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. The disease appears on leaves as dark-green, velvety spots. The fungus overwinters in fallen leaves and reinfects trees during rainy weather in the spring. Their presence is frequently accompanied by large numbers of ants.