Table of Contents
Septoria leaf spot is a common fungal disease that affects tomato plants and other members of the Solanaceae family, such as potatoes and eggplants. It is caused by the fungus Septoria lycopersici, which can survive on plant debris and weeds in the soil. The disease can reduce tomato yields significantly by defoliating and weakening the plants.
Symptoms and Signs of Septoria Leaf Spot
The first symptoms of the Septoria leaf spot usually appear on the older, lower leaves and stems of the tomato plants when they are setting fruit. The symptoms may also occur on the petioles, calyxes, and blossoms, but rarely on the fruits.
The symptoms include:
- Small, circular lesions (1/16 to 1/8 inches in diameter) with dark brown margins and lighter gray or tan centers. A yellow halo may surround the lesions.
- Small black dots (pycnidia) in the center of the lesions. These are the fruiting bodies of the fungus that produce spores.
- Yellowing and wilting of the infected leaves. The lesions may coalesce and cause the entire leaf to die.
- Reduced growth and fruit production of the plants. The fruits may also be exposed to sun scalding due to a lack of foliage protection.
Conditions that Favor Septoria Leaf Spot
Septoria leaf spot thrives in warm (60 to 80 F), humid, and rainy conditions. The fungus spores can be spread by water splashing from the soil, as well as by insects, people, and equipment that touch the infected plants. The disease is more severe in seasons with frequent rainfall and moderate temperatures.
Prevention and Control of Septoria Leaf Spot
There are several cultural and chemical methods that can help prevent and control the Septoria leaf spots on tomato plants. These include:
- Planting resistant or tolerant tomato varieties. Check the seed catalogs or labels for information on disease resistance.
- Practicing crop rotation. Avoid planting tomatoes or other solanaceous crops in the same area for at least three years.
- Removing infected plant debris and weeds from the garden. Dispose of them in the trash or burn them to prevent the fungus from overwintering in the soil.
- Spacing and staking the plants to improve air circulation and reduce leaf wetness. Avoid overhead watering or water early in the day to allow the leaves to dry quickly.
- Applying organic or synthetic fungicides as soon as symptoms appear or before rainy periods. Fungicides containing copper or potassium bicarbonate can help prevent the disease from spreading. Follow the label directions for application rates and intervals.
