Bitter rot is commonly found on apples in Missouri during the harvest season, especially on minimally-sprayed trees. This disease was first noted in 1856 and was a major disease before the use of fungicides. In organic orchards infected with bitter rot, severe crop loss can occur within a few weeks of warm, wet weather. This fruit rot is caused by the fungus, Colletotrichum gloeosporioide. Another phase of this organism, known as Glomerella leaf spot, also causes necrotic, irregularly-shaped lesions on apple foliage that can defoliate trees. Leaf spot is problematic on Gala, but it can also be found on other apple cultivars, including Golden Delicious, Pink Lady, and Fuji.
Infection of this disease can occur as early as bloom and first appears as tiny, brown flecks. As fruit develops, infected fruit have small, circular, brown lesions that enlarge. Once the lesions are 1 to 3 cm in diameter, concentric circles formed by acervuli (fungal fruiting bodies) are visible on the fruit surface. As the fruit develops, the rotted area enlarges. When the fruit is bisected, a v-shaped area of decay is visible in the flesh of the apple. Other common fruit rots, such as white rot or black rot, do not cause this v-shaped pattern.
Colletotrichum survives year-round on buds, fruitlets (not removed by thinning), mummified apples, colonized dead wood, and cankers. Also, overwintering leaves and prunings left on the ground are a source of infection the following spring when rainfall occurs. Foliar infections develop at temperatures between 61° and 86°F in as few as two hours during wet conditions. The optimum temperature for fruit infection is 79° F during at least five hours of wetness, although it can occur during or after bloom under favorable environmental conditions. However, fruit infection is most common from mid- to late summer.
To control bitter rot, use sanitation practices in combination fungicide treatments. Remove infected fruit or mummified apples and cankers on limbs throughout the growing season. Also, prune and remove any wood infected fire blight as these shoots can be colonized with Colletotrichum and reinfect the tree. When Colletotrichum infection occurs early in the season, this primary infection can provide abundant inoculum for a secondary outbreak of the disease. Fungicides applied from bloom to harvest can be used to control bitter rot, but the disease is difficult to control after the fruit is infected. For fungicide recommendations, refer to the on-line Midwest Fruit Pest Management Guide.
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REVISED: October 31, 2019