Why Does My Basil Look Moldy?

Downy Mildew

• Downy mildew is caused by fungal spores called peronospora. Fungal spores quickly germinate and spread during cool, humid weather. This disease can easily spread to your entire basil crop through the wind. Other factors that contribute to its spread include close basil spacing and overhead watering. Because this disease can wipe out your entire basil crop, recognizing early signs may mean the difference between a few infected plants or a dead crop.

Symptoms

• Gardeners should monitor their plants during humid conditions. Look for a moldy growth underneath leaves. Powdery mildew is another fungal disease that can produce moldy looking growth, but the growth is typically on the tops and bottoms of leaves. Also, the downy mildew causes leaves to yellow. Yellow leaves are generally a sign that the plant is experiencing nutrient problems such as lack of nitrogen. Test your soil with a home testing kit to rule out lack of nitrogen in the soil.

Fungicides

• Because downy mildew is a new fungal disease, there are few fungicides on the market. Gardeners may use a fungicide that contains phosphorous acid. Also, full spectrum bactericide and fungicide may be used to control the disease. Avoid using a fungicide that is chemically formulated to treat powdery mildew, because it provides little treatment for your basil plants. Refrain from using a fungicide on water-stressed basil plants.

Cultural Management

• Good cultural practices may prevent downy mildew. Choose planting locations that receive full sunlight. Provide your basil plants with enough spacing at the time of planting to reduce the risk of downy mildew. Give basil plants at least six to 12 inches of space. Water your herbs at their base rather than watering from overhead. If water splashes on leaves, take out a soft towel and gently wipe off the water.