Tomato Diseases: How To Fight Septoria Leaf Spot

by Matt Gibson

About Septoria Leaf Spot

Septoria leaf spot, also known as septoria blight is a common disease of the tomato plant, which also affects other members of the plant family Solanaceae, namely potatoes and eggplant. The disease is caused by the fungus Septoria lycopersici, and is known to affect crops in different regions all around the world. The disease won’t always kill your tomato plants, but it can do some pretty severe damage to the crops, defoliating them, weakening them, and if not treated, Septoria leaf spot will take over the entire plant, keeping them from being able to produce mature fruit. 

Causes And Symptoms of Septoria Leaf Spot

The fungus spores of Septoria lycopersici hide out on top of the soil and wait for the right conditions to come along. The spores enter your garden by either wind or water transportation and live on top of the soil. In warm, damp conditions, spores get splashed up onto the underside of leaves and attach themselves to the foliage. If spotted before the infection gets too serious, leaf spot can be treated and the plants can be salvaged.

Check the underside of older leaves often, as that is where Septoria symptoms will first appear. TIny circular spots appear on the underside of the leaves. The spots are grey and brown, and grow larger, merging together as the disease progresses. If you use a magnifying glass, you will notice the dark brown, pimple-like fruiting bodies of the fungus. Spots may also appear on the stems, blossoms, and calyxes, but Septoria leaf spot rarely affects the fruit of the plant directly.

If left untreated, Septoria leaf spot will eventually cause all of the leaves of the plant to yellow, dry out, and fall off of the plant. This weakens the tomato plant and the lack of leaves offer no shade to the fruit in the hot summer sun, which leads to fruit scalding. Without leaves, the plant will no longer attempt to produce tomatoes, Unfortunately, septoria leaf spot spreads quickly

Treatment and Control of Septoria Leaf Spot

As soon as you see signs of Septoria leaf spot, take action, and remove all infected leaves immediately. Wash your hands thoroughly before continuing to work in the garden around uninfected plants. Clean and disinfect your pruning shears or scissors after removing infected leaves.

Organic fungicides can go a long way towards treating and preventing fungal infections like septoria leaf spot. Fungicides containing copper and potassium bicarbonate will help contain the fungal disease and keep it from spreading. Start spraying as soon as you notice symptoms of septoria leaf spot. Continue treating according to the directions on the fungicide label.

If organic fungicides aren’t standing up to the task, you may be forced to consider chemical control options. One of the most effective chemical fungicides is also one of the least toxic, the compound is called chlorothalonil, and it is sold under the names Fungonil and Daconil. 

Commonly Asked Questions Septoria Leaf Spot

How do you treat Septoria leaf spots?

Remove diseased leaves, improve air circulation around plants, use drip irrigation instead of overhead watering or water soil directly, taking care to keep leaves as dry as possible. Pull up any weeds you see and mulch around the base of the plant. Practice crop rotation, alternating where you plant tomatoes, never planting tomatoes in a location where tomatoes, potatoes, or eggplant was grown in the previous year. Spray with fungicidal sprays that contain copper or potassium bicarbonate. Use chemical controls, like Fungonil, if the infestation is severe.

What causes Septoria leaf spot?

Fungal spores from the fungus Septoria lycopersici attach to the underside of low lying leaves of the tomato plant. In warm, moist, humid environments, the fungal infection invades the plant, first by attacking the leaves, and then by attacking the stems and flowers of the plant, This disease doesn’t affect the fruit directly, but damages the plant to the point where it can no longer produce mature fruit if left untreated.

Will Septoria kill tomato plants?

Septoria can severely weaken and eventually kill plants if left untreated. Septoria leaf spot is not generally lethal, but if the disease is allowed to progress into its later stages, it can kill entire plants. Generally, Septoria leaf spot will not kill your tomato plants if time is taken to treat the plants.   

How does Septoria leaf spot spread?

Septoria leaf spot spreads by spores which can be carried into your garden by wind or water, and can overwinter on infected plant debris or garden materials (such as support structures and trellises). As with most fungal diseases, septoria leaf spot thrives in moist, warm, humid environments.

Is leaf spot contagious?

Yes, bacterial leaf spot is highly contagious. Humid environments caused by warm, wet conditions can cause leaf spot to infect clusters of susceptible plants.

How do you treat Septoria leaves?

As soon as you notice leaves that are affected by Septoria leaf spot, each leaf should be removed and discarded immediately. Do not compost Septoria infected leaves, as they will harbour the spores and reintroduce the disease into your garden when you use the compost. After all infected leaves have been removed, wash your hands and any equipment used to remove infected foliage.

Does Septoria leaf spot affect fruit?

Septoria leaf spot generally doesn’t attack fruit directly. The fungus tends to target the leaves, then the stems and flowers, ignoring the fruit of the plant. However, the disease causes the leaves of the infected plant to fall off, leaving the fruit exposed to direct sunlight rays, causing it to scorch and ruin. The plant also becomes so weak due to the disease, that it can no longer produce fruit.

Have you seen signs of Septoria Leaf Spot on your tomato plants? We are looking for photos. If you have photos, we would appreciate if you could send them our way.

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