Save Your Tomatoes From Septoria Leaf Spot

Septoria leaf spot is a very common tomato disease. 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 them, rendering them unable to bear fruit to maturity. Septoria leaf spot causes black spots on tomato leaves with small fruiting bodies (raised bumps) in the center, resulting in yellow leaves that eventually turn brown and die.

Learn what Septoria leaf spot is and how to identify, treat, and prevent it when growing tomatoes.

What Is Septoria Leaf Spot?

Septoria leaf spot is a fungal disease that causes black or brown spots on the older, lower leaves of the plant but not on the fruit. The disease can quickly spread upward on the plant, affecting new growth, producing less healthy or flavorful fruit, and overall, making the plant susceptible to sun scorch. The disease becomes more prevalent when the garden has warm, humid conditions.

Symptoms of Septoria leaf spot are often confused with early blight. Septoria leaf spot appears as lesions on older leaves dotted with tiny brown-black dots. Early blight shows as larger, slightly sunken lesions embedded with concentric rings.

Identifying Septoria Leaf Spot on Tomato Plants 

Septoria leaf spots start 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. A yellow halo may surround the spot.

As the disease develops, the spots will get larger and may merge. If you view them through a magnifying lens, you may see the fungus's fruiting bodies, which look like dark brown pimples. This is one of the symptoms that distinguishes 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. It may also appear on the stems, blossoms, and calyxes (starfish-shaped green leaves on top of each tomato fruit). The good news is that it rarely affects the fruits.

If untreated, Septoria leaf spot causes the leaves to turn yellow and eventually dry out and fall off. This weakens the plant, sends it into decline, and causes sun scalding of the unprotected, exposed tomatoes. Without leaves, the plant will not continue producing and maturing tomatoes.

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 on and in the soil, which can overwinter.

Septoria leaf spot is spread to plants by water splashing up on them from the soil, as well as insects, people, and equipment that come in contact with the fungus. Warm (60 to 80 degrees Fahrenheit), humid 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. But note that 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. Wash your hands and cutting tools thoroughly before you prune tomato plants that are not infected.

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 as Fungonil and Daconil).

How to Prevent Septoria Leaf Spot on Tomatoes

You probably won't be able to avoid Septoria leaf spot altogether. Given the ideal conditions, it is prevalent and 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 this fungus is carried by seeds, but err on the safe side and don't save seeds 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 can overwinter 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. Cage tomatoes (disinfect cages yearly) to prevent leaves from touching the ground or other plants.

Avoid Overhead Watering

Water aids the spread of Septoria leaf spot. Water the plant at the base only to keep it off the leaves as much as possible. Of course, keeping the rain off your plants is impossible, but every little bit helps.

Provide Room For Air Circulation

Leave proper spacing between your tomato plants so there is good airflow. Stake them so they are not touching the ground and not all bunched together. Good air circulation is essential during damp and rainy periods.

Mulch Below the Plants

A layer of mulch will help prevent spores on the ground from splashing up onto the lower leaves.

Use Baking Soda and Vegetable Oil

Spray a natural fungicide solution on healthy tomatoes to prevent pests and fungal diseases. Mix 1 heaping tablespoon of baking soda, 1 teaspoon of vegetable oil, a few drops of dish soap, and one gallon of water in a spray bottle. Reapply weekly, in the morning or evening, to help your plants stay healthy.

Plant Next Year's Tomatoes in a Different Section

In small gardens, rotating crops is not always practical, so good cleanup and sanitation become even more important. But if possible, plant your next crop in a different area of your garden.