Septoria leaf spot 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.
Septoria lycopersici is seedborne and also survives at least 1-2 years in soil. Septoria leaf spot is favored by moderate temperatures, high humidity and rain or overhead irrigation.
Septoria leaf spot starts on lower leaves as small, circular gray lesions (spots) with dark borders. Fungal lesions enlarge, coalesce, and cause leaves to yellow and die. Lesions usually appear when the first fruit begins to form. Tiny black pycnidia (fungal fruiting bodies) can be seen in the lesions. Favored by wet weather.
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 and pruners 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.
Rotate away from tomato for 2 or more years.
Control the presence of horsenettle and other weeds.
Remove crop debris from planting areas (particularly tomato crop debris).
Reduce the amount of time plants are wet, when possible.
Stake plants to improve air circulation and drying of leaves.
Use mulches or plastic to reduce contact between leaves and soil.
Avoid working in fields when leaves are wet.
Scout plants often and remove infected plants.
Clean tools and equipment after leaving a field.
Use fungicides listed as effective against Septoria leaf spot.
Staking and mulching plants and pinching off infected leaves help keep leaf spot diseases in check.
Improve air circulation around plants.
Keep leaves dry by watering at the base of the plant.
Do not save seeds from infected plants.
Leaf spots should not affect the amount of fruit your plants produce.
There’s many prevention techniques that work against this and other fungal diseases.
First, remove all fallen plant debris from your garden beds. Fallen leaves can harbor spores and allow them to overwinter. Be sure to clean your hands and sterilize tools you use to remove debris if you suspect fungal disease may be present.
Keep weeds in check. This is especially true with septoria lycopersici. Many solanaceous weeds can harbor fungal spores. Diseased weeds will spread the disease to nearby plants.
Mulch around your plants. This has a dual effect. It prevents soil splashing up onto the leaves, which can spread spores to your plant. It also reduces the likelihood of weed development. A 3″ to 4″ layer of mulch will also aid in water retention in the soil, which is an added bonus!
Water the soil, not the plants. Drip or soaker hose irrigation will reduce the likelihood of spore spread.
Rotate crops. Septoria fungi can survive in diseased plant debris, weeds, or perennials for up to 3 years. Crop rotation can reduce the likelihood of reinfection year after year.
Ensure there’s proper airflow. Plants that are tightly packed together are more at risk from diseases. Trimming excess foliage will provide better airflow. It also reduces the ease of spore spread, as leaves are spaced further apart. Staking plants or securing them in sturdy tomato cages can help with airflow as well.
Keep pests at bay. Remember, pests may accidentally carry spores from plant to plant. Reducing your pest population will also reduce spore transmission.
Select disease-free varieties. While tomatoes are all susceptible to septoria, other plants have resistant hybrids. Picking a variety that is resistant to septoria will reduce its occurrence.
Remove and destroy damaged material. If you see signs of reappearing fungal damage, trim it off before it can spread. If you remove it before it forms spores, it won’t have time to spread further. Be sure to sterilize any pruning tools you use to remove damaged leaves and wash your hands before touching the healthy parts of the plant.
