The frequent rains this spring have 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. 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 may curl up, become distorted in shape, turn yellow and fall off. Highly susceptible crabapple varieties may lose most of their leaves by mid-summer. The premature leaf drop weakens the trees somewhat, but usually doesn't kill them. The damage is mainly aesthetic. Heavily defoliated trees are unattractive.
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 varieties.
When selecting crabapples, gardeners should avoid 'Almey,' 'Dorothea,' 'Hopa,' 'Pink Perfection,' 'Radiant,' 'Royalty,' and 'Vanguard.' All of these varieties are highly susceptible to apple scab. Heavy defoliation can be expected by mid-summer after a rainy spring. 'Spring Snow' is a popular crabapple variety because it produces little or no fruit. Unfortunately, it is also severely susceptible to apple scab.
The following crabapple varieties possess good to excellent resistance to apple scab. These varieties experience little leaf drop even when weather conditions are favorable for apple scab. White-flowering crabapple varieties that are resistant to apple scab include 'Adirondack,' 'Bob White,' 'David,' 'Donald Wyman,' Golden RaindropsÃ‚Â®, Harvest GoldÃ‚Â®, Red JewelÃ‚Â®, and Sugar TymeÃ‚Â®. Excellent pink to red-flowering varieties include 'Adams,' 'Louisa,' 'Prairifire,' 'Profusion,' and 'Purple Prince.'