Although the weather this weekend promises to be lovely, recent rains and the likelihood of more to come is ideal for the development of foliar diseases in the landscape. The most common diseases on trees and shrubs affect the foliage as spots, blotches, and blisters. Although unsightly, most foliar diseases do not greatly impact the health of the plant, and chemical inputs are rarely necessary.
Disease development in the landscape is often favored by abundant moisture and cooler temperatures. Diseases we may expect to see during the early part of the growing season include a variety of leaf spots on many ornamental trees and shrubs, anthracnose of shade trees, scab and rust on apple and crabapple, Sphaeropsis shoot blight and canker, juniper, tip blight, ivy anthracnose, oak leaf blister, horse chestnut leaf blotch, Volutella blight of pachysandra, ash rust, and Rhabdocline and Swiss needle casts of Douglas fir.
Management of springtime foliar diseases benefits from a few basic strategies:
- Reduce leaf wetness and humidity in plantings (improve air-flow through proper spacing and weed management, irrigate during early morning hours, avoid overhead watering);
- Remove sources of pathogen inoculum (rake up leaf litter, remove severely affected plants, prune away affected tissue during dry weather);
- Improve plant vigor to help reduce disease severity.
Although spring-time diseases require chemical inputs only when troublesome, pesticides are labeled for management of each of these diseases. When using a fungicide, always check the label for host, timing, and rates. Thorough coverage is often necessary for good control.
Remember: the environment drives the foliar disease process, so if the weather is wet, expect to see symptoms of many of the following diseases in the weeks to come.
