Key Features
- Yellow, gray or purple spots
- Yellow halo around leaf spots
- Tufty, gray growth under spots
Symptoms
There are many pathogen species that cause downy mildew on many different annual and perennial hosts. The variability in symptoms, lesion characteristics and severity of infection makes this disease difficult to diagnose. In general, grayish-brown, yellow, or purple leaf spots develop, often on the underside of leaves. These spots are surrounded by a yellow halo and are usually stopped at leaf veins which gives them a blocky appearance that can be easily misdiagnosed as bacterial leaf spot. Spots expand and grow together, often resulting in defoliation. A white to gray fuzzy-looking mass of spores may be seen on the underside of affected leaves when viewed with a magnifying lens. Stem infections are present in some cases; these purplish-brown spots may have white tufts of spores and mycelia emerging from their center. Defoliation results when severe infections occur in coleus and impatiens. Making the distinction between powdery and downy mildew is important as the fungicides effective against one are not usually effective against the other.
Biology
This disease is caused by fungus-like organisms (Oomycetes), commonly called 'water molds.' These pathogens over-winter as thick walled spores on dead plant parts and leaf litter. These pathogens produce 'swimming' spores that are capable of moving towards plants in standing or thin films of water. Cool, wet, high humidity weather is required for infection and favors downy mildew development. Dense canopy growth and tight plant spacing encourages spore production on the lower leaf surface. Spores produced during the season can reinfect the plant or be splashed to nearby plants.
Management Recommendations
Reduce overhead water and keep leaves as dry as possible. Remove and destroy symptomatic leaves, shoots, or plants. Replace highly susceptible plants with less susceptible varieties or species. Do not replant in areas with a history of downy mildew. Some copper-containing fungicides are labeled for homeowner use against downy mildew. Alway read and follow the product label. Plants not included on the label may be damaged by the fungicide, particularly in hot weather.
Effective Pesticides
Active Ingredients include: Copper salts, Phosphorous acid salts
Resources
