The disease appears first as yellow spots on the upper surfaces of leaves. These spots enlarge, turn orange, and develop small black specks in their centers. Blisters form in the leaf tissue under the spots. Severely infected leaves become distorted a nd fall from the plant. Fire Blight Fire blight is a bacterial disease that may severely damage susceptible varieties of flowering crabapple.
Some varieties are immune to fire blight.
The bark on infected twigs becomes dark and sunken. A milky or brownish ooze may form on the infection. Cankers form where the disease enters a large branch or trunk. The bacteria overwinter in these cankers and a source of infection the following year. Apple Scab Apple scab is a fungus disease that infects the leaves and twigs.