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White rot

Botryosphaeria dothidea (Moug.) Ces. & De Not.

Fruit lesions become visible 4–6 weeks before harvest, and appear as small, circular, slightly sunken tan to brown spots, sometimes surrounded by a red halo on yellow-skinned fruit. On red-pigmented fruit, the halo appears dark purple to black. Expanding lesions develop in a cylindrical fashion to the fruit core, unlike bitter rot lesions, which tend to be V-shaped. Rotted fruit appear tan to light brown (A) and scattered clumps of black fruiting structures develop on the surface of fruit in the late stages of white rot. The fruit may become soft and watery in warm weather. Fruit rot developing under cooler conditions is firmer and has a deeper tan color, similar to black rot caused by B. obtuse.

White rot cankers begin as small, sunken, reddish brown lesions in spring and enlarge throughout the season, often girdling the infected limb. Pimple-like fruiting bodies develop on canker surfaces 4–8 weeks after infection (B). Shoot dieback may appear above the canker, particularly when the branch is girdled; the appearance of yellow foliage in late May to early June on infected limbs is one of the more striking symptoms of the disease. In the northeast, white rot infection may remain superficial, surviving in the outer bark without damaging the phloem or cambium until drought stress allows the fungus to penetrate and kill these tissues.

Distribution

Widespread; occurs throughout eastern North America.

Similar Species

The fruit rot phase can be confused with bitter rot or black rot, but can be differentiated as described above. The canker phase can be confused with other canker diseases.

Management

Avoid wounding or pruning during periods of drought stress. Remove and destroy infected branches, cankers and other sources of inoculum, such as mummified fruit. Applications of fungicides for control of summer diseases such as apple scab, fruit rots, sooty blotch and flyspeck decrease canker symptoms.

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