Landscape - Diseases and Disease Control

The following is an article on diseases and disease control in the landscape by Bob Mulrooney, UD Extension Plant Pathologist.

Under Delaware conditions foliage diseases of trees and shrubs rarely threaten plant health but are unsightly and reduce the aesthetic value of our ornamentals. Repeated defoliation or high levels of infected foliage can reduce growth and weaken plants. This stress can make them more susceptible to insect pests or adverse weather such as drought. So disease control may be needed even if the plant is not in immediate danger. Each disease situation should be evaluated for the importance of the host plant in the landscape, the threat to plant health that it poses, and the acceptable level of plant damage or infection. Keep records of disease outbreaks. Record keeping allows the plant owner to establish a history, which indicates patterns and establishes priorities in making disease control decisions. If foliage disease control is warranted, consider replacing the plant with a resistant cultivar if available, e.g. crabapples resistant to scab; or apply a fungicide. Sometimes replacing the plant with one that has fewer pest problems is the best solution. When fungicides are needed, apply before disease symptoms appear. Prevention is the key to control with fungicides and bactericides. For trees and shrubs, most infections occur during wet, warm spring weather so the first fungicide control spray is generally aimed at bud-break and repeated 10-14 days later. Be sure to check the product label for timing and rates. Often when I report that a certain disease has been found in the landscape, the opportunity for chemical control is past. Remember three factors must be present in order to have a disease 1) a susceptible host plant 2) a disease causing organism (pathogen) and 3) suitable weather conditions for infection. If you can affect one or more of the legs of this disease triangle you can eliminate or reduce diseases in the landscape.

Written by Bob Mulrooney, Extension Plant Pathologist in a past issue of the Ornamentals Hotline.