The cool, wet weather this spring has created ideal conditions for the development of apple scab on susceptible crabapple varieties. Scab is caused by the fungus Venturia inaequalis. Initial symptoms are small olive-green to black spots on the foliage. Eventually the leaves turn yellow and fall from the tree. Highly susceptible crabapple varieties may lose most of their foliage by midsummer. The heavy defoliation will weaken the trees somewhat, but will usually not 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, when growth first appears. Repeat at 7 to 10 day intervals until about the middle of June. Sanitation also plays an important role in controlling apple scab. Raking and destroying the leaves as they fall helps control the disease next season. The best way to avoid apple scab problems is to plant scab resistant varieties. The following crabapple varieties perform well in Iowa and also possess good to excellent resistance to apple scab.
|Species or Variety||Flower Color||Fruit Color||Ht/Wd||Tree Form|
|'Candied Apple'||pink||dark red||15/15||weeping|
|'Pink Satin'||pink||dark red||15/15||rounded|
|'Silver Moon'||white||dark red||25/15||upright-oval|
|x zumi 'Calocarpa'||white||red||15/20||horizontal|
|x zumi 'Winter Gem'||white||red||15/12||upright-spreading|
|x zumi 'Wooster'||white||orange-red||12/15||horizontal|
* 'Indian Magic' and 'Snowdrift' possess good resistance to apple scab. However, they will lose considerable foliage whenweather conditions are ideal for scab.
Gardeners should avoid 'Almey,' 'Dorothea,' 'Hopa,' 'PinkPerfection,' 'Radiant,' 'Royalty,' and 'Vanguard.' All thesevarieties are highly susceptible to apple scab. 'Spring Snow' isone of the few crabapple varieties that produces little or nofruit. Unfortunately, it is also severely susceptible to applescab.
If a crabapple tree is in your future landscape plans,select a scab resistant variety at your local garden center ornursery.
This article originally appeared in the June 23, 1995 issue, pp. 1995 issue, pp. 92-93.