Remove the trees that are highly susceptible and select other less susceptible disease-resistant crabapples. Alternatively, apply fungicides as leaves begin to emerge, at two weeks and again four weeks after the first application. Frog-eye leaf spot is found commonly on many flowering crabapples and its effect, from heavy defoliation to no impact, depends upon susceptibility to this fungus. The best course of action is to select crabapples which are resistant or tolerant to this disease. Fireblight This devastating disease is caused by a bacterium, Erwinia amylovora.
Symptoms appear as death of new terminal shoots in late spring or early summer.
These shoots appear to be scorched by fire. Fireblight often progresses down through the shoot and forms a canker in the older tissue. Cankers are typically sunken areas that are dark brown to purplish in color. An orange or amber gum may ooze from these infected parts. As the bark dies, the area becomes slightly depressed.