AMES, Iowa — The cool, rainy weather in recent weeks has aided the establishment of newly planted annuals, vegetables, perennials, trees and shrubs. The cool, rainy weather also has been favorable for the development of foliar diseases on some trees.

In this week’s yard and garden article, Richard Jauron, horticulture program specialist with Iowa State University Extension and Outreach, offers some tips.

For more information, contact ISU Extension and Outreach horticulture specialists at hortline@iastate.edu.

Why is my sycamore tree dropping its leaves?

The leaf drop is likely due to anthracnose. Anthracnose is a common fungal disease of sycamore, ash, maple, oak and other trees. Anthracnose is most severe in years with cool, wet spring weather. While anthracnose may cause extensive defoliation, it does not cause serious harm to healthy, well-established trees.

Symptoms of anthracnose on sycamores include brown blotches on leaves, death of young buds and shoots, and leaf drop. In cool, wet springs, affected sycamores may lose most of their initial foliage.

Fortunately, the sycamore trees will continue to produce additional leaves and shoots through early summer. Foliage that develops in late spring and early summer should not become infected as warmer, drier weather suppresses anthracnose. Most sycamores should have a good canopy of leaves by late June or early July.

Since anthracnose does not cause serious harm to sycamores, fungicide treatments are rarely warranted.

My crabapple has begun to drop some of its leaves. Why? 

The leaf drop is probably due to apple scab. Apple scab is a 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.

Sanitation 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. 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 laborious and not practical. The best way to prevent apple scab is to select and plant scab-resistant crabapple cultivars.