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Apple Scab Diagnostic Guide

Venturia inaequalis

Apple scab, caused by the fungus Venturia inaequalis, is not considered a serious threat to crabapple or apple trees; however, repeated annual infections can weaken the tree making it more susceptible to other insect or disease problems. Apple scab is more of an aesthetic issue because of the early defoliation and blemishing to the leaves and fruit that it causes. Heavy infections can reduce growth and apple yields. Apple scab is dependent on cool, wet weather. If spring weather is dry, scab may not be a problem.

Trees at Risk

Apple scab affects apple fruit trees and flowering crabapples.

Apple

Crab Apple

Symptoms of Damage

  • Heavily infected leaves may become yellow and result in premature defoliation from late spring through late summer.
  • Brown to olive irregular-shaped spots on the leaves in late spring.
  • Spots become dark and velvety in appearance.
  • Fruit may also be infected and display similar symptoms to those on leaves
  • The velvety appearance of the leaf spots is the result of spore production on the leaf surface.

Apple Scab SignsApple Scab Signs

Apple Scab Signs

Microscope image of apple scab lesion.

Biology

  • The pathogen overwinters in infected leaves on the ground from the previous year.
  • In spring, overwintered spores (ascospores) mature and are discharged over a period of 5 to 9 weeks.
  • Wind and splashing rain carry spores from infected leaves to new growth on nearby trees where new infections begin.
  • In late summer or early fall the primary infections produce secondary spores and create new infections, which can continue through the growing season during wet periods.
  • Infections occur most rapidly between 55 and 75 degrees when leaves or fruit remain wet for a continuous 6-9 hours.

Treatment Strategy

The need for fungicides depends on the susceptibility of the tree species and weather conditions. Since apple scab is dependent on wet weather, fungicide applications may be needed for control during wet seasons. Proper sanitation, pruning, and watering, may suffice during dry seasons. Fungicide applications are preventive only and need to be timed properly for effective control. In most parts of the country crabapples will require 2-4 applications at 14 day intervals in the spring.

It is important that the trees are treated in the spring time as this is when the infection occurs and not in midsummer when the symptoms begin to appear.

Other Treatment Practices

  • Avoid wetting the foliage with irrigation during the growing season.
  • Rake and remove fallen leaves from yard in autumn to reduce spores.
  • Each winter, prune suckers and branches throughout the crown to improve air circulation.
  • Foliar Spray using Myclotect Gallon

    Notes: Recommended option. Label allows for residential fruit trees and turf
    Dosage: 6 oz./100 gallons water
    Timing: Begin sprays at leaf expansion when temps are 68 degree with continuous leaf moisture for 6+ hours.
    Re-Treatment: 14 days
    Myclotect Gallon
    Myclotect Gallon
    $169.00
  • Foliar Spray using Propiconazole 1.3ME T&O

    Notes: Common fungicide widely used by professionals
    Dosage: 4 oz/100 gallon water
    Timing: Begin sprays at leaf expansion when temps are 68 degree with continuous leaf moisture for 6+ hours.
    Re-Treatment: 14 days
    Propiconazole 1.3ME T&O
    Propiconazole 1.3ME T&O
    $88.20
  • Foliar Spray using T-Bird 4.5L

    Notes: Common fungicide widely used by professionals
    Dosage: 20 oz./100 gallons water
    Timing: Begin sprays at leaf expansion when temps are 68 degree with continuous leaf moisture for 6+ hours.
    Re-Treatment: 14 days
    T-Bird 4.5L
    T-Bird 4.5L
    $176.00
  • Foliar Spray using T-Bird 85WDG

    Notes: Common fungicide widely used by professionals
    Dosage: 0.8 lbs./100 gallons water
    Timing: Begin sprays at leaf expansion when temps are 68 degree with continuous leaf moisture for 6+ hours.
    T-Bird 85WDG
    T-Bird 85WDG
    $99.50

Treatment Expectations

Spraying for apple scab is difficult. If timing is incorrect, or if rain washes away fungicide applications, control will be lower. In perfect conditions control may be aesthetically pleasing, but the disease will always persist. The focus must be on suppression, not eradication.

A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org

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