Rust

• Crabapples are vulnerable to the fungal infection known as rust. It produces yellow to orange spots on the tops of leaves and minute, cup-like growths on the undersides. When juniper and crabapple are planted in close proximity, it is difficult to break the disease cycle, as they pass this fungus back and forth. Monitor nearby junipers for galls -- reproductive bodies the fungus grows on junipers, which then release spores to nearby plants.
  
  

Scab

• Apple scab fungus produces the disease known as scab on crabapples. This condition is most common after wet weather, producing feathery brown to olive-colored leaf spots, which can then crust over into a scab. Severe infections cause leaves to yellow and drop. If the infection reaches the fruit before they mature, the tiny crabapples will become disfigured.
  
  

Powdery Mildew

• Nearly all trees and shrubs are susceptible to powdery mildew, but crabapples are particularly vulnerable, along with lindens, catalpas and chokecherries. Powdery mildew fungus produces millions of spores on leaf surfaces, which are visible as white growths. Small black dots might also be seen; these are a modified spore body that, unlike its white counterparts, survives the winter. Fungus that causes powdery mildew thrives in humid conditions.
  
  

Control Methods

• Though the chemical chlorothalonil can be used on crabapples to reduce rust, one of the best control methods is separating crabapples and junipers upon planting and to remove juniper galls before they have a chance to release spores. Chlorothalonil can also be used to help control the apple scab fungus, but prevention here is also key. The fungus overwinters on fallen leaves, so raking up and disposing of fallen leaves and fruit will help prevent infection. Occasional or late-season powdery mildew infections do not need to be treated, as they will recede on their own or die back in cold temperatures. However, frequent infections can be controlled with a wettable sulfur spray available at gardening stores. Mix this at a rate of 2 to 4 lbs. per 100 gallons. Tender foliage may sustain damage from the spray, particularly in hot weather.