Basil (Ocimum spp.) is grown as an annual everywhere, although it may live for up to two years in frost-free climates. Multiple fungal diseases and a bacterial disease can cause spots on the leaves of container- and garden-grown basil. One of these fungal diseases is fairly new to basil, and has become a serious problem for basil growers all over the United States.
Downy mildew in basil was first noticed in 2007 in Florida. The fungus is brought into gardens on seeds and seedlings but can also be blown from garden to garden or plant to plant. This disease begins at the base of basil plants and moves up as it spreads. The first symptom is yellow areas on the leaves between major veins along with black and brown spots. As the disease progresses, the whole leaf may turn yellow. Purple-gray fungal spores form on the bottoms of infected leaves. Eventually, infected leaves drop and the whole plant begins to look unwell as though it is lacking in nutrients or has been sunburned.
Sweet basil (Ocimum basilicum) is highly susceptible, although red-leaved varieties (Ocimum basilicum purpurescens), 'Queenette' Thai basil (Ocimum basilicum 'Queenette') and 'Eleonora' (Ocimum basilicum 'Eleonora') are only moderately susceptible. Lemon and lime varieties (Ocimum citridorum) are also only moderately susceptible. 'Blue Spice', 'Blue Spice F1' and 'Spice' (Ocimum americanum hybrids) are less susceptible, but they look and taste quite different from sweet basil.
Other Leaf Spot Diseases
Three other fungal diseases, Alternaria spp., Cercospora spp. and Colletotrichum spp., cause black or dark brown spots on basil leaves but do not produce the purple-gray spores on the undersides of leaves. They are prevented and treated in the same way as downy mildew.
Bacterial leaf spot, also called basil shoot blight, produces watery brown or black spots with a yellow halo. Basil contracts bacterial leaf spot through wounds in the leaves or stems. Fungicides will not work on bacterial leaf spot. Pinch off infected plant parts right away and throw them in the trash.
All of these diseases are brought into the garden the same way as downy mildew.
Prevention and Treatment With Fungicides
To help prevent fungal diseases on container- and garden-grown basil before they start, begin spraying the basil with potassium bicarbonate fungicide when warm, humid weather is predicted. A common dilution rate is 2 teaspoons per gallon of water, but there are multiple potassium bicarbonate products on the market. Follow the manufacturer's instructions carefully.
Mix it up in a hand-held or hand-pump pressure sprayer and spray the basil plants, thoroughly coating the tops and bottoms of the leaves as well as the stems. Wear protective eyewear, clothing and gloves and be careful not to inhale the fungicide. If a container-grown basil plant is being grown in an enclosed area, take it outdoors to a shady spot for treatment. Repeat treatments every seven to 10 days, as long as the weather remains warm and humid.
This fungicide treatment can be used to treat infected plants, but it is unlikely to be effective on severely infected plants. Remove infected leaves on plants that have just begun to show symptoms before treating with potassium bicarbonate. Remove and dispose of severely infected plants. Put infected leaves and plants in the trash.
Do not treat with regular baking soda. It will be ineffective and will leave behind a toxic salty residue.
Prevention Through Good Cultural Practices
Prevention is the key to beating these diseases.
Inspect seedlings before purchasing them and buy fresh seed each year rather than saving it from year to year. Wash your hands before handling the plants or seeds.
Give basil plenty of room and sunlight. Plant multiple plants far enough apart so they will have 6 inches of space between them when they reach their mature width. Increased air circulation helps the leaves stay dry and can help prevent the spread of disease from one plant to the other.
Water basil from below the leaves with a soaker hose or watering can. Water in the morning so leaves will dry throughout the day.
Remove infected plant parts immediately and place them in the garbage. Never put infected leaves and stems into a compost pile. Remove dead plant material and debris from around basil plants as soon as it is noticed.
Spread fresh mulch over the soil beneath garden basil plants to help keep fungal and bacterial spores from splashing up onto the plants when it rains.