Septoria leaf spot of tomato
Septoria leaf spot on tomato leaves.
Photo: U of MN Department of Plant Pathology
Septoria leaf spot is caused by the fungus Septoria lycopersici. Septoria can affect many parts of tomato plants, commonly causing leaf spots and stem lesions. The fungus can survive winter on diseased plant debris or on perennial weeds such as nightshade. In the spring, the fungus produces spores that are windblown or rain-splashed to healthy tomato leaves, causing primary infections.
Symptoms usually begin to appear on the lower leaves after fruit set. Initially round, yellow spots develop. Later, these spots enlarge and turn brown to gray. Tiny black fruiting bodies (pycnidia) eventually form in the center of the leaf spots. These pycnidia produce spores which cause secondary infections, usually in an upward direction, throughout the plant. Heavily infected leaves turn yellow, then brown and fall from the plant. Exposed fruit, due to defoliation, may be damaged by overexposure to the sun (sunscald).
- Do not plant tomatoes in the same location for 3-4 years
- Reduce moisture on the tomato leaves by using drip irrigation or directing water at the base of the plant. Water tomatoes early in the day so leaves dry quickly in the sun.
- Stake plants to improve air circulation around the plant.
- Apply mulch around the base of plants to minimize water splash.
- Pinch off infected leaves and remove them from the garden (never remove more than 1/3 of a plant's foliage).
- At the end of the season remove all plant debris from the garden, or in large gardens till it under to speed up break down of plant material.
- Copper, Chlorothalonil or Mancozeb fungicides can be applied to protect healthy leaves if cultural control practices do not reduce disease enough to produce a good crop of tomatoes.
Fungicide applications may be necessary during rainy seasons. Read the label carefully and apply only as directed. Fungicides currently labeled for use are:
- Bordeaux Mixture
- Chlorothalonil (Daconil 2787)
- Liquid Copper
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