The record rainfall this spring has created ideal conditions for the development of apple scab on crabapples. Apple scab is caused by the fungus Venturia inaequalis and is a serious problem on susceptible crabapple varieties (cultivars). Scab appears on leaves as roughly circular, velvety, olive-green spots on both the upper and lower leaf surfaces. The spots eventually turn dark green to brown. Margins of these spots are feathery rather than distinct. Heavily infected leaves curl up, become distorted in shape, turn yellow, and fall from the tree. Highly susceptible crabapple cultivars may lose a majority of their leaves by mid-summer. The premature leaf drop weakens trees, but usually doesn't kill them. The damage is mainly aesthetic. Heavily defoliated trees are unattractive.
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on June 14, 2013. The information contained within may not be the most current and accurate depending on when it is accessed.