Late Blight of Tomatoes

Home gardeners and commercial tomato producers in the Eastern and Southern US face a potential tomato crop failure this summer. The problem is called late blight and is caused by a fungus.

What are the symptoms?

There are several different fungal diseases of tomatoes, including early blight and septoria leaf spot. But no other fungus has the specific combination of symptoms on leaves, stems, and fruit that identifies late blight. If your tomato plants have all three of the following symptoms you can be sure you are dealing with late blight.

First, determine how big the spots are. Late blight spots are large. They grow rapidly, enlarging to engulf the leaf or stem in just a few days. As long as the leaf tissue is moist, the spots will be very dark, purplish-black. When the tissue dries out the spots become dark brown. The spots generally do not develop yellow haloes. If you see dark brown or purplish-black spots on the leaves that are about the size of a quarter and that get significantly larger (seemingly overnight) you should suspect late blight. Other fungal diseases cause leaf spots that are smaller in size, do not grow as fast as late blight, and are lighter in color.

Second, look for cottony-white mold on the spots, especially on the underside of leaves. In dry weather you may not see this white mold on the plant, so put an infected leaf inside a plastic bag with a piece of moistened paper towel, then seal the bag. Cottony white spores will develop within 12 hours on the tissue inside the plastic bag.

Third, look for greasy-looking, brown, firm patches on the tomato fruit. These can appear on unripe, green tomatoes as well as ripe, red ones. They are often on the stem end of the fruit but can be anywhere.

What does late blight do?

Like many other fungi, late blight fungus (Phytophthora infestans) produces millions of airborne spores that drift on the breeze. When they settle on a susceptible host plant (leaf, stem, or fruit), the spores germinate and the fungus begins to grow down into the plant’s tissues. The fungus digests the cells of the host plant as it grows, first turning them black and then brown as the cells die. The initial spots are small, but they grow larger, quickly engulfing the entire leaf. The leaf wilts and dies, hanging on to the sick plant. Soon, the entire plant dies and the fruit is ruined.

Each of the spots on leaves and stems produce millions of microscopic spores that look like cottony white fuzz. Each tiny spore is a potential new infection that can devour healthy tissue. These spores are carried to healthy plants by wind, wind-driven rain, irrigation water, tools, and people. If the weather is cool and moist, the disease spreads rapidly through many plants in the nightshade family (Solanaceae: tomatoes, potatoes, peppers, and eggplants, for example).

What is the solution?

Prevention is the only effective way to protect your plants from late blight. This disease spreads quickly, so vigilance is called for. Check your tomato plants (and their relatives) frequently for symptoms. If your plants are not sick, and late blight is in your area, protect your plants with a spray that is certified for use on organic food crops. Remember that you intend to eat the fruit of these plants. Preventive treatments include Neem oil, copper, or sulfur. Read the labels to learn how to use each product. Choose the product that best suits your circumstances.

These products create conditions on the surface of leaves, stems, and fruit that prevent late blight spores from germinating and infecting your plants. As such, they protect healthy plants. They cannot cure sick plants. Unfortunately, there is no magic bullet; not even modern synthetic fungicides can cure a plant with late blight.

If your plants are only slightly infected--with only a few lesions (spots) on leaves or stems--you must protect the remaining healthy tissue. To do so, spray Neem oil, copper, or sulfur, allow the plants’ foliage to dry, then sanitize. To sanitize means to remove infected tissue from the plants, the ground, and your entire garden. Put all the infected material into a plastic bag, seal it, and discard it in the trash. Do not compost it. You will have to be vigilant and search for new infections every day. You will also need to apply the spray again, especially if rain washes the material off your plants.

If your plants are already seriously ill with late blight you should pull up the infected plants, roots and all. Put them in a plastic bag and seal the bag tightly. Put the bag in the sun for a couple of days. The sunlight and the heat will help kill the spores. Then discard the plants, with the bag, in a landfill. Do not compost the sick plants. Get rid of them. The fungus can live in your compost pile and will be a source of new infections.

Late blight fungus can live in the soil and may even overwinter. Next year put unrelated plants where your tomatoes and their relatives are this year. Plant corn, cabbage, or squash in that location and move your tomatoes to a completely new spot. You may have a disappointing tomato crop this year, but look forward to a bumper crop next year.

David Deardorff and Kathryn Wadsworth, authors of What's Wrong with My Plant (And How Do I Fix It?), available in November