Early Blight

Common Name: Early blight of tomatoes

Scientific Name: Alternaria solani

Symptoms

One of the most severe tomato diseases affecting home gardeners, early blight can affect the stems, leaves, and fruit of tomato plants. It can also cause damping-off in seedlings. It doesn't occur in arid dry regions but is most active in warm, wet or rainy conditions.

Plant symptoms: Brown circular "bull's eye" rings appear as 1/2-inch diameter leaf spots. The spots appear first on lower, older leaves. As the disease progresses, stems and fruit may become infected. Leaves turn yellow and wither, starting at the base of the plant, until the whole plant is affected.

Fruit symptoms: Similar to the leaf spots, the spots on fruit appear as dark brown concentric rings with the presence of dark, dusty spores. As the disease progresses, the spots on more mature fruit may appear larger, dark, dry, and sunken.

Causes of Early Blight

The fungus survives in infested soil and plant residue. Therefore, planting successive years in the same soil can cause a recurrence of the disease. It is caused by different fungi than late blight. The disease can be carried by infected seed and can be spread by wind, water, insects, humans, and equipment. The spores that land on plants will affect the leaves when they are wet.

The Alternaria fungus is most severe on plants stressed by a heavy load of fruit or low nitrogen fertility and is worse during the rainy season.

Control of Early Blight

A combination of practices can help keep the fungus under control. The best method is prevention. Once it's started the disease is very difficult to control. Fungicide may be your only option.

Preventative measures:

Rotate your crops by not planting tomatoes in the same soil where tomatoes, peppers, potatoes, eggplant have been planted for at least two years.
Stake and mulch tomato plants to keep foliage from contact with the soil.
Plots surrounded by grasslands are better protected because grasses don't host the disease.
If watering from overhead, do it early in the day to allow the sun to dry the leaves.
Use disease-free certified seed.
Use fresh deep soil with good drainage.
Sterilize the soil with hot water before planting.
Inspect tomato seedlings before planting and destroy any that show disease signs.
Increase the amount of organic matter in the soil. In particular, well rotted manure will increase fertility and decrease nematodes.
Use "green manure" nitrogen fixing legumes in the off-season to increase fertility of your soil for tomatoes.

Chemical control:

First check with your county agricultural extension office for their recommendations and to find out what chemicals are legally allowed.
When early symptoms of the fungus are detected, apply protectant fungicides (carbamates, clorotalonil, and cuprics) to the entire plant.
Use intervals of seven days if the conditions are wet or ten days if the weather is dry.
Rain or overhead irrigation will wash off the fungicide treatments so fungicide will have to be reapplied.

For more information on the Alternaria fungus and other tomato diseases, contact your county Extension office.