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Advanced Apple Scab #876369

Asked July 08, 2024, 3:36 PM EDT

Hello, I have 4 mature crabapple trees (or similar) and 2 of the 4 have had 3 years of serious apple scab. One is moderate. This includes fruit dropping constantly (thousands of little apples), and in the last week the leaves on 2/3 trees have completely yellowed with yellow/brown/black spots and are leaves are dropping as if it's October. They have stunning blossoms each spring, and appear hearty and healthy, initially. These trees are at least 25 years old. Would you recommend completely removing the seriously diseased trees at this point? If I would do that, what would need to be done to ensure no disease would pass on to new trees? (grinding stump, etc.) Also, if I plant replacement ornamental apple trees, would I need to have more than one? I have 4 ornamental trees and 3/4 have apple scab, one has none, and one is moderate, 2 are very bad.

Washington County Minnesota

Expert Response

Apple scab is prevalent this year - probably due to the long wet season we have had. We are getting a lot of questions about it.

Usually, Apple Scab is mostly cosmetic and trees recover from it. Having Apple Scab for several years can weaken a tree as they don’t have the leaf cover to photosynthesize. It is caused by a fungus and trees can be reinfected several times over the season when leaves don’t have time to dry out. 

The best defense is good clean up. The fungal spores live in the leaf litter and during times of rain the spores shoot up into the trees to reinfect them. Clean up all leaf debris and either burn, bury or compost it.  Bag it and put it in your city compost - not your home compost.  A home compost pile will not get hot enough to kill the spores. 

You will have to decide if your trees need replacing. You could work on good clean up this summer and fall and see how your trees are next spring before you make this decision.  If you choose to do this, there are Scab - resistant varieties on the market. You will find a list in the following link. You would not need to grind out the stumps. 

https://extension.umn.edu/plant-diseases/apple-scab

Deb Reierson Replied July 09, 2024, 7:53 AM EDT

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