Understanding and controlling apple scab disease

Hey there, time traveller!
This article was published 20/09/2014 (4011 days ago), so information in it may no longer be current.

This has been a terrible season for ornamental crab apple diseases. I wrote about fire blight a few weeks ago, but I could have easily written about apple scab fungus disease. Although apple scab does make the fruit unsightly, owners of crab apple trees and occasionally apple trees complain about the discoloured leaves that fall during summer. The leaves usually turn prematurely yellow, often with darker blotches or with orange coloured patches.

Apple scab first appears on the undersides of young leaves in the spring as they open up and become targets for infection. As leaves mature, the spots turn shades of brown to olive green. The infected leaves become yellow or orange-yellow as they die, and then fall from the tree. The dropping of these premature coloured leaves is an important symptom of apple scab disease.

Often I find that the badly infected crab apple trees are not in full sunlight. For a good part of the day they are shaded by buildings or larger trees or by the dense leaf canopy in the infected tree. Ventilation can be poor under these conditions, especially during hot humid periods or rainfall events. These are the conditions under which apple scab fungi thrives.

As open-grown crab apple trees rarely show apple scab symptoms, it is important to prune dense, shaded branches within the tree and overhanging branches of nearby larger trees to allow more sunlight into the crab apple tree.

Fungicidal treatments are also effective in controlling this disease. Spray dormant lime sulfur fungicide on the entire tree before the leaves open up in April. You will have to do this yourself as professional spray applicators are not approved to use this product. Later in early June and into early July the tree can be sprayed twice about two to three weeks apart by yourself or professionally with an approved copper sulfate fungicide. If it rains within 24 hours of spraying fungicidal application you will need to redo the spraying. Be sure your professional spray applicator is aware of this condition before you engage a company to do this work.

It will be bothersome to collect the fallen discoloured leaves on a regular basis, but this is considered to be a very important component of disease sanitation as you strive to significantly reduce or even stop the disease cycle. Using a lawn mower to collect the leaves makes the task easier. If you do not collect and get rid of those diseased leaves they will re-infect the tree in the following late spring and summer.

 

Michael Allen M.Sc.F., RPF (ret.) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts, a Manitoba company that provides objective assessments of the condition and the care required for trees and shrubs on home and business landscapes. He is available to visit homes and gardens. He can be reached at 204-831-6503 or 204-223-7709 viburnumtrees@shaw.ca His web site is

www.treeexperts.mb.ca