I. Introduction: The bitter rot fungus is almost worldwide in distribution and causes an especially important disease in the southern areas of the U.S. epiphytotics (outbreaks) can occur rapidly and losses can be severe, especially under prolonged warm, wet weather. Several host species can be affected. On peach and nectarine, the same fungus causes a disease known as anthracnose, and on grape it causes ripe rot. The primary hosts, however, are apple and pear. The canker phase of the disease is rare; therefore, the discussion below is limited to fruit infection.

II. Symptoms: The disease occurs in orchard blocks beginning in July through August, however, its appearance varies with the climatic conditions during any particular season. Fruit infection can occur early in the season but symptoms do not develop until the fruit begins to mature. The rot begins as a small, light brown, circular lesion. As lesions enlarge, they change to a dark brown and form sunken or saucer-shaped depressions. The number of lesions per fruit may vary from one to many. When lesions reach about one inch (25 mm) in diameter, fruiting bodies of the fungus appear near the center of the lesion. Under humid conditions, large numbers of spores are produced in a creamy mass, salmon pink in color (photo 2-33), which are often arranged in concentric circles. Under dry conditions, the spore mass appears crystalline. The rotted flesh beneath the surface of the lesion is watery, appearing in a V-shaped pattern in cross section that narrows toward the core (photo 2-34). The fruit decays rapidly as it ripens and eventually shrivels into a mummy that may remain attached to the tree throughout the winter.

III. Disease Cycle: The fungus overwinters in mummified fruit, in cracks and crevices in bark, and in cankers produced by the bitter rot fungus or by other diseases, such as fire blight. Jagged edges of broken limbs are also ideal sites. The bitter rot fungus is one of the few rot organisms that can penetrate unbroken skin of fruit. Although penetration is direct, wounds can be colonized rapidly by the fungus. Spores are waterborne and are released during rainfall throughout the growing season. Fruit infection can occur early but is more common from mid to late season. Often, the first infections appear in cone-shaped areas within the tree beneath mummies or a canker. Factors which determine the time of appearance of bitter rot are the maturity of fruit, temperature and humidity, and the presence of disease in the area. The optimum conditions for disease development include rainfall, relative humidity of 80 to 100 percent, and warm temperatures. Infection can occur in as little as five hours at 79 F (26 C).

IV. Monitoring: Old fire blight cankers, winter-injured wood, and dead prunings left in the tree often serve as sources of inoculum. Remove dead 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 mid-season and continuing through preharvest, observe 25 fruit on each sample tree. The most disease (photos 2-33, 2-34) is likely to be found in low areas of the orchard where drying may be slower. 'Nittany' fruits are very susceptible to bitter rot and may provide an early indication of the disease.

V. Management:  Application of fungicides on a 10- to 14-day schedule from petal fall through harvest is the most effective means for disease control.  More frequent applications may be necessary under conditions favorable for disease development.  Some cultural practices may help reduce inoculum and thus reduce the incidence of the disease.  Cultural practices include removing mummified fruit, dead wood, and fire-blighted twigs from trees and removing newly infected fruit during the growing season.  Although apple cultivars do not vary widely in their susceptibility, the disease is often more severe on Empire, Freedom, Golden Delicious, Fuji, Granny Smith, and Arkansas Black (see table of apple cultivar susceptibility to Colletotrichum spp.).

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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