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.
II. Symptoms. 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. Twig symptoms may be confused with the nutritional
disorders measles and internal bark necrosis. Fruit lesions are small, black, circular and
slightly sunken (photo 2-44). Leaf lesions begin as red halos with light green centers,
and become tan to brown with purple borders. Severe leaf infection may cause defoliation.
Differences in cultivar susceptibility have been noted, with 'Rome Beauty' and 'Grimes
Golden' being more susceptible than 'Yellow Transparent' and 'York Imperial'.
III. Disease Cycle. This wet weather fungus
overwinters and produces conidia in old bark lesions. The optimum temperature for growth
is 82F (28C). Leaf infections may become evident in midsummer, and fruit and stem lesions
slightly later. The incubation period is three to six months on fruit and three to ten
months on bark. If fruit is harvested relatively early in the season, infection may occur
after the last preharvest fungicide application, allowing spread of the disease to
unprotected new growth and new trees.
IV. Monitoring. Beginning in the dormant season,
look for bark lesions on ten shoots per tree in blocks where this minor disease has been a
problem. Record the number of shoots with lesions. At midseason, black pox Infection may
occur during warm wetting periods. Monitor for leaf and fruit (photo 2-44) infections
which may appear in problem areas by midsummer. The incubation period is three to six
months on fruit and three to ten months on bark.
V. Management. Disease-free planting stock
is important to help prevent spread of the disease. Most fungicides used in the
cover sprays at 14-day intervals for summer diseases will control black pox. On
early-maturing apple cultivars, postharvest sprays may be needed to reduce the buildup of
the disease on leaves and twigs after harvest.
Text prepared by K.S. Yoder
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