Sooty blotch and flyspeck are caused by different fungi that commonly occur together on the same fruit. The sooty blotch fungus causes superficial, black spots or blotches up to 1/4 inch or larger. These spots may merge, causing most the apple to appear blackened. The flyspeck fungus causes clusters of shiny round dots, each of which is about the size of a pinhead. Pm- 673, "Recognizing common apple diseases in Iowa," shows a color photo of disease symptoms. Both diseases cause superficial damage since the growth affects the cuticle of the fruit. Vigorous rubbing will remove the blackened appearance.
These diseases are most prevalent when summer temperatures are moderate and when extended wet periods occur. Several cultural practices will aid in the control of sooty blotch and flyspeck. Pruning, which facilitates drying by improving air flow, helps to prevent extended wet periods. Proper thinning of fruit is also important. Tightly clustered fruits create a moist environment and also prevent thorough fungicide penetration.
An all purpose pesticide spray (e.g. Ortho's Home Orchard Spray) may be used in disease control, beginning in mid-June. Applications should be made every 10-14 days. Read the label for timing, rates, and safety precautions. Sulfer is the only fungicide available for organic growers, but is less effective than the Captan fungicide contained in Home Orchard Spray.
This article originally appeared in the October 14, 1994 issue, p. 145.