Spores of the fungi are windblown into and throughout the orchard; fruit infection can occur any time after petal fall but is most prevalent in mid- to late summer. Photo by K. Peter.
They cause surface blemishes that detract from fruit appearance, lowering 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, which are olive green to black, 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 apple and pear. The diseases are spread by these overwintering hosts. Spores of the fungi are windblown into and throughout the orchard; fruit infection can occur any time after petal fall but is most prevalent in mid- to late summer.
There are several disease models, which are variations on the original model published by Brown and Sutton (1995), to predict sooty blotch and flyspeck infection periods. For the Brown and Sutton model, leaf wetness hours greater than 4 hours starting 10 days after petal fall are counted. The threshold to start treatment begins approximately at 220 hours of leaf wetness.
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 this disease in Pennsylvania. Summer fungicide applications should not be extended beyond 14-day intervals. Cultural controls include removing alternate hosts such as brambles from the orchard and surrounding hedgerows. Dormant and summer pruning that opens up the tree canopy and facilitates air movement and the drying of fruit after rainfall helps control these diseases. Thinning to separate the fruit clusters also helps prevent disease.
No cultivar resistance to these diseases is known. Prune trees annually to improve air circulation and reduce the potential for disease. Cool fruit after picking to retard disease development.