Fruit Pathology - Black Pox Black pox, Helminthosporium papulosum I. Introduction.
Black pox is a minor fungal disease which can affect apple bark, fruit, and foliage.
The disease is more common from southern Virginia southward than in the northern mid-Atlantic region. The same fungus causes blister canker on pear. Infection first appears on current season twigs as well-defined, conical, shiny black lesions which enlarge by the end of the first season. Extensive secondary lesion enlargement may girdle small, heavily infected twigs. Smoothbarked twigs remain susceptible for several years and twig infection is cumulative throughout this period.