Early blight on Plants

Symptoms

Early blight begins with small black or brown spots on leaves, stems, and fruit. The spots are usually about 0.25 to 0.5 inch (6–12 mm) across. On leaves, they are leathery and show a concentric ring pattern. You will see them first on older leaves. At the center of a leaf lesion, the rings form a bull's-eye pattern.

As the spots expand, the tissue surrounding them turns yellow. A leaf with concentric rings and a yellow surround matches the description given for this disease. Stems can develop lesions similar to those on the leaves. These stem lesions can sometimes girdle the plant.

Fruit shows the disease as well. Spots on fruit are sunken and dry, and they sometimes have a concentric pattern. Large fruit lesions can involve nearly the entire fruit. A single fruit can be affected across nearly all of it. Infected fruit frequently drops. Look for these signs on tomato, potato and eggplant, the plants listed as hosts.

Causes

The cause is the fungus Alternaria solani. It survives in the soil on residue of infected tomatoes, potatoes, and nightshade weeds. Nightshade weeds belong in that list of residue sources. The soil residue of infected tomatoes and potatoes is where the fungus survives.

Spores spread by wind or splashing water. Weather shapes how the disease develops. Cool, humid weather for several days after rain favors disease development. The fungus needs free moisture for spores to germinate and infect. Wet leaf surfaces are needed for the fungus to germinate and grow. Leaves that stay wet give the fungus what it needs to grow.

Disease development stops in dry, hot weather. Dry, hot weather and cool, humid weather after rain sit at opposite ends of the conditions listed for this disease. Rain, humidity and leaf wetness are the conditions tied to the disease. The hosts named for this disease are tomato, potato and eggplant.

Diagnosis

The facts on this page name no look-alike diseases, so the best test is how well your plant matches the full description of early blight. Begin with the older leaves, where the leathery spots with concentric rings appear first. A bull's-eye pattern at the center of a spot, with yellow tissue around it, fits the description.

Next, check the stems for lesions similar to the leaf spots, which can sometimes girdle the plant. Then look at the fruit for sunken, dry spots, sometimes with a concentric pattern. Signs on leaves, stems and fruit all belong to the same description.

Then consider the conditions: early blight needs wet leaf surfaces, and cool, humid weather for several days after rain favors it. If the weather has been dry and hot, remember that disease development stops in those conditions. Check the host as well, since this disease is recorded on tomato, potato and eggplant. Also look at the soil for residue of infected tomatoes and potatoes, and for nightshade weeds, where the fungus survives.

Treatments

Start with cultural steps. Destroy nightshades and volunteer tomato and potato plants to remove sources of infection. Rotate crops properly so infected plant debris has time to decompose before the next susceptible crop. Plant disease-resistant tomato cultivars where available.

Eradicate weeds and volunteer tomato plants, and space plants so they do not touch each other. Avoid wetting foliage with irrigation water. Remove infected lower branches and leaves and dispose of them properly, without adding them to compost. When you prune out infected leaves, do not remove more than a fifth of the plant's total leaf mass at once. Disinfect pruning tools with a diluted household bleach solution between cuts to avoid spreading the pathogen.

For organic gardening, apply a biological fungicide containing Bacillus subtilis for suppression, beginning when plants are still young. Check that the specific product is labeled for organic production. A copper fungicide is another organic option, and fixed copper is the active ingredient. Not all copper products are approved for organic production, so check the label of the specific product.

Chemical options include chlorothalonil and mancozeb fungicides. Apply either one following label directions when environmental conditions favor the disease and at the first sign of symptoms. A fungicide of any kind is applied once conditions become favorable for the disease, following label directions. Treatment is often unnecessary where the disease is uncommon. The product label is the legal instruction, so follow it for the crop and site you are treating.

Prevention

Prevention is about conditions in the bed. Keep foliage dry by avoiding wetting leaves with irrigation water, since the fungus needs wet leaf surfaces to germinate and grow. Space plants so they do not touch each other.

Rotate crops so infected plant debris has time to decompose before the next susceptible crop. Remove weeds, nightshades and volunteer tomato and potato plants, which are sources of infection. Residue from infected tomatoes, potatoes and nightshade weeds is where the fungus survives in soil.

Choose disease-resistant tomato cultivars where available. Watch for cool, humid weather lasting several days after rain, which favors the disease. Remove infected lower branches and leaves and dispose of them, keeping them out of compost.

Plants most often affected

Tomato, potato and eggplant are the hosts recorded for early blight. Volunteer tomato and potato plants and nightshade weeds are named as sources of infection. The fungus survives in the soil on residue of infected tomatoes, potatoes, and nightshade weeds. Disease-resistant cultivars are named for tomato where available.

• Garden Tomato