Answer: Two fungal diseases come to mind; flyspeck and sooty blotch. While these diseases don?t affect yield or quality below the surface, they can badly discolor apple skin. Apples affected by sooty blotch have greenish or black splotches on their skins. Flyspeck appears as clusters of tiny black dots.
Flyspeck and sooty blotch are caused by different fungi. The environmental conditions that favor the development of these fungi are so similar, however, that the diseases often occur together. The fungi that cause these diseases overwinter on the twigs and branches of apple, pear, and other woody plants. About two to three weeks after petal fall, they are spread to the fruit by wind and splashing rain. The spores germinate best at temperatures of 60-80?F and develop at a relative humidity of 90-95 percent.
These diseases are managed by orchard sanitation and the use of fungicides. Some cultural practices may help prevent the diseases and/or reduce the severity of sooty blotch and flyspeck. These include dormant and summer pruning to open up the tree canopy and thinning or separating the fruit clusters. Both diseases are difficult to control in orchards with restricted air movement.
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