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.
Apple Scab and Flowering Crabapples
Apple scab may be prevented by the application of fungicides, such as chlorothalonil, from just prior to bloom until about the middle of June. Infections are less likely to occur with the arrival of warmer, drier weather in early summer. For most home gardeners, however, controlling apple scab with fungicides is simply not practical. Sanitation also plays a role in controlling apple scab. Raking and destroying the leaves as they fall may reduce the severity of the disease next season as the fungus overwinters on partially decayed leaves. However, the best control for apple scab is to plant scab resistant cultivars.
When purchasing a crabapple, select a cultivar that possesses good to excellent resistance to apple scab. White-flowering crabapple cultivars that are resistant to apple scab include 'Adirondack,' 'Bob White,' 'David,' 'Donald Wyman,' Golden Raindrops®, Harvest Gold®, 'Professor Sprenger,' Red Jewel®, Malus sargentii, and Sugar Tyme®. ('Spring Snow' is a popular white-flowering crabapple cultivar because it produces little or no fruit. Unfortunately, it is highly susceptible to apple scab.) Excellent pink to red-flowering cultivars include 'Adams,' 'Louisa,' 'Prairifire,' 'Profusion,' and 'Purple Prince.'
If a crabapple tree is in your future landscape plans, select a scab resistant cultivar at a local garden center or nursery.