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Apple Scab

University of Minnesota Extension, Stearns County News
July 16, 2014       
           
Source:  Beth Berlin, Extension Educator-Horticulture
University of Minnesota Extension
Stearns & Benton Counties

 

Apple Scab
By Beth Berlin, University of Minnesota Extension

ST. CLOUD, Minn. (7/16/14) — Apple trees and crabapple trees went through a tough winter, cold wet spring, and now seem to be having some issues with their leaves.  Apple scab could be the culprit.  This spring’s conditions, moist and cool, were perfect for an infection to occur.

Apple scab is caused by the fungus Venturia inaequalis, which will overwinter in infected leaves or leaf litter on the ground.  The spores eject from last year’s leaves, and quickly mature in spring and will infect new buds. Although the infection occurred earlier in the spring, the symptoms often don’t become prevalent until mid-summer.  Due to the wet spring conditions, apple scab is affecting many apple and crabapple varieties this year, including some that normally are quite resistant.

Symptoms of infection usually start as olive green to brown spots on the leaves with an irregular or feathered edge. As the infection spreads, the spots merge together and look dark brown or even velvety in appearance.  In heavy infections the leaves may turn yellow and drop off the tree; leaving the tree quite bare. Fruit can also be infected; common symptoms are also olive green to brown spots on young fruit, which can later become hard and black or corky, making the fruit inedible. Severely infected fruit may be deformed and crack open.

Typically apple scab does not seriously harm the tree; however keeping good records of frequency and severity will help make proper management decisions. For repeated infection can cause the tree to be more susceptible to winter damage.

To minimize the effects of apple scab one should practice persistent sanitation of leaf litter. Rake all fallen leaves and burn, bury, or dispose of them in a sanitation disposal.  Additional management practices include pruning.  Well pruned trees provide air movement, allowing leaves to dry off more quickly and not provide the desired environment for the fungus to grow. Finally minimizing other physiological stress such as watering during drought periods will help keep the tree healthy.

If replacing or planting new, consider the variety.  Resistant varieties are available, although as mentioned even these can be infected on very wet years.  Some varieties are considered immune and do not require any fungicide sprays at any time. Immune apple varieties include Dayton, Freedom, Liberty, McShay, Pixie Crunch, Pritine, Redfree, and William’s Pride. Resisteant flowering crabapple varieties include: Prairiefire, Ann E., Coralburst and Red Jewel.

Unfortunately there aren’t management practices that can be done mid-season when symptoms are most prevalent.  Fungicide application timing is critical to the success of preventing infection.  Product for the homeowner includes captan, lime-sulfur, and powdered or wettable sulfur. This provides a protective coating that will inactivate any spores that land on foliage. Repeat applications need to be applied in timely intervals according to the fungicide label.  Commonly this is when the leaf buds are swollen and begin to open, exposing a half-inch of leaf tissue, and again after blossom petals fall.  This preventative effort breaks the disease cycle and will prevent further infection later in the season.  However if the cycle is not controlled, the leaf and fruit infection will occur and will be a constant threat throughout the entire season.

Monitoring your crabapple and apple trees for apple scab is important.  Follow the management practices indicated to help ensure the health of your trees for years to come. For more information on apple scab visit http://www.extension.umn.edu/garden/yard-garden/fruit/apple-pest-management/apple-scab/

 

PHOTO credit:  Beth Berlin

Contacts

Beth Berlin
Extension Educator, Horticulture
(320) 255-6169
adam0062@umn.edu
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