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They cause only surface blemishes that detract from fruit appearance and lower fruit quality and market value. Sooty blotch also shortens fruit storage life because of increased water loss. Sooty blotch is a disease complex caused by several unrelated fungi. Flyspeck is caused by the fungus Zygophiala jamaicensis.
Sooty blotch appears on fruit surfaces as sooty or cloudy blotches with indefinite borders. These blotches are olive green to black and can be removed by rubbing vigorously. Flyspeck looks like true "flyspecks," characterized by sharply defined, small, black, shiny dots in groups of a few to nearly 100 or more.
Both fungi overwinter on the twigs of many woody plants, as well as on apple and pear trees. The diseases are spread by these overwintering hosts. Spores of the fungi are windblown into and throughout the orchard, and fruit infection can occur anytime after petal fall. It is most prevalent in mid- to late summer. Disease outbreaks are favored by extended periods of above-normal summer temperatures, combined with frequent rainfall and high humidity. These diseases usually appear on fruit late in the season.
Routine fungicide sprays normally control these diseases in Pennsylvania. Cultural controls include the removal of alternate hosts such as brambles from the orchard and surrounding hedgerows. Dormant-season and summer pruning that opens up the tree canopy and facilitates air movement and the drying of fruit after a rain period will help control these diseases. Thinning to separate the fruit clusters also will help prevent disease.
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