Fungus causes crabapple tree to lose leaves

Q. I have a flowering crabapple tree that is losing leaves. All during the spring the tree flowered and was beautiful. Now, the leaves turn yellow then fall. I've watered and fed regularly with tree food stakes. The dead leaves sometimes have black spots on them. I've checked for insects and there are none. What do you recommend?A. From the symptoms it sounds as if your crabapple has apple scab caused by the fungus Venturia inaequalis that also infects eating apples. The fungus can live through winter within infected leaves and spread in the spring when spores are released during rainfall, are carried by wind, and infect new leaves. In a severe scab year trees may lose most leaves by early July.The disease can be controlled by planting scab resistant cultivars and by a combination of cultural and chemical controls.Resistant cultivarsThe best way to avoid apple scab is to plant cultivars with high resistance such as Japanese Flowering, Prairie Maid, Prairiefire, Red Jewel, Sinai Fire, and Sugar Tyme. Check at a local nursery for cultivars that do well in your location.Cultural controlsBecause the fungus resides on fallen leaves, rake and destroy leaves soon after they fall and before they dry and crumble. Fungus can still be released and spread from tiny leaf fragments. Prune trees in late winter to maintain an "open" structure that allows for good air circulation and drying.Chemical controlsSpraying crabapple trees annually with a fungicide in April and May is very effective. A total of three or four sprayings (on a seven- to 10-day schedule) in spring usually controls apple scab. The first spraying should be done just before the blossoms open. You can spray the tree yourself but, depending on the size of the tree, it may be best to hire a tree service. Before the spraying is done get an accurate diagnosis from an arborist or take leaf samples for diagnosis to your local Extension Office.Fungicides effective in controlling apple scab include: captan (sold as Captan); chlorothanonil (sold as Daconil 2787); thiophanate-methyl (sold as Halt); and myclobutanil (sold as Immunox.) Check with a nursery to find a product available and recommended for your area. If you do the spraying yourself, make sure to follow all the directions on the container.Even though fungicides are chemicals they do not harm honeybees and can be applied when the tree is blooming.bull; Provided by Mary Boldan, Mary Moisand and Donna Siemro, University of Illinois Extension Master Gardeners. Send questions to Ask a Master Gardener, c/o Friendship Park Conservatory, 395 W. Algonquin Road, Des Plaines, IL 60016, (847) 298-3502 or via e-mail to cookcountymg@sbcglobal.net.