Septoria Leaf Spot

Yellow spots on leaves may indicate Septoria leaf spot.

Septoria leaf spot on tomato (Scot Nelson) Public Domain

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This fungal disease is very destructive and it affects celery, chicory, cucumber, and other cucurbits, along with asters, carnations, chrysanthemums, verbena, and various trees and shrubs. Septoria leaf spot is one of the most destructive tomato diseases I know.


Like other leaf spot diseases, Septoria reduces photosynthesis and the flow of important nutrients through the vascular bundles, leaving plants to wither and die. 


Warm, wet weather is all this fungi needs to set up housekeeping in your garden. And remember, that wetness can be caused by poorly placed sprinklers, leaky hoses, and overhead watering, just as easily as the weather. Temperatures between 60°F and 80°F are ideal for fungal growth. Knowing what to look for can help you protect your plants.

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Types of Septoria

Septoria is a family of fungi. Different subspecies affect different plants. The most common types of Septoria, followed by their host plants and symptoms, include:

• Septoria apiicola - celery; tiny yellow round spots grow together and become brown and may be angular; black fruiting bodies are commonly visible [While also known as late blight, this is not the same late blight of tomato, caused by Phytophthora infestans.]
• Septoria cucurbitacearum - melons, squashes, cucumbers; round, brown spots
• Septoria lycopersici - tomato; 1/8” water-soaked spots grow to 1/4” grayish-tan spots with dark edges; black fruiting bodies and yellow halos may be visible.
• Septoria petroselini - parsley; sunken, angular brown spots, may have reddish edges; petioles may have lesions

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Symptoms are first seen in older leaves. The disease spreads upward into newer growth. As the spots spread, leaves turn yellow, die, and fall off. This leaf loss reduces plant vigor and increases the chance of fruit being damaged by sunburn. Severe infections can result in complete defoliation.

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Septoria leaf spot lifecycle

Septoria fungi travel on the wind and in rain, so it’s something you need to monitor for regularly. Spores come into contact with host plants and send out thready hyphae, which enter plants through cracks and injury sites. Spores overwinter in the soil and on infected plant debris.


How to control Septoria leaf spot

As with many other diseases, prevention is far easier than treating. These tips will help prevent Septoria leaf spot in your garden:

• start with clean, disease-free seeds and seedlings
• shop for disease resistant cultivars [Indicated with an “S” on the label.]
• prune plants for good air flow
• use tomato cages or trellises
• avoid overhead watering
• preventative sprays of fixed copper may help
• mulch around plants (without touching stems) to reduce the splash-back factor
• use a 3-year crop rotation
• remove weeds in the nightshade family
• potassium bicarbonate can be applied

If Septoria leaf spot is seen, remove infected leaves right away and throw them in the trash. Also, sanitize any tools that may have come into contact with infected plants and avoid working around plants when they are wet.