WSU Extension

Hortsense

Crabapple : Scab
(revision date: 1/22/2016)


Biology
Scab of crabapples is caused by the same fungus which causes scab on apple and hawthorn. The disease infects during wet weather in the spring and initially causes small, pale, watersoaked spots on the leaves. The spots enlarge and darken, first to olive-green then to black. Leaves may become distorted and often drop, sometimes resulting in severe defoliation of susceptible trees. Scab can also affect fruit, with pinhead spots enlarging to velvety olive then black blotches. The disease is most favored by cool, wet conditions and overwinters in infected plant debris.
Management Options

Non-Chemical Management
  • Many crabapple cultivars are resistant to scab including 'Bob White', 'Centennial', *'Christmas Holly', 'David', 'Indian Summer', 'Jewelberry', *'Molten Lava', 'Ormiston Roy', *'Professor Sprenger', 'Profusion', 'Red Jewel', and *'White Angel'. Malus floribunda, and M. Tschonoskii are also resistant. (*rated as good to excellent scab and powdery mildew resistance. See EB1809.)
  • Plant in full sun.
  • Avoid overhead irrigation.
  • Rake and destroy fallen leaves.
Select non-chemical management options as your first choice!

Chemical Management

Pesticide recommendations are for flowering (non-edible) crabapples. Apply at pre-pink, pink, calyx. If scab was a problem last year, begin at green tip. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bonide Fung-onil Multi-Purpose Fungicide Conc
    Active ingredient: chlorothalonil  |  EPA reg no: 60063-9-4
  • Bonide Infuse Systemic Disease Control
    Active ingredient: propiconazole  |  EPA reg no: 100-773-4
  • ferti-lome Liquid Systemic Fungicide II R-T-Spray
    Active ingredient: propiconazole  |  EPA reg no: 53883-184-7401
  • GardenTech Daconil Fungicide Conc
    Active ingredient: chlorothalonil  |  EPA reg no: 67572-82-71004
  • Ortho Max Garden Disease Control Conc
    Active ingredient: chlorothalonil  |  EPA reg no: 239-2522
  • Spectracide IMMUNOX Multi-Purpose Fungicide Spray Conc
    Active ingredient: myclobutanil  |  EPA reg no: 9688-123-8845
  • This list may not include all products registered for this use.
Images
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Caption: Apple scab on leaves and young fruit
Photo by: R.S. Byther
Caption: Scab on crabapple
Photo by: R.S. Byther
Caption: Apple scab on fruit
Photo by: R.S. Byther
Caption: Apple scab on leaf
Photo by: R.S. Byther
Caption: Apple scab
Photo by: R.S. Byther