Thai Basil Plant Disease

Growing Conditions

• Several diseases that attack Thai basil are caused by the conditions in which the plants are growing. Leaf spots infect Thai basil plants that are subjected to overhead irrigation or do not receive adequate air circulation. Help prevent leaf spots by watering plants only at the base and allowing a minimum of 12 to 18 inches spacing between plants for adequate air flow. Gray mold appears as a fungal growth and is the result of a combination of cool temperatures, high humidity and bad air flow around the plants. Root rot in Thai basil plants is caused by poor soil drainage. When planting Thai basil, place it in a well-drained, sunny location in your garden.

Soil Problems

• Thai basil plants are also susceptible to diseases that come directly from the ground in which you plant them. Soil-borne diseases include fusarium wilt, which is caused by a fungus that assaults the stem tissue keeping the plant from taking in water, and bacterial wilt, which infects the roots and destroys the plant’s tissue. Both diseases cause stunted growth and eventual death of the Thai basil plants. If you plant Thai basil every year, rotate it to a different spot in your garden to prevent soil-borne diseases from attacking your new plants. Provide optimal soil conditions to help ward off diseases by maintaining a pH of 6 to 7 and incorporating organic matter to provide nutrients for your plants.

Seed Infections

• Ensure that your Thai basil is completely disease free before collecting seeds from it to save for use in propagating next year’s plants. The seeds from Thai basil plants that have been infected with certain diseases should not be saved because those diseases will affect any new plants started from them. Do not save seeds from Thai basil plants that have the fusarium wilt fungus or bacterial leaf spot, as both diseases are seed-borne. Collect disease-free Thai basil seeds from the plant’s seed capsules and store them in small containers or plastic baggies until you are ready to plant them.

Preventing Diseases

• Thai basil plants will thrive if planted from disease-free seed, in healthy soil and at an appropriate location in your garden. Start your Thai basil seedlings using only seed that you know is free of diseases and incorporate organic matter or compost into the garden prior to planting to reduce the potential for soil-borne pathogens. As your Thai basil plants grow, check them regularly for any signs of disease and remove infected plants to prevent the spread of disease to others.