Brownish Yellow Spots on a Holy Basil Plant

Downy Mildew

• Downy mildew was once uncommon in basil stands, but is gaining considerable ground. At first, leaves may yellow partially or completely, starting near the center, but soon grey-purple fungal bodies develop on the undersides of yellowed areas and quickly turn brown as they mature. Downy mildew is believed to be transmitted via seed and through infected plant debris but needs warm temperatures and high humidity to thrive. Neem oil is an effective fungicide against downy mildew and should be mixed at a rate of 2 tablespoons per gallon of water and applied liberally to all plant surfaces. Repeat treatment weekly until you can see the disease has stopped spreading. Before using neem oil, thoroughly read all labels.

Fusarium Wilt

• Fusarium wilt is a common fungal disease of basil that causes the plant to slowly wilt and die. Fusarium bodies enter the plant's water transport tissues and make their way through the system -- eventually multiplying so fast they clog transport tissues, causing sections of the plant to suddenly yellow, brown and die. Plants may wait to show symptoms until they've reach 6 to 12 inches high. No treatment for Fusarium wilt exists; infected plants must be pulled and destroyed. Crop rotation helps starve out fungal bodies that remain in the soil; choosing Fusarium wilt-resistant cultivars such as the fragrant Genovese varieties "Aroma 1" and "Aroma 2" will make gardening easier where Fusarium is a frequent problem.

Bacterial Leaf Spot

• Bacterial leaf spot is usually caused by transferring pathogens from infected seedlings onto the leaves of unaffected plants, either by splashing or on gloves and tools. Spots on leaves begin as light, water-soaked areas that may remain between leaf veins. Soon, these spots brown and stem rot may occur. If the leaf spots aren't widespread, reducing the humidity by decreasing waterings and hand-picking leaves showing disease may be enough to stop the bacteria. Otherwise, mix a copper fungicide in a sprayer at a rate of 2 fluid ounces per gallon and apply to all the leaves and stems of sick plants. Reapply every seven to 10 days for best results. Read the product label carefully before applying copper to your plants.

Fungal Leaf Spot

• Several fungal leaf spots, including Cercospora, Alternaria and Colletotrichum appear on basil leaves. These spots will vary in appearance but typically cause spotty yellowing followed by browning of affected areas. Spots may retain a yellow halo even after the centers have died, and the brown areas may fall out, giving leaves a shothole appearance. More aggressive leaf spots can lead to defoliation, dieback, lesions and plant death. Removing diseased plants is highly recommended for the aggressive leaf spots, but if your plant is only slightly spotty and not racing to its death, try applying a healthy dose of neem oil, mixed at a rate of 2 tablespoons per gallon. Repeat this treatment weekly until you no longer see new spots forming. Plucking highly diseased leaves before treatment may help to reduce pathogen spread.