Sooty blotch (smoky blotch)
Sooty blotch fungus is a common problem in apple trees with poor air circulation or where humidity is high during the cool season. The fungus Gloeodes pomigena is responsible for the dark, smutty discoloration that makes affected apples look impossibly damaged. Fortunately for growers, sooty blotch on apples is a surface disease only; it may make your apples hard to sell at market, but if you’re eating them at home or canning them for later, a thorough wash or peel will remove all of the fungus.
Sooty blotch fungus requires temperatures between 65 and 80 degrees Fahrenheit (18-26 C.) and relative humidity of at least 90 percent to initiate germination. Under ideal conditions, infection can take place in under five days, but typically requires 20 to 60 days in an orchard setting.