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Black Rot of Grapes

Posted on April 23, 2024 by cakaiser2014

Black rot is a common disease of grapes in Kentucky. Due to the prevalence of this disease, it is also one of the most economically important diseases of grapes.  Black rot can affect all young, developing, above ground plant tissues. However, fruit infections are the most destructive. Without adequate disease management, both home and commercial vineyards may suffer significant yield losses.

Black Rot Facts

  • Leaf symptoms first appear as small, round, reddish-brown spots that enlarge to 1/8 to 1/4 inch diameter (Figure 1). These spots may coalesce into larger blotches (Figure 2). Enlarged spots develop dark margins with light brown to tan centers that may contain small, black spore-bearing fungal fruiting structures (pycnidia).
Figure 1: Leaf symptoms are first observed as small, round, reddish-brown spots. (Photo: Nicole Ward Gauthier, UK)
Figure 1: Leaf symptoms are first observed as small, round, reddish-brown spots. (Photo: Nicole Gauthier, UK)
Figure 2: Leaf spots coalesce into larger blotches. (Photo: Nicole Ward Gauthier, UK)
Figure 2: Leaf spots coalesce into larger blotches. (Photo: Nicole Gauthier, UK)
  • Fruit symptoms begin as soft, light brown spots (Figure 3) that enlarge to envelop entire berries. Infected fruit then shrivel into black, wrinkled mummies (Figure 4).
  • Symptoms on shoots, petioles, and tendrils appear as irregular or diamond-shaped, sunken, tan to brown lesions. Black fungal fruiting structures may be observed in lesions.
  • Disease is favored by warm, moist periods. Infection may occur throughout the growing season; however, as plants mature, disease susceptibility decreases.
  • Caused by the fungus Guignardia bidwellii.
  • The pathogen survives winter in mummies, fallen leaves, and stem lesions.
Figure 3: Fruit symptoms begin as soft, light brown spots that enlarge to envelop the entire berry. (Photo: Clemson USDA CES, Bugwood.org)
Figure 3: Fruit symptoms begin as soft, light brown spots that enlarge to envelop the entire berry. (Photo: Clemson USDA CES, Bugwood.org)
Figure 4: Infected fruit shrivel into black, wrinkled mummies, which may stay attached to the cluster or fall to the ground. (Photo: Nicole Ward Gauthier, UK)
Figure 4: Infected fruit shrivel into black, wrinkled mummies, which may stay attached to the cluster or fall to the ground. (Photo: Nicole Gauthier, UK)

Management Options

  • Select cultivars with reduced susceptibility to black rot.
  • Plant in good sites with appropriate air circulation and leaf drying characteristics.
  • Avoid planting in low lying, poorly drained sites.
  • Train vines onto high trellises to promote air circulation and allow for better fungicide penetration.
  • Remove weeds and basal sprouts for increased air movement.
  • Utilize disease prediction models to evaluate risk for infection. The University of Kentucky provides a Black Rot of Grape Disease Prediction Model
  • Remove and destroy all diseased plant tissues.
  • Fungicides may be applied preventatively beginning at bud break (after 1/2 inch new growth) and continue through berry maturity. Homeowners may use fungicides that contain the active ingredients mancozeb or myclobutanil. Contact a county Extension agent for more information on fungicide use.

Additional Information

  • Backyard Grape Disease & Pest Management Using Cultural Practices (with Low Spray, No Spray, & Organic Options) (PPFS-FR-S-24)
  • Black Rot of Grape (PPFS-FR-S-16)
  • Black Rot of Grape – Ag Weather Center Disease Prediction Model (University of Kentucky)
  • Commercial Fruit Pest Management Guide (ID-232)
  • Disease and Insect Control Programs for Homegrown Fruit in Kentucky (ID-21)
  • Effectiveness of Fungicides for Management of Grape Diseases (PPFS-FR-S-18)

By Kim Leonberger, Plant Pathology Senior Extension Associate and Nicole Gauthier, Plant Pathology Extension Specialist

Posted in Fruit
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