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Septoria Leaf Spot

Plant disease · Septoria spp.

Description​

Septoria leaf spot is a fungal disease caused by species of Septoria. It affects many garden plants; the species responsible depends on the host. On tomatoes, the usual cause is Septoria lycopersici. The disease develops most readily during warm, humid weather when leaves remain wet. Spores spread from infected plant debris or leaves to healthy foliage in splashing rain and irrigation water. Spots often appear first on older, lower leaves, then spread upward as conditions favor repeated infections. Heavy infection can kill leaves and leave plants with too little foliage to grow well or protect developing fruit from sun. On tomatoes, the fungus primarily infects leaves rather than fruit. Although the spots are permanent, a plant can continue to produce healthy new growth if spread is controlled. Knowing which plant is affected matters because other leaf-spot diseases can look similar and may call for different management.

Symptoms​

  • Spots on leaves
  • Yellowing leaves
  • Leaves dropping

How to Identify​

Look for numerous small, roughly circular spots on older leaves, often with pale tan or gray centers and darker brown margins. Tiny black dots within mature spots are the fungus’s spore-producing structures; a hand lens can help reveal them. Surrounding leaf tissue may turn yellow before the leaf drops. On tomatoes, early blight usually makes larger spots with conspicuous concentric rings, while bacterial spots may look water-soaked and lack the black dots. Other fungal leaf spots can be difficult to distinguish by appearance alone. Check the pattern across the plant and recent wet weather, and ask a local extension service for a diagnosis if the distinction affects treatment.

Prevention​

Start with healthy plants and, where available, varieties with resistance appropriate to the crop. Give plants enough space for air to move through the foliage, and prune or stake them where that is standard practice. Water at soil level, preferably early in the day, rather than wetting the leaves. Mulch to reduce soil splash onto lower foliage. Remove fallen, diseased leaves and clear crop debris at the end of the season; the fungus can persist on infected material. Avoid handling plants while they are wet, since this can help move spores. For annual crops, rotate away from susceptible hosts when practical, and control volunteer plants that could carry infection. Inspect lower leaves regularly during humid weather so you can act before the disease spreads widely.

Treatment​

Begin by removing fallen leaves and the most heavily spotted foliage. Bag or discard it according to local guidance rather than leaving it beneath the plant. Avoid stripping off so many leaves that the plant becomes stressed or fruit is exposed to direct sun. Improve airflow and switch to watering at soil level; these steps help protect new growth but will not erase existing spots. If the infection is mild or the season is nearly over, monitoring and sanitation may be enough. When disease is spreading and the crop is worth protecting, consider a fungicide labeled for Septoria leaf spot on that particular plant. Copper-based products are an option for some crops, but they can damage foliage and accumulate in soil. Fungicides work mainly by protecting uninfected tissue, so follow the label’s timing, application, and safety directions exactly. Remove badly affected annual plants after harvest and clean up their debris.

Plants Affected​

Black-eyed Susan, Celeriac, Celery, Parsley, Parsley Root, Parsnip, Tomato

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