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Leaf drop in flowering crabapple trees

By Staff | Aug 23, 2013

Apple scab is caused by the fungus Venturia inaequalis. The fungus can survive through the winter within infected leaves and fallen fruit. In early spring, spores of the fungus are released into the air when leaves and infected plant debris become wet. Spores are then carried by wind to the newly developing apple leaves and cause leaf and/or fruit infection. Symptoms are very subtle at first, beginning as olive green blotches on leaves. These blotches later turn brown, causing leaves to yellow and drop. Lesions can also appear on fruits, starting as olive green to brown spots and later developing into shallow, corky scabs. Symptoms are more pronounced in the inner canopy and in other shady portions of the tree. This disease cycle repeats itself annually and is more prevalent in seasons when growing conditions are favorable to disease development.

Once leaf spots are visible, the window of opportunity for controlling apple scab has passed and we have lost the battle for this year. Homeowners should not be overly concerned about leaf drop at this time of the year- especially in established trees- as the leaves have pretty much done their job for the year and the tree will survive. It just may look a bit unsightly for a while.

Control of Apple Scab

Since fallen fruits and leaves harbor the scab fungus, rake and destroy them before they become brittle and break into tiny fragments that are difficult, if not impossible, to rake. Prune crabapples in late winter prior to bud break to maintain an “open” tree. A well-pruned tree allows better air circulation, faster drying conditions, and provides for better penetration of spray materials.

Crabapple varieties that are susceptible to apple scab need to be sprayed with fungicide each year on a regular schedule to prevent infection. Homeowners should look for a fungicide that contains either the active ingredient captan or chlorothanolil which can usually be found at most home and garden supply stores. The most critical time to apply fungicides is spring (April and May) as the first blossom buds begin to appear. Generally a minimum of 3 to 4 sprays are required for adequate control of apple scab. These sprays are generally at pre-bloom, the start of bloom, mid-bloom, and full-bloom. Fungicide applications are made on a 10-14 day schedule until dry weather occurs. Since fungicides act as a protective coat on the leaf surface, fungicide applications are most effective in preventing infection when applied just before a prolonged wet period occurs, not after.

It is important to note that fungicides vary in their formulation and percent active ingredient therefore it is critical to follow all label directions regarding amounts of pesticide to use, methods of application, safety warnings, and preharvest intervals. While fungicides are not harmful to honeybees and may be applied during bloom, there are a number of specially formulated, general purpose garden pesticides that contain the above listed fungicides in addition to one or more insecticides. General purpose pesticides containing an insecticide SHOULD NOT be sprayed during bloom as insecticides are toxic to honeybees. Honeybees are beneficial insects and necessary for pollination of flowers, garden plants, and crops.

Fall Webworm

Starting to Appear

Fall webworms have begun to emerge and a number of calls have come into the office from concerned homeowners regarding the presence of these unsightly pests and their nests. Although the silken nests that are appearing in the trees of yards and public grounds this time of year can be very unsightly, fall webworms cause more of a nuisance than a threat to tree health.

Fall webworm (Hyphantria cunea) will eat the foliage of many trees and shrubs. There are a number of trees that are likely hosts for fall webworm including: alder, aspen, birch, chokecherry, cottonwood, elm, fruit trees, maple, pin cherry, poplar, willow, and other hardwood trees, but chokecherry, cherry, birch, and elm leaves seem to be preferred.

There is no need to worry about the defoliation caused by the feeding of fall webworms. The leaves have pretty much done their job for the year and you can let the webworms eat. When they get full, they will leave the nest and find a sheltered place to form a cocoon and spend the winter. The trees will be fine. Often times the defoliation is more distressing to the homeowner than it is to the tree.

Since the caterpillars do their feeding within the web, control with insecticidal sprays is difficult as the pesticide must contact the webworm larvae. This can be achieved by opening the nest- a tedious task- before applying the insecticide. Insecticides containing the active ingredients: carbaryl (Sevin), malathion, bifenthrin, and permethrin can kill fall webworms. Safer insecticides like Bacillus thuringiensis var. kurstaki, horticultural oils, and insecticidal soaps can also be used, but are most effective before the worms become enclosed inside their webs.

It is justifiable to spray young trees to protect them from webworms since these trees don’t have a lot of leaves and they need them to overcome transplanting shock and to prepare for winter. It is hard to justify killing webworms in tall trees such as elms and birches. These mature trees are under minimal stress due to the feeding of webworms and you are more likely to get the toxic insecticide on you when spraying.

Some gardeners choose to prune out nests as soon as they appear, but it is not totally necessary and can cause more harm to the tree in terms of the open pruning wound being an entry point for other infections. The use of propane blow torches to “fry” the worms is not recommended for obvious fire risk potential and can also cause more damage to the tree than the webworms themselves.

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