Question: I love basil and have grown it for years without problems. This year I don’t know what happened. Some leaves started looking a little yellowish. Shortly after black spots appeared on the leaves. Some of the basil plants are older and a couple are quite young but all are infected. What is happening? R.C.
Answer: Basil downy mildew is a widespread problem this summer. I’ve seen infected basil samples from home growers with a few plants and from commercial producers with hundreds of plants.
The first symptom of basil downy mildew is sections of light yellow discoloration between large leaf veins. As the disease progresses the yellow sections turn very dark brown to black. Infected leaves may be cupped, puckered, or curled downward. When you turn an infected leaf over, the underside will have fluffy gray mass of spores. (Downy mildew and powdery mildew are different diseases with different causative organisms. The fuzzy spore structures in downy mildew are only on the underside of the leaf in contrast to powdery mildew diseases which produce white to gray fuzz on the upper and lower leaf surface.)
The first case of basil downy mildew in Wisconsin was identified in 2010. Since then, the disease has occurred every year. The pathogen is spread on seed, on infected plants, and by wind. Like many windblown diseases, the spores can travel miles from the source. Plants infected with basil downy mildew may not show symptoms of disease during cool weather. Symptoms and damage increase in warm, humid weather when the disease-causing organism thrives.
Basil downy mildew infects culinary and ornamental varieties of basil. All cultivars of sweet basil (Ocimum basilicum), such as Genovese, Italian Large Leaf, Cinnamon, Opal Purple Variegated and others, are very susceptible to this disease. Red-leafed basil (Ocimum basilicum var. purpurescens) cultivars and lemon basil (Ocimum citridorum) cultivars show some resistance to downy mildew. Spice, Blue Spice, and Blue Spice Fl are resistant. Resistant varieties of basil don’t have the same flavor as sweet basil so aren’t good culinary substitutes.
Downy mildew is best managed by prevention. Plant disease-free seed. Don’t save seed from infected plants. Watch transplants closely for initial symptoms of downy mildew. Destroy infected plants by burning or burying. Increase space between plants to improve air movement which helps keep leaves dry. Use a soaker hose instead of overhead sprinkler to water. Fungicides are available but not recommended for home gardeners with few plants.
Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email Larson at firstname.lastname@example.org or call her at 262-857-1942.