Key Features
- Leaf drop or dieback
- Black, velvety spore masses
Symptoms
Alternaria leaf spot symptoms begin as small brown spots, often with a yellow halo, and develop into irregular lesions up to an inch across. Over time, these leaf spots may develop a target-like pattern of rings. Severely infected leaves turn brown, curl, wither and die. Although common on annuals and perennials, few Alternaria diseases have been reported on woody ornamentals.
Biology
Alternaria spores are produced in the spring on previously infected leaf litter and plant debris. Spores are wind blown and splashed to susceptible hosts. Infections occur when relative humidity is high, temperatures are warm, or the plant is wet. These new infections will produce spores that cause leaf spots on nearby susceptible plants.
Management Recommendations
Whenever possible, reduce overhead watering and keep leaves as dry as possible. Correct fertilization, irrigation, and soil drainage will increase plants' natural defense against the disease. Remove and destroy all Alternaria infected plants and plant debris to reduce the spread of disease and the number of overwintering spores. Several fungicides are labeled for homeowner use for the control of Alternaria leaf spot. These chemicals can only prevent new infections and must be continually applied while temperatures are between 55 and 80 degrees F. Always read and follow the directions on the product label.
Effective Pesticides
Active Ingredients include: Chlorothalonil, Copper salts, Myclobutanil, Propiconazole, Sulfur
Resources
- Not satisfied with ID? Contact the Purdue Plant and Pest Diagnostic Lab
- Sign Up for the Purdue Landscape Report
