Early blight on tomatoes — symptoms and treatment

Severity: medium

What is Early blight

Early blight of tomato is caused by the fungus Alternaria solani. It marks the leaves with characteristic target-spot lesions — brown patches crossed by concentric rings, like the growth rings in a cut log. It does not stop at the leaves: the same fungus lesions the stems and rots the fruit at the stem end. Early blight seldom kills a plant outright, but it strips the canopy from the bottom upward, and a plant that loses its leaves loses the machinery that fills its fruit.

Symptoms

The disease starts low and old. The first lesions appear on the oldest leaves nearest the soil as dark brown spots, and as each enlarges it develops the concentric rings that earn the disease its "target spot" nickname, often with a yellow halo. Spots merge, the leaf yellows and dies, and the bare front creeps up the plant. On the stems the fungus produces dark lesions; on the fruit it goes for the stem end, rotting the shoulders where the calyx meets the skin.

• Early: dark brown spots on the lowest, oldest leaves.
• Progressing: spots expand into ringed target lesions; leaves yellow and drop; defoliation climbs.
• Advanced: dark stem lesions; rot on the fruit at the stem end; bare below, leafy only at the top.

Two diseases mimic this one. Target spot also rings its lesions and marks fruit at the stem end. Septoria leaf spot also starts low, but its spots are many, small and grey-centred — not few, large and ringed.

Causes and conditions

Alternaria solani survives between crops on infected tomato debris and in the soil, which is why the lowest leaves are the first casualties: rain and overhead irrigation splash soil and spores upward onto the foliage closest to the ground. From those lesions new spores ride wind and splashing water to the leaves above. Infection needs the leaves to stay wet, so warm, humid weather, heavy dew and a dense canopy that dries slowly all push it along.

Treatment

Mulching and staking — cultural

Timing: at planting and throughout. Mulch around the plants to prevent soil splash, which is how the fungus reaches the leaves in the first place. Stake or cage plants so the foliage is held off the ground and air moves through it. Remove the lower leaves.

Chlorothalonil — chemical

Timing: at first symptoms. Apply the contact fungicide at first symptoms on the lower leaves, while the infection is still confined to the bottom of the plant. Alternate with systemic fungicides. Pre-harvest interval: 7 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Mulch under the plants so rain cannot splash soil-borne spores onto the foliage.
• Stake or cage every plant and space rows so the canopy dries quickly after rain and dew.
• Water at the base rather than over the leaves, and water early in the day.
• Strip the lowest leaves so no foliage touches the soil.
• Clear and destroy tomato debris after harvest, and rotate away from tomatoes and their kin.

Frequently asked questions

Can I eat tomatoes from a plant with Early blight? Yes. Fruit with no rot on it is fine — discard fruit that has rotted at the stem end. If you have sprayed chlorothalonil, respect the pre-harvest interval of 7 days.

Is it too late once the lower leaves are gone? No, but you are behind. A contact fungicide protects healthy tissue rather than curing existing lesions, so the goal is to halt the defoliation front before it reaches the leaves feeding your fruit.

Will it spread to my other plants? It moves readily from tomato to tomato and to related crops such as potato. Unrelated vegetables are not at risk.