Prune limbs before mid-June to avoid removing next year's flowerbuds. Although crabapples are susceptible to a number of diseases, the four of significance are cedar-apple rust, fire blight, scab and powdery mildew. All are endemic to Nebraska. Scab, cedar-apple rust and powdery mildew are less common in western Nebraska where fire blight is the predominant disease. Scab, followed by cedar-apple rust, is the major crabapple disease in both central and eastern Nebraska.
Fire blight is the most difficult to control of the four diseases.
It is also the most devastating because infection of the large scaffold limbs and trunk eventually kills the tree. Scab, cedar-apple rust and powdery mildew, on the other hand, usually don't directly kill trees, but instead render them aesthetically unattractive. Repeated infection, however, makes them more prone to winter injury and general decline. Cedar-Apple Rust Figure 1. Spores released from these tendrils in May and June are blown to crabapples where they primarily infect leaves, although occasionally fruits and twigs become infected.