Apple Scab on my Flowering Crab Tree?

(Question)

Last year our flowering crab tree looked beautiful, as usual, when the flowers opened in mid May. The leaves came on afterwards as always, but in late June the leaves turned yellow and brown and by mid July they had all dropped off and the tree looked dead. No fruit grew. This year it looks like the blossoms are forming again. I looked up the symptoms last year and apple scab seemed to be what is hurting the tree. Is there something I can use to get rid of the Apple scab without harming the tree, the soil, or any creatures who wish to land or live in my tree?
Thanks for your time.

Barbra Lee

 

(Answer)

 

Crab apple trees certainly are beautiful when in flower, so I can understand you wanting to keep your tree healthy and blossoming year after year.

Apple scab is caused by the fungus Venturia inaequalis. The severity of the disease varies from situation to situation, and although it can cause severe leaf drop, it rarely kills a tree. However, if severe defoliation occurs for several years, the tree could be weakened, making it vulnerable to other problems. Apple scab can also infect hawthorn and firethorn.

Fungal spores overwinter on infected fallen leaves and fruit. Spring wind and rains carry the spores on to the young leaves. Cool, moist springs favor infection. Ten to twenty days after infection, new spores are released which infect new leaves. These secondary infections can continue throughout the summer until conditions become too hot and dry. Wet leaves are needed for the fungal spores to germinate and infect the leaf.

Integrated Pest Management Strategies

  1. Live with the disease. Apple scab rarely kills trees and the severity of infection will vary from year to year depending upon climatic conditions. Give more attention to young trees that may not establish well if prematurely defoliated. Also take note of any trees that become weakened through several years of defoliation and act accordingly.
  2. Keep plants healthy. A tree that is watered during dry periods will be better able to withstand periodic defoliation.
  3. Clean up and destroy infected leaves. The fungus overwinters on infected leaves; therefore, it is important to rake up and destroy leaves in the fall. Since the spores also travel on the wind, cleaning up leaves may be only partially effective. Do not compost leaves or infected plant material.
  4. Prune trees to allow good air circulation. Allowing for better air movement will allow leaves to dry more quickly and help prevent infection from occurring. Do not use overhead watering.
  5. Apply fungicidal sprays to susceptible trees if clean foliage is necessary or if defoliation reduces fruiting on eating apples. Sulfur can be used but it must be applied before the fungal spores germinate. Fungicidal sprays must be applied before infection as a preventive to be effective. Please note that apple scab resistance to some fungicides has been found in Ontario. For more information on this, refer to the Government of Ontario Ministry of Agriculture, Food and Rural Affair’s webpage on Managing Apple Scab and Powdery Mildew:

http://www.omafra.gov.on.ca/english/crops/hort/news/orchnews/2013/on-0413a7.htm

  1. Replace the tree with a variety resistant to apple scab. Resistant varieties of eating apples include: ‘Florina’, ‘Freedom’, ‘Gavin’, ‘Jonafree’, ‘Liberty’, ‘Macfree’, ‘Nova Easygro’, ‘Novamac’, ‘Priam’, ‘Prima’, ‘Priscilla’, ‘Redfree’, and ‘Sir Prize’.

Strategies 1, 2, 3, 4, and 6 are organic approaches.

 

We have received previous questions about apple scab, which can be found via these links (or using the search term “apple scab” on our Ask a Master Gardener website):

http://www.torontomastergardeners.ca/askagardener/control-of-apple-scab/

http://www.torontomastergardeners.ca/askagardener/crab-apple-tree/

http://www.torontomastergardeners.ca/askagardener/backyard-trees-infestation/

Also, here is a link to previous question, which includes best practices on caring for fruit-bearing trees:

http://www.torontomastergardeners.ca/askagardener/young-apple-tree-new-leaves-deformed/

 

Good luck with the care of your crab apple tree, and I hope that you are able to enjoy many years of beautiful blossoms.

Thank you for contacting the Toronto Master Gardeners.