This alternate host disease can distort the foliage, fruit, and twigs of apples and crabapples, but is mostly a cosmetic injury. For ornamental crabapples in areas of high visibility, it might warrant treatment.
Sometimes the more conspicuous form of this disease is the galls that form on juniper and cedar. Spores are released from here to infect the crabapple in the early spring. These galls do not need treatment, but preventative foliar treatments are available for susceptible crabapples.