what causes fire blight crabapple
A. Judging by your description and the branch sample you sent, your crabapple (Malus spp.) is suffering from a common disease known as fire blight. This bacterial disease, caused by the bacterium Erwinia amylovora, affects many plants in the rose family, including apple, cotoneaster, crabapple, hawthorns, pears, pyracantha and serviceberries. Fire blight enters susceptible plants through wounds and through natural openings, such as the nectaries of flowers.