Problem Identifier / How to Get Rid of Septoria Leaf Spot

Septoria Leaf Spot Symptom
Symptoms of Septoria leaf spot, a fungal disease.

Septoria leaf spot is the Septoria lycopersici fungus. This common disease attacks tomato plants in addition to cannabis, eggplant, and potatoes. Septoria spreads rapidly, weakens plants, and prevents fruit from reaching maturity. Septoria leaf spot fungal spores hide until the conditions are ideal. The fungus lives in weeds and debris from plants in the soil, spreading with wind and water.

Septoria will travel until finding ideal conditions, then access the plant from the soil. The most favorable conditions for the development of septoria leaf spot are damp, warm areas with temperatures between 60- and 80-degrees Fahrenheit. The first symptom is circular spots appearing on older leaves, usually on the undersides and toward the base of your plant.

The spots are approximately 1/8th to 1/16th of an inch in diameter with tan or light gray centers and a deep brown margin. As Septoria leaf spot progresses, the spots increase in size and often merge. Under magnification, the fungus resembles deep brown pimples. Despite generally appearing on older leaves, Septoria can also develop during every stage of the life of your plant.

If you leave Septoria leaf spot untreated – for instance – on a tomato plant, the leaves will eventually yellow, become dry and fall off. Your plant will be weakened and begin to decline, with the exposed tomatoes becoming susceptible to scalding from the sun. Once the leaves are gone, your plant will be unable to produce mature tomatoes. Septoria leaf spot will advance extremely quickly.

How to Prevent Septoria Leaf Spot

  • Clean the area of any diseased plants and debris to help prevent Septoria leaf spot.
  • Overwatering your plants will help the disease spread.
  • Leave space between your plants to ensure good air circulation.
  • Use mulch to stop Septoria leaf spot spores in the soil from reaching the lower leaves.

How to Treat Septoria Leaf Spot

All infected leaves must be removed immediately. Always wash your hands and clean any tools thoroughly before touching your healthy plants. Organic fungicides containing potassium or copper bicarbonate are an effective treatment provided the Septoria leaf spot is not too far advanced. Chemical treatments are not recommended since you may not be able to eat your tomatoes. If you are treating cannabis plants, remove and destroy the infected lower leaves.

Septoria Leaf Spot
Close up of septoria leaf spot on a cannabis leaf.

If your cannabis plants are flowering, removing the leaves will weaken your plant and negatively impact bud quality. You can improve the circulation in your grow room by adding more fans. Make certain there is space between your plants. To treat Septoria leaf spot on cannabis, all bushy sections of your plant should be trimmed. The leaves should not touch the other plants.

If your plants are outdoors, elevating them slightly will improve air circulation. Septoria leaf spot spores germinate and spread with excess moisture. Keep your soil moist but not wet. Rake outdoor soil for the removal of infected vegetation. The most common hosts of Septoria leaf spot spores are horsenettle and nightshade. Any weeds potentially hosting the disease need to be removed.

Septoria leaf spot thrives in wet, hot conditions. Decrease the humidity level and temperature in your growing space.

Does Septoria Infect Seeds?

Yes, Septoria leaf spot spores can infect your seeds.

What Tomato Varieties Resist Both Septoria Leaf Spot and Early Blight?

You can look for hybrid tomato plants grown to have resistance to septoria leaf spot. The labels on the plants should have the letter S shown after the name of the tomato variety. According to a publication of the Clemson Cooperative Extension, Tomato Diseases and Disorders, there are two plant varieties that exhibit resistance to Septoria leaf spot. They are Jasper hybrid, which is a cherry tomato, and Iron Lady hybrid.

Clemson lists 17 varieties, including Jasper and Iron Lady, that are also resistant to early blight. Some seed vendors use the letter code EB to designate early blight resistance; others use AB. Keep in mind that resistance does not mean immunity.

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