Septoria Leaf Spot

Yellow spots on leaves may indicate Septoria leaf spot.

​This fungal disease is very destructive. It affects celery, chicory, cucumber and other cucurbits, and tomatoes.

Septoria leaf spot on tomato (Scot Nelson) Public Domain

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

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

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 the following:

• Septoria apiicola – celery; tiny yellow round spots grow together and become brown and can 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.]
• S. cucurbitacearum – melons, squashes, cucumbers; round, brown spots
• S. lycopersici – tomatoes; 1/8” water-soaked spots grow to 1/4” grayish-tan spots with dark edges; black fruiting bodies and yellow halos may be visible.
• S. petroselini – parsley; sunken, angular brown spots, may have reddish edges; petioles may have lesions

Symptoms occur first 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 risk of sunburn damage. Severe infections can result in complete defoliation.

Septoria leaf spot lifecycle

Septoria fungi travel in the wind and rain, so monitor regularly. Spores land on 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 them. 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 airflow.
• 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.
• Avoid working around plants when they are wet.
• Use a 3-year crop rotation.
• Remove weeds in the nightshade family.
• Potassium bicarbonate can be applied.

If Septoria leaf spot appears, remove infected leaves and throw them in the trash. Also, sanitize any tools that may have come into contact with infected plants.