Q: My basil gets brown spots on some of the leaves. Not too cold or windy. I thought it was being burned by too much sun, but you say 6 hours of full sun per day are needed. What could be causing the brown spots? Some of the spots are on the edges and some are right in the center of the young leaves.
A: There are several possibilities here, and most depend on the size, shape and coloring of the spots. In general you will want to prune the affected leaves and improve air circulation with further pruning, adjusted position, changing your watering habits or staking.
Basil can suffer from a wide variety of afflictions during the growing season.
Photo by Laura Taylor/Courtesy Flickr
Brown and black spots on the leaves and streaks on the stem: These are likely symptoms of bacterial leaf spot, which occurs when soil infected with Pseudomonas cichorii is splashed onto the leaves during watering. There is no direct cure for bacterial leaf spot, but damage can be minimized by modifying watering practices to eliminate splashing and increasing air circulation around the plant by increasing spacing or adjusting placement. You will also want to remove the affected leaves.
Watering from above can also result in brown discoloration of the leaves. If water sits on the leaves it can act as a lens for sunlight, resulting in the leaves being burnt. This is most likely to happen if you water in the afternoon, when the sun is at its hottest. Watering in the morning or evening and at soil level may correct the problem. Additionally, if your area has low levels of humidity, the basil may be getting too much heat and sun for the amount of water it receives. Additional watering or adjusted placement may help.
Brown or black spots on older or lower leaves may be a symptom of potassium deficiency. This deficiency is most common in chalky or sandy soils. Other symptoms include a “scorched” look to leaf edges and yellow coloration between veins. A potassium fertilizer may help, but will not immediately correct the problem. One homemade potassium fertilizer in a comfrey tea, and others include seaweed and composted banana peels. Another possibility is that the potassium levels in the soil are fine but the plant is not getting enough water to properly absorb the nutrients.
Fungal or bacterial disease can also cause discoloration and spotting (black, brown or gray, sometimes ringed with yellow). Fungal conditions are often accompanied by gray, purple or white fuzz, but a particular diagnosis is needed before you can choose the right treatment. Take a sample to a local nursery or send detailed photos to your local extension office to determine the type of disease, then use an appropriate organic fungicide or antibiotic soap or spray.
If you know of another problem brown leaf spots could indicate, let us know what it is and what to do about it in the comments!