Black pox of apple (blister canker of pear)

Disease

Black pox of apple (blister canker of pear)

Helminthosporium papulosum Berg.

Distribution: Most common from the southern edge of the mid-Atlantic region southward, but has been reported as a problem as far north as NJ.


On apple, conical, smooth, shiny black swellings are evident on current season's growth. As lesions age, they become ovoid with raised borders (A). On leaves, lesions begin as small, circular green spots surrounded by a red halo. Lesions expand to 1.5–6 mm in diameter and the center of the lesions turn brown. On apple fruit, lesions are approximately 3–9 mm in diameter and appear shiny black, round and sunken (B). On pear branches, fully developed lesions are generally circular, and the interior of the lesion cracks, exposing darkened wood beneath the bark (C).

  • Crops Affected: Apples, Pears

    Management

    The disease is typically controlled with fungicides that target control of apple scab and summer diseases.

    Similar Species

    Infection of the twigs can be confused with the disorder internal bark necrosis (or measles). This disorder, however, is primarily a problem on Delicious and its sports.