Septoria leaf spot, also called Septoria blight, is a very common disease of tomatoes. It is caused by a fungus (Septoria lycopersici) and can affect tomatoes and other plants in the Solanaceae family, especially potatoes and eggplant, just about anywhere in the world. Although Septoria leaf spot is not necessarily fatal for your tomato plants, it spreads rapidly and can quickly defoliate and weaken the plants, rendering them unable to bear fruit to maturity.
How Septoria Leaf Spot Spreads
Fungus spores are very good at hiding out and waiting for ideal conditions. They can travel great distances, in hopes of finding those conditions. The Septoria lycopersici fungus lives on the fallen tomato plant debris and weeds that are on and in the soil. It is spread to the plants by both water and wind, usually splashing up on the plants from the soil. Warm (60 to 80 F), damp conditions are the most favorable for Septoria leaf spot to develop and that's when you should be most watchful. If caught early, it can be controlled.
Symptoms of Septoria Leaf Spot on Tomato Plants
Septoria leaf spots start off somewhat circular and first appear on the undersides of older leaves, at the bottom of the plant. They are small, 1/16 to 1/8 inches (1.6 to 3.2 millimeters) in diameter, with a dark brown margin and lighter gray or tan centers.
As the disease develops, the spots will get larger and may merge together. If you view them through a magnifying lens you may see the fruiting bodies of the fungus, which look like dark brown pimples. This is one of the symptoms that distinguish Septoria leaf spot from other leaf spotting diseases.
Although the symptoms usually occur in the older, lower leaves, the disease can develop at any stage in the tomato plant's life. They may also appear on the stems as well as the blossoms and calyxes. One small bit of good news—they rarely affect the fruits.
If untreated, Septoria leaf spot will cause the leaves to turn yellow and eventually to dry out and fall off. This will weaken the plant, send it into decline and cause sun scalding of the unprotected, exposed tomatoes. Without leaves, the plant will not continue producing and maturing tomatoes. Septoria leaf spot spreads rapidly.
How to Treat Septoria Leaf Spot
There are a few options for treating Septoria leaf spot when it appears; these include:
- Removing infected leaves. Remove infected leaves immediately, and be sure to wash your hands thoroughly before working with uninfected plants.
- Consider organic fungicide options. Fungicides containing either copper or potassium bicarbonate will help prevent the spreading of the disease. Begin spraying as soon as the first symptoms appear and follow the label directions for continued management.
- Consider chemical fungicides. While chemical options are not ideal, they may be the only option for controlling advanced infections. One of the least toxic and most effective is chlorothalonil (sold under the names Fungonil and Daconil).
How to Prevent Septoria Leaf Spot on Tomatoes
You probably won't be able to avoid Septoria leaf spot altogether. It is very widespread and, given the ideal conditions, it will try to take hold in your garden. However, there are some precautions you can take to lower the likelihood that Septoria leaf spot will occur.
- Use disease-free seed. There's no evidence that it is carried by seeds, but err on the safe side and don't save seed from infected plants. Thoroughly processing the tomato seeds you are saving will also help rid the seeds of lingering diseases.
- Start with a clean garden. Dispose of all affected plants. The fungus does not remain in the soil, but it can over-winter on the debris of diseased plants. So it's important to dispose of all the affected plants far away from the garden and the compost pile. Keep in mind that it may have spread to your potatoes and eggplants, too.
- Avoid overhead watering. Water aids the spread of Septoria leaf spot. Keep it off the leaves as much as possible by watering at the base of the plant only. Of course, it's impossible to keep the rain off your plants, but every little bit helps.
- Provide room for air circulation. Leave some space between your tomato plants so there is good airflow. Stake them so that they are not touching the ground and not all bunched together. Good air circulation is especially important during damp and rainy periods.
- Mulch below the plants. A layer of mulch will prevent spores on the ground from splashing up onto the lower leaves.
- Plant next year's tomatoes in a different section of your garden. In small gardens, it's not always practical to rotate your crops, so good clean up and sanitation become even more important.