APPLE SCAB ...

Apple scab picking on Ky trees

Apple scab most consistently serious disease of homegrown apples in Ky.

Figure 3: Older fruit lesions turn dark brown to black, develop a corky (“scabby”) appearance, and frequently become cracked as fruit enlarge. (Photo: Kim Leonberger, UK)

LEXINGTON, Ky. — Apple scab is the most consistently serious disease of homegrown apple and flowering crabapple in Kentucky. The most noticeable losses on apple result from reduced fruit quality and from premature drop of infected fruit. Scab also causes a general weakening of the host when leaves are shed prematurely. Summer defoliation of flowering crabapple due to scab invariably results in fewer flowers the next spring. Resistant cultivars and fungicides are available; however, sanitation is a critical step in prevention and management.

Apple Scab Facts

  • Leaf symptoms begin as olive-green to brown spots (lesions) with indefinite, feathery margins (Figure 1) on upper and/or lower surfaces. As disease progresses, lesions become more distinct, develop a greenish-black, velvety growth, and then thicken, and bulge upward (Figure 2).
  • Infected fruit develop symptoms similar to those on leaves. Older lesions turn dark brown to black, develop a corky (“scabby”) appearance, and frequently become cracked as fruit enlarge (Figure 3). If infections occur on young fruit, uneven growth near “scabs” may cause fruit to be deformed.
  • Heavily infected leaves and fruit may drop prematurely.
  • Hosts include apple, crabapple, hawthorn, and mountain ash.
  • Primary infection occurs during periods of continuous leaf wetness from bud break until 2 to 4 weeks after petal fall.
  • Subsequent infections result from a second spore type (conidia) that are produced in lesions throughout the remainder of the season.
  • Caused by the fungus Venturia inaequalis.
  • The apple scab fungus overwinters in fallen leaves.
Figure 1: Scab lesions initially have indefinite, feathery margins. (Photo: Kim Leonberger, UK)
Figure 1: Scab lesions initially have indefinite, feathery margins. (Photo: Kim Leonberger, UK)
Figure 2: Older foliar lesions become more distinct, develop a greenish-black, velvety growth, and then thicken, and bulge up. (Photo: Kim Leonberger, UK)
Figure 2: Older foliar lesions become more distinct, develop a greenish-black, velvety growth, and then thicken, and bulge up. (Photo: Kim Leonberger, UK)

Management Options

  • Select varieties that are tolerant or resistant to apple scab.
  • Prune trees to improve air circulation.
  • Maintain plant health with proper nutrition and irrigation practices.
  • Destroy fallen leaves and fruit by burning or burying. Commercial orchards can mow or apply nitrogen to aid in breakdown of leaf tissues.
  • Apple scab risk throughout the season can be determined by disease development models. Visit the UK Ag Weather Center site for additional information.
  • Homeowners may apply fungicides containing copper, mancozeb, sulfur, or captan. Preventative application should begin at green tip (typically late March) and continue until 2 to 4 weeks after petal fall. Fungicides can also be used to treat infections after they occur (Note that fungicides are less effective when used in this manner). For additional information on homeowner management of apple scab using fungicides please see Backyard Apple Disease & Pest Management Using Cultural Practices (with Low Spray, No Spray & Organic Options) (PPFS-FR-T-21).
  • Commercial growers should refer to Midwest Fruit Pest Management Guide (ID-232) for recommended fungicides.

Additional Information

  • Apple Scab (PPFS-FR-T-13)
  • Fruit, Orchard, and Vineyard Sanitation (PPFS-GEN-05)
  • Backyard Apple Disease Management Using Cultural Practices (with Low Spray, No Spray & Organic Options) (PPFS-FR-T-21)
  • Simplified Backyard Apple Spray Guides (PPFS-FR-T-18)
  • Disease and Insect Control Programs for Homegrown Fruit in Kentucky including Organic Alternatives (ID-21)
  • Commercial Midwest Fruit Pest Management Guide (ID-232)

Click here to visit the University of Kentucky Pest News blog.

— Kim Leonberger, University of Kentucky Extension Associate and Nicole Ward Gauthier, Extension Specialist

For more news from Kentucky, click here.

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