ISSUE 11 July 10, 2003
The crabapple trees on campus are showing signs of apple scab infection this week. This is a fungal disease, caused by Venturia inaequalis. Initial infections will appear as olive green areas of discoloration on the underside of the leaves on apple trees.
Both leaf surfaces may eventually shows lesions. Numerous infections on young leaves may lead to leaf distortion, such that the leaves become curled, twisted, or dwarfed. Lesions on the leaves initially appear similar to that on leaves, but as they progress may become corky and brown. Fruit may develop unevenly and crack. If the infections occur late in the season on fruit, they will appear more as pin point black lesions.
Management for apple scab actually begins in the fall with good sanitation. Rake up and destroy or completely remove fallen fruit, twigs, branches, and leaves. Protectant fungicides can help minimize damage and loss from this disease. For ornamental crabapples, Captan, chlorothalonil (Daconil and Ortho MultiPurpose Fungicide), and thiophanate-methyl (Cleary’s 3336) are some of the products available. For edible apples, Captan, thiophanate-methyl (Cleary’s 3336), and All-Purpose Fruit Sprays that include one of the above ingredients will provide good control of the disease if used according to label instructions and recommendations.
Many shade trees are starting to show symptoms of infestation with eriophyid mites. These pests attack many different hosts and result in a variety of symptoms, including leaf distortion (fig. 2), blisters, scales, erinea (hairy growths), and gall formation (figs. 3,4). Symptoms are specific to the host. These pests generally are not detrimental beyond causing an aesthetic distraction, since enough foliage remains unaffected during the season to maintain a healthy tree.
NDSU Plant Diagnostician