Across the area, crabapple trees have been dropping leaves this summer. What causes these trees to drop their leaves mid-summer?
The answer is a fungal disease called apple scab that infects crabapple leaves early in the spring. As the fungus grows in the developing leaves, it causes purplish-brown spots, often clustered along the leaf veins.
The spots can grow up to a half inch in diameter, with feathery margins when young and more distinct margins as they mature. Most people don't notice the disease until these infected leaves turn yellow and fall off the tree. Some people rake the fallen leaves out of their yards daily for weeks, until the tree is almost completely bare by midsummer. The crabapple that was spectacular while blooming in spring has become an eyesore by August.
Apple scab fungus spends the winter in the fallen leaves, and in the spring produces spores that can infect the new crop of leaves. Besides crabapple, the disease also affects apple, pear, hawthorn and mountain ash. Apple scab is the most economically important disease of commercial apple orchards.
Why do some crabapple trees get hit with scab every year, while others seem to be unharmed most years? Just like many plant diseases, apple scab severity is largely dependent on the weather. Apple scab is favored by cool, wet weather in the spring, when the fungus infects the new leaves.
Crabapple cultivars vary greatly in their ability to fend off the apple scab fungus. Some popular cultivars, such as "Spring Snow", are very susceptible to apple scab, and they lose their leaves nearly every year. Other cultivars, such as "Prairiefire" are resistant. Even a resistant variety can get apple scab if the weather is favorable for disease, but in most years it will be disease-free.
Although naked trees in summer are ugly, apple scab does not kill crabapples and usually does not seriously hurt them. But we plant crabapples to be pretty, so what can we do to manage apple scab? First, choose a resistant cultivar when planting a new crabapple tree.
There are many choices of resistant cultivars with beautiful spring blossoms. Realize that resistance is relative, and a "resistant" cultivar can still become diseased under some conditions--but it will have fewer disease problems than a susceptible cultivar.
Since the apple scab fungus survives the winter in fallen leaves, raking up and destroying those leaves at the end of the season can help to minimize problems next year. Keeping trees well spaced and pruned to promote airflow through the canopy can also help. Sometimes fungicide sprays are used to prevent infection of susceptible cultivars in the spring. However, the sprays must be repeated, and they are ineffective once symptoms appear.
Starting with a resistant cultivar is the best way to ensure that your crabapple tree stays beautiful and leafy throughout the summer.
For more information on apple scab fungus and other diseases which effect trees in your area contact our local county extension office. In Cherokee the office is located at 209 Centennial, Suite A. The phone number is 712-225-6196.