Early Blight

Dark brown spots on your tomatoes? It’s probably early blight.

“Bullseye" pattern leaf lesion of early blight (USDA Cooperative Extension) CC BY-SA 3.0

Cool spring temperatures and too much rain or other moisture create the perfect storm for this fungal disease. The early blight fungus (Alternaria solani) is a disease of the nightshade family, which means your potatoes, eggplants, bell peppers, and chili peppers are equally susceptible, as are other plants in the nightshade family, such as petunias and blue potato bushes. If similar lesions occur later in the season, it is probably late blight, also known as the dreaded potato blight.

Early blight was once a disease found only on the east side of the Rocky Mountains. Sadly, that is no longer the case. The early blight pathogen now occurs everywhere host plants have been grown and can result in up to 30% of your potatoes and 79% of your tomato crop ending up in the trash bin.

Early blight symptoms

Small black or brown spots, usually 1/4 to 1/2 inch in diameter, may appear on fruit, leaves, and stems. These spots may have a concentric ring pattern or bullseye. Fruit spots are dry, sunken areas, most commonly seen near the calyx (flower). Spots on leaves feel leathery. Symptoms are seen on older leaves first. Stem lesions do not have the same circular bullseye pattern.

Early blight stem lesion (USDA) CC BY-SA 3.0

This pathogen can also cause collar rot and damping off, which usually kills seedlings. As the disease progresses, leaf loss can significantly reduce fruit production. Honestly, the fruit that grows doesn’t look particularly appetizing. Infected potatoes either rot in the ground or storage.

Early blight lifecycle

Fungal spores overwinter in the soil and on infected fruit and plant debris. Rain and overhead irrigation splash spores onto plants, where they begin reproducing.

To reduce the chance of early blight in your garden:

• Plant resistant cultivars.
• Remove nightshade family crops after they stop producing.
• Remove nightshade family weeds.
• Throw infected plants in the trash.
• Avoid overhead watering.
• Employ a 3-year crop rotation plan.
• Use mulch around plants to reduce spore mobility.

Fixed copper or sulfur sprays can provide fungicidal benefits in heavy infestations. Also, healthy plants are less likely to become infected, so feed and water your plants properly, give them enough space to reach full size, and help them avoid physical injuries, which provide entry points for early blight fungal spores.