If you grow tomatoes, you have almost certainly run afoul of the fungus that causes the disease known as early blight.
What Is Early Blight?
Early blight is a fungal disease caused by . It can occur at any time during the growing season, high humidity and temperatures above 75°F cause it to spread rapidly.
The fungus overwinters in the soil, and spores can be spread by wind, water, insects, and even on your clothes or shoes.
If you catch an outbreak early enough, you may be able to save your crop. The tomatoes are still edible, particularly if the disease is mostly confined to the foliage.
You may have also heard of a disease known as late blight.
To avoid any potential confusion, I want to stress that early blight is not the same thing as late blight – the devastating disease responsible for the Irish potato famine.
Late blight is caused by a fungus-like water mold, .
If you think early blight is bad, late blight is much worse. It is generally fatal to both tomatoes and potatoes; it can spread for miles, and it was largely responsible for the death of one million Irish people (although the reasons for that famine were highly political and not just agricultural in nature).
You can learn more about late blight blight in this guide (coming soon!)
The first sign that your plants are infected with early blight is usually the appearance of dark brown spots on the lower leaves. This disease usually progresses from the bottom of the plant to the top.
These are not just any spots. As they grow larger, they form concentric rings that resemble a bull’s-eye, and the rest of the leaf gradually turns yellow.
Your plants may lose a lot of their leaves, resulting in sun scald on the fruits.
If the stems develop lesions, they will be slightly sunken. As they grow in size, they will develop the same concentric markings that appear on the leaves.
If the spots are near the ground, they can girdle the stems and prevent the plant from thriving by limiting the plant’s uptake of water and nutrients.
Part of the reason that there is pretty much no escape from this fungus is that it overwinters in the soil and in infected plant debris. It is often introduced into gardens via contaminated seed or transplants.
The lower leaves become infected via contaminated soil – either from direct contact or from rain splashing fungal spores onto the plant.
Flea beetles sometimes transmit the pathogen as well as they travel from plant to plant.
The fungal spores require free water, such as rain or heavy dew, or at least 90% humidity to germinate. They are not very fussy about temperature, and will germinate in a wide range of temperatures, from 47 to 90°F.
After the spores have germinated, lesions can form on infected plants in as little as five days. When the spores are present in a garden or field, they can be spread by wind, equipment, insect pests, or human contact.
Resistant Varieties Can Still Become Infected
Many tomato cultivars exhibit some resistance to early blight. However, resistance is not the same thing as immunity.
Regardless of the cultivar that you have selected, your plants can still be infected with early blight if you do not take precautions to prevent the disease.
Cornell University’s Department of Plant Pathology provides an extensive listing of resistant varieties.
A popular resistant cultivar is ‘Cloudy Day,’ a hybrid, indeterminate cherry type that produces 4- to 5-ounce fruits.
Ready to harvest in 70 days, you can find packets of 25 seeds available at Burpee.
If you see “EB” listed on tomato seed packets or in catalogs, that stands for “resistant to early blight.” Keep an eye out for this helpful notation when you are searching for resistant varieties.
Fortunately, even though tomato plants may not be immune to early blight, you can take measures to minimize the chances of infection.
There are a number of steps you can take during the growing season – or even before it begins – to limit the damage that this fungus may do to your plants.
These are the most effective ways to prevent it from taking hold in your garden:
Rotate Your Crops
When you harvest a bumper crop one year, it is so tempting to plant in the same spot the following season.
However, if tomatoes are the crop in question, restrain yourself! You increase the chance of developing an early blight infection if you grow tomato plants in the same place in consecutive years.
Wait at least two years before planting in the same location again, since the spores can persist in the soil and any partially decomposed plants for a year to follow.
This is true even if you didn’t see any symptoms of early blight, since the pathogen can start building up without your knowledge.
Purge Nightshades and Volunteer Tomato Plants
Plants in the same family as tomatoes (solanaceous plants, or nightshades), and volunteer tomato plants can also serve as hosts for species. These can pass the infection on to your tomato plants.
This is true for edible crops such as potatoes, as well as hairy nightshade, black nightshade, and horse nettle, so be vigilant and keep these weeds out of your garden.
When you remove the weeds and volunteer plants, make sure you destroy them – do not place on your compost pile.
Keep Your Plants Dry
The spores require a lot of moisture to germinate.
Tomato plants are used to growing in dry climates, so they are unusually sensitive to water on their leaves, which makes them more prone to fungal infections than many other crops.
Take every precaution you can to minimize the amount of moisture on your tomato plants. In the presence of spores, this can help to prevent a fungal infection from getting worse. Try to avoid working with or around your plants in wet weather.
Using drip irrigation instead of watering from overhead will help to keep your plants dry.
Stake Your Plants
When your plants are staked, this helps to create better airflow around them, helping them to stay dry.
Florida weave to support your tomato plants.
Remove Infected Plants
If you see signs of early blight on one or two of your plants, it’s best to pull them up and destroy the debris immediately, do not place it on the compost pile.
You’ll then need to inspect the rest of your crops to check for symptoms, and treat accordingly, as discussed below.
Even if they aren’t showing any symptoms, if one plant has been infected, then it’s likely that the infection has already spread to your other plants. There are a few treatment options available.
The first is a modern version of the classic Bordeaux Mixture, Bonide® Copper Fungicide Dust, and it’s available from Arbico Organics.
The original Bordeaux mixture contained copper sulfate and lime and could sometimes be toxic to plants.
However, this formulation with copper alone is much safer, and is approved for use on edibles.
Use either diluted as a spray, or dust the powder onto the foliage, every three days.
Another option is the biofungicide Serenade® available on Amazon.
This is a strain of the bacterium that can help prevent the infection from spreading.
While the copper fungicide dust will kill existing infections, Serenade® decreases the likelihood that any remaining fungi will spread throughout your crop.
See our guide to learn more about controlling plant pathogens with .
A variety of fungicides are effective against early blight, but the fungi that cause this infection are becoming resistant in some areas.
One advantage of this fungicide is that it acts on multiple targets in the fungus at the same time, so resistance is less likely to develop.
In other words, if a fungicide targets one specific aspect of the organism’s metabolism, resistance develops much more quickly than if the chemical targets a number of different biochemical processes at the same time.
You may want to apply fungicide as a preventative measure. If so, apply it at fruit set, or just before, and reapply every 7 to 14 days, avoid harvesting your plants within 7 days of application.
Always follow the manufacturer’s recommendations when you use chemical treatments.
There Is Hope Against This Ubiquitous Pathogen
Despite its frequent presence on tomato plants, there are steps you can take to minimize the risk of initial infection and subsequent spread of early blight.
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Helga George, PhD
One of Helga George’s greatest childhood joys was reading about rare and greenhouse plants that would not grow in Delaware. Now that she lives near Santa Barbara, California, she is delighted that many of these grow right outside! Fascinated by the knowledge that plants make chemicals to defend themselves, Helga embarked on further academic study and obtained two degrees, studying plant diseases as a plant pathology major. She holds a BS in agriculture from Cornell University, and an MS from the University of Massachusetts Amherst. Helga then returned to Cornell to obtain a PhD, studying one of the model systems of plant defense. She transitioned to full-time writing in 2009.