By Richard Jauron
Crabapple trees come in many cultivars and are often excellent additions to a home landscape. However, they can be susceptible to issues that can be tough to decode, just like many other trees.
Iowa State University Extension and Outreach horticulturists can help decipher questions about crabapple trees and the problems that surround them. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.
My crabapple has begun to drop some of its leaves. Why?
The leaf drop is probably due to apple scab. Apple scab is a fungal disease caused by the fungus Venturia inaequalis. Cool, wet weather in spring favors apple scab development. Crabapple cultivars differ in their susceptibility to apple scab. Some cultivars are very susceptible to the disease, while others are resistant to apple scab.
Apple scab appears as velvety, olive-green to black spots on crabapple leaves. Heavily infected leaves turn yellow and fall from the tree. Highly susceptible crabapple cultivars may lose a large percentage of their leaves by mid-summer. Fortunately, apple scab does not kill affected trees. The damage is mainly aesthetic.
Apple scab can be prevented by applying fungicides from bud break through mid-June. For most home gardeners, however, controlling apple scab with fungicides is not practical. Sanitation also plays a role in controlling apple scab. Raking and destroying the leaves as soon as they fall may help reduce the severity of the infection next season. However, the best way to prevent apple scab is to plant scab-resistant crabapple cultivars.
What is responsible for the yellow spots on my crabapple leaves?
Cedar-apple rust is likely responsible for the yellow spots on the crabapple leaves. Cedar-apple rust is a fungal disease. The fungus requires both a crabapple or apple and a cedar (juniper) to complete its life cycle. Crabapple and apple cultivars differ in their susceptibility to cedar-apple rust. Some cultivars are susceptible, others are resistant.
On cedar, the fungus produces reddish brown galls that are up to golf-ball size on young twigs. During wet spring weather these galls swell and begin to push out bright orange gelatinous tubular structures. Wind carries fungal spores from these gelatinous structures to susceptible crabapple or apple cultivars. On crabapples and apples, cedar-apple rust produces yellow spots on the foliage. Cedar-apple rust on crabapples and apples is most severe when there are frequent rains in spring.
Fortunately, cedar-apple rust does not cause serious harm to crabapples. No control measures are necessary.
Several new shoots on my crabapple have turned brown and wilted. What is the problem?
Fire blight is probably responsible for the wilted shoots on your crabapple. Fire blight is caused by the bacterium Erwinia amylovora. Plants susceptible to fire blight include apple, crabapple, pear, hawthorn and cotoneaster.
Symptoms of fire blight appear in spring within a few weeks of bloom. Succulent, new shoots turn dark brown to black and wilt. The wilted shoots somewhat resemble the top of a shepherd’s crook or candy cane. Small droplets of amber-colored bacterial ooze often can be seen on blighted shoots when the weather is warm and humid. Under favorable environmental conditions, shoot infections may continue to move down the branches and kill large portions of the tree.
The best way to avoid fire blight is to select crabapple, apple and pear cultivars that are resistant to the disease. Also, avoid heavy pruning and fertilization that promote excessive shoot growth. Succulent, rapidly growing shoots are more susceptible to fire blight infections.
To control fire blight, remove all infected shoots and destroy them. Late winter (February or March) is the best time to prune out infected branches. Make pruning cuts 10 to 12 inches below diseased areas. Also, disinfect tools between cuts by dipping them in a 10 percent chlorine bleach solution. Infected shoots also can be pruned out in late summer (after the crabapple has stopped growing). Late summer pruning should only be done in dry weather.
Source : iastate.edu