Apple Scab is a foliar disease caused by the fungus Ventura inaequalis. It can affect wild and cultivated apple and crabapple trees. The fungus spends the winter season in the infected leaves that lay on the ground after being shed by the tree. Spores from the fungus are discharged and carried to the budding branches of the tree during wet conditions in the spring. Once infected, the leaves will show olive-green, yellow-tinged, or brown circular lesions. These lesions will form new spores which can be transferred to other areas of the tree in the right conditions causing multiple infection cycles in a season.
Treatment in the early spring is key to managing this disease. A fungicidal treatment at bud break can be effective in breaking the disease cycle. The spores need the leaves to continue to survive and reinfect the tree. Raking, or vacuuming, and destroying any infected leaves that were shed can help control the disease. Keeping trees well pruned can also be beneficial by encouraging airflow and cutting down on the moisture that the fungus requires to spread.
If left unmanaged the tree will continue to weaken with the feeding season being cut short from early leaf drop. This weakness could leave the tree more susceptible to other diseases or insects. Scab-resistant varieties of apple trees are now available which can help reduce the need to control this disease.