Both bacterial and fungal diseases can affect basil plants. If left untreated, these diseases can cause malformed or discolored foliage and, in some cases, the eventual death of the plant. Checking plants regularly for signs of disease and following proper practices can prevent outbreaks of many basil diseases.
The fungal disease downy mildew spreads by infected seed or wind dispersion and attacks basil plants during wet, humid conditions. Basil leaves turn yellow or brown and a layer of fuzzy, gray growth appears on the undersides of the leaves. Choose fungicides labeled to fight basil downy mildew and apply according to the instructions. Remove heavily infected plants and plant new basil in a different area of your garden. To prevent an outbreak of the disease, avoid overhead watering and space basil plants at least 2 feet apart to improve air circulation.
Fusarium wilt spreads through the soil and attacks the xylem of basil plants, causing a blockage that prevents the plants from receiving water. Basil plants infected with this disease experience yellowing or browning of leaves. In the advanced stages, leaves wilt and the plant will suddenly die. Remove and discard all infected plants. Plant new basil away from the contaminated soil or choose disease-resistant varieties such as Nufar, Aroma 1 or Aroma 2. If growing basil in containers, discard the soil and thoroughly clean the pots.
Gray mold typically affects basil plants after they have been harvested, entering through the cut stems. This disease causes leaves to die and fall from the plant. The plant will die if the disease moves into the main stalk. Avoid harvesting basil on rainy days and avoid overhead watering for 48 hours after harvesting. Cut out any diseased foliage and dispose of it. Use rubbing alcohol to disinfect your pruning tool between each cut to prevent the spread of the disease.
Leaf spot diseases cause small spots to form on basil leaves. These spots eventually merge and cause the entire leaf to turn brown. Fungal diseases thrive in wet conditions, so avoid overhead watering. Cut back any infected foliage and dispose of it. Apply potassium bicarbonate fungicides to fight minor outbreaks.
Root rot diseases cause the tips of basil stems to die before infecting the rest of the plant. Pythium root rot causes a shedding of the outer layer of the roots. Rhizoctonia root rot causes brown spots to form on the roots. These spots eventually merge, causing the entire root system to turn brown. Remove the infected plants and the surrounding soil. Provide sufficient drainage by adding compost to the soil and rotate basil crops to a new location each year.
Bacterial wilt enters the root system of basil plants from the soil and attacks the xylem, prohibiting water movement. The leaves will wilt, but remain green. Dig up and discard any diseased plants, as well as the surrounding soil. Rotate the location of basil within your garden every year to avoid this disease.