

Management:  Removing dead wood, mummies, and cankers from the trees are important cultural practices that may help reduce the incidence and severity of the disease.  Current-season prunings should be either removed from the orchard and burned or raked and then chopped with a flail or rotary mower.  Piles of prunings on the orchard perimeter can serve as sources of fungal spores.  The main method of control is application of fungicides from silver tip through harvest.  Apple cultivars do not vary greatly in their susceptibility to B. obtusa; however, Empire and Cortland may be slightly more susceptible than others.

Chemical control - commercial growers

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Text prepared by J.W. Travis, J.L. Rytter, and A.R. Biggs

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