Early Blight – A Fungal Disease

Alternaria solani

Early blight affects members of the nightshade family, most particularly tomatoes and potatoes, though it may occasionally affect peppers and eggplant. It is distributed world wide.

Early blight is a fungus, and the spores, which you’ll eventually see on the fruits and tubers of the affected plants, are dark and velvety.

Early blight isn’t as devastating as late blight. It’ll most likely stunt the growth of your plants and damage some of the produce, but you may still be able to harvest some undamaged fruits or tubers.

Tomatoes

Ringed brown spots of early blight found on tomato leaves. Photo credit: Clemson University - USDA Cooperative Extension Slide Series

Ringed brown spots of early blight found on tomato leaves.
Photo credit: Clemson University – USDA Cooperative Extension Slide Series

Early blight is likely to appear when the plant blossoms or when the fruit begins to set.

The earliest warning you’ll have that your tomatoes have early blight will likely be brown, ringed spots (or, more accurately, you’re likely to see concentric circles) on the lower foliage. The spots may increase in number and size until they coalesce.

There may occasionally be a ring of yellow around the spots. Eventually the rest of the leaf will turn yellow, then brown, curl up, and drop from the plant. (See picture to the right. There is also an excellent example here.)

If the leaf turns yellow, but doesn’t curl up and turn brown, and you don’t find any of the characteristic brown spots on the leaves, it’s more likely your tomatoes have a nitrogen deficiency, and not early blight.

If enough leaves die and fall off, it can expose the tomatoes to the sun, causing sun scald.

Stems can also develop lesions, which will appear brown or black, and sunken.

The fruits of the tomato will first show signs of the disease at the stem end. The color around the attachment to the stem will look grayish-green, and the inside of the fruit will begin to rot. If you cut the fruit open, you’ll find dark, rotting, unpleasant-smelling tomato mush. The fruit may show the same ringed spots.

Seedlings may become girdled (a ring around the stem will appear sunk in) which is known as “collar rot”. Early blight can also cause damping off in seedlings.

Potatoes

Early blight also causes leaf spotting on potato plants, which are similar in appearance to those that appear on tomato leaves.

The tubers will also be affected, becoming puckered and showing small, dark spots or lesions. Early blight lesions on potatoes are characteristically dry and leathery — not a typical moist, moldy rot. The potato is more likely to shrivel up than rot through.

Do not eat affected potato tubers or tomato fruits.

Transmission

Early blight is a fungus, and produces spores, so anything that can carry a spore can spread early blight – most particularly the wind and rain, and a gardener working in the garden when there’s moisture on the leaves. Moist, warm environments are ideal for early blight spore formation.

The disease can also be spread by using seed from infected plants. Only use clean seed from disease-free plants. Remove any volunteer potatoes or tomatoes that show up in your garden – they may have sprouted from seeds dropped by diseased plants that grew the previous year.

Some tomato varieties are somewhat resistant to early blight, though no tomato variety is 100% resistant.

Prevention & Control of Early Blight

“An ounce of prevention is worth a pound of cure.”

If early blight is found on any plants, it’s best to remove them immediately from your garden. Allow the plant material to dry, and then burn the plant matter.

If burning plant material is not feasible, after plant material has thoroughly dried, then bury the plant matter.

To prevent early blight increasing and spreading from one garden season to the following year, be sure to remove all crop debris from your garden at the end of the year. A good, thick wood-chip mulch, which covers the soil and prevents splashing of soil onto leaves during routine watering or when it rains, can do a lot to help prevent the spread of disease. You may even want to remove some of the bottom leaves, though that would open wound sites where the spores could enter, so only prune the lower leaves when the weather is going to be hot and dry for several days.

Ensuring proper circulation of air in and amongst your plants will help prevent early blight, and/or control its spread, if it does occur. Don’t plant plants too close, particularly in humid climates. Make sure leaves are able to dry as quickly as possible when they do get wet.

Try to keep plants dry to prevent the occurrence and spread of disease.

Try to keep plants dry to prevent the occurrence and spread of disease.

Do not work in your garden when there is still water on your plants from rains, watering, or morning dew.

Don’t water with sprinklers. Always water at ground level, taking care not to splash water on the leaves or stems. Try to water in the morning, when any splashed water will be able to dry more quickly in the heat of the day. If you water in the evening, the plant may remain damp all night and be much more likely to become diseased.

As always, one of the best ways to prevent diseases and help your plants resist them when they do occur is to make sure your plants have the nutrition they need. Strong, healthy plants can do a lot in fighting their own battles. Make sure your plants have consistent moisture, as well as other nutrients.

Early blight will be able to enter through small wounds openings that are already on your plants. Try to prevent damage occurring to your plants so early blight doesn’t even have a place to take hold.

Because early blight can be such a troublesome disease (though not as devastating as its cousin Late Blight), crop rotation is advisable (a 3-year rotation), especially if it is prevalent in your area. (Talk to some experienced gardeners in your area – they’ll know if early blight is a particular problem where you live.) This may not be practical in small gardens.

Keep your garden clear of weeds, especially those that might be carriers of the disease, such as black nightshade (worth getting rid of anyway due to being highly poisonous).

It is possible that fumigating your soil (which can be done naturally with mustards) may help rid a section of your garden of early blight. If your plants are consistently affected with early blight, it might be worth a try.

If you try using fungicides, just know that once the leaves turn yellow and begin to fall off, the fungicide isn’t likely to be effective anyway. Always follow the instructions when using fungicides.

Further Reading Resources:

  1. APS: Early Blight of Potato and Tomato
  2. University of Maine – Cooperative Extension: Fact Sheets – Early Blight of Tomato
  3. Colorado State University Extension: Tomato Early Blight
  4. Virginia Cooperative Extension: Early Blight of Tomatoes

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