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 almost all their leaves by midsummer. The premature leaf drop will weaken the trees somewhat, but will usually not kill them. The damage is mainly aesthetic. Heavily defoliated trees are unattractive. However, heavy defoliation in several successive years can weaken susceptible crabapples and predispose them to additional damage, such as winter injury.
Apple scab may be prevented by the application of fungicides, such as captan and chlorothalonil, starting as soon as leaf growth appears until about the middle of June. Infections are less likely to occur with the arrival of warmer, drier weather in early summer. Sanitation also plays an important role in controlling apple scab. Raking and destroying the leaves as they fall will help control the disease next season. The fungus overwinters on partially decayed leaves. Pruning to open up the tree canopy and speed drying of the foliage may also be helpful. However, the best control for apple scab is to plant scab resistant varieties. There are numerous crabapples that are resistant to apple scab and need no spraying for disease control.
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 midsummer after a rainy spring. 'Spring Snow' is one of the few crabapple varieties that 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 require no spraying for disease control. White-flowering crabapple varieties that are resistant to apple scab include 'Adirondack,' 'David,' Harvest Gold , Lancelot , Red Jewel[TM], 'Sugar Tyme,' and Malus x zumi 'Calocarpa' (redbud crab). 'Donald Wyman' and 'Snowdrift' are excellent white-flowering crabapples that do experience moderate disease problems in wet years. Excellent pink to red-flowering varieties include 'Adams,' Camelot , 'Prairifire,' 'Profusion,' and 'Purple Prince.'
If a crabapple tree is in your future landscape plans, select a scab resistant variety at your local nursery or garden center.
This article originally appeared in the May 28, 1999 issue, p. 70.