Watch out Crabapples! Apple Scab is a Major Threat!

As seen above, apple scab produces lesions on leaves, flowers, fruit and on young succulent shoots. (photo courtesy of Wikipedia)

As seen above, apple scab produces lesions on leaves, flowers, fruit and on young succulent shoots. (photo courtesy of Wikipedia)

Plant diseases love Spring! As temperatures warm and tender new growth emerges, conditions are ideal for pathogens to settle in and take up residence in our favorite trees. It is now when many plants are susceptible and treatable for diseases, such as Dothistroma needle cast of Austrian Pine, Rhizosphaera needle cast of Blue Spruce, rust diseases on hawthorn and pear, and for the purpose of this article: the aesthetically devastating apple scab on flowering crabapple.

What is apple scab and what does it do?

  • Apple scab is caused by the pathogen Venturia inaequalis.  It is a common disease targeting the rose family of plants which includes crabapple, apple, pear, hawthorn, mountain ash, cotoneaster and pyracantha.
  • Apple scab produces lesions on leaves, flowers, fruit and on young succulent shoots.  When infected areas are numerous, leaves can become curled and distorted and severe infections can lead to complete defoliation of trees and shrubs.
  • Several years of early leaf loss generally results in poor growth, reduced bloom and increased susceptibility to winter injury and insect predation.

How does the disease work?

  • Apple scab overwinters in fallen diseased leaves and in the soil.  Disease development is favored by wet weather and cool temperatures that generally occur in central Ohio in the Spring and early Summer.  Fungal spores are moved from the ground and carried to trees by raindrop splash and wind where they make their way to leaves, flowers, and fruit.  
  • During wet periods, newly emerging crabapple leaves are extremely susceptible to infection.  The longer the leaves remain wet, the more significant the infection can be. 10 - 20 days after initial infection, new spores are released which infect new leaves.  Repeated infections can continue through the summer until environmental conditions become too hot and dry for the pathogen.

What can be done to protect my crabapples?

  • For most cultivated varieties of crabapple, fungicides must be applied preventatively to successfully manage apple scab.  Fungicide sprays should begin when the first green leaf tips are emerging with additional sprays being applied in roughly 2-week intervals.  The number of fungicide applications needed can vary with weather conditions but generally, we recommend 3 separate applications.
  • Other cultural practices that can increase the effectiveness of fungicide applications when managing apple scab are:
  1. Raking up and destroying fallen leaves to reduce locations where the apple scab pathogen can overwinter
  2. Space crabapple trees generously to increase air circulation around trees
  3. Prune out crossing limbs, vigorous upright sprouts and suckers to increase airflow and sunlight penetration into the canopy (this can help foliage dry faster after rain or dew)
  4. Avoid overhead irrigation, if plants need water, apply water directly to the root zone.

Additionally, much breeding and selection work has been performed to introduce scab resistant crabapples into the landscape.   A short list of some of the most resistant cultivars includes:

  • Coralburst
  • Donald Wyman
  • Prairiefire
  • Purple Prince
  • Red Jewel
  • Royal Raindrops
  • Sugar Tyme

Other common fungal diseases of plants that can be managed through the use of our fungicides include:  Oak Wilt, Dutch Elm Disease, Powdery Mildew, Anthracnose and Verticillium wilt.