Scab on Crabapples

Why are the leaves falling from my crabapple? This has been a common question. The answer in most cases is scab. Scab is a disease caused by the fungus Venturia inaequalis. In early stages, spots on occur on leaves as small olive-green to black areas. The spots may coalesce, especially along the midvein of the leaf. Infected leaves will eventually turn yellow and fall from the tree. The extended wet conditions this spring have favored the development of the disease.

To diagnose scab, examine fallen leaves or the leaves remaining on the tree for the presence of leaf spots. The disease is called scab because infected fruit on apple trees typically show raised brown areas that have a scabby appearance.

To control scab:

  1. Rake and remove fallen leaves. The scab fungus overwinters in apples leaves on the ground.
  2. Properly prune the tree canopy for better ventilation and faster drying of leaves.
  3. When planting crabapples, select varieties that are resistant to scab.
  4. Fungicides can be used to protect scab susceptible crabapples. Sprays need to be applied when growth first appears and repeated at 7 to 10 days intervals. Thorough coverage is essential. Fungicides for scab control include Daconil 2787, Cleary's 3336, Funginex, or products that contain captan, maneb, or mancozeb. Carefully read the manufacturer's label.

This article originally appeared in the June 9, 1995 issue, p. 85.

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