Late blight: What does it look like?

This tomato plant is very sick with late blight, a fungus disease.

If your tomato crop failed miserably in the summer of 2009, take heart, you are not alone. Thousands of people faced the same disaster. And it doesn’t mean you’re a failure as a gardener. A widespread fungus disease called late blight caused the problem and cool, wet weather made it worse.

How can you tell if your tomatoes are infected with late blight? Check the leaves and stems for large brown spots, look for cottony white spores on the spots, and see if the fruit has greasy brown patches. The combination of all three symptoms identifies late blight.

Large brown spots.

Late blight spots are large, dark, and without yellow haloes.

First, determine the size and color of the spots. Late blight has large spots that 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. 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.

Cottony-white mold.

Cottony white spores develop on the underside of the leaf.

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 itself, so do a bit of forensic detective work. Put an infected leaf inside a plastic bag with a piece of moistened paper towel, then seal the bag. If it’s late blight, cottony white spores will develop within 12 hours on the tissue inside the plastic bag.

Greasy brown patches.

Spots on fruit are firm and brown.

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.

The presence of all three of these symptoms (large, dark spots; cottony-white mold; and greasy brown patches on the fruit) confirms the worst; you have late blight. Our next blog, Late blight: How do I fix it?, gives you some tips on how to deal with it.

David Deardorff is a botanist and plant pathologist. Kathryn Wadsworth is a writer and naturalist. Visit their website. They are the authors of “What’s Wrong With My Plant? And How Do I Fix It?” from Timber Press.

Late blight: How do I fix it?

Late blight is a devastating fungus disease of tomatoes and potatoes. Our previous blog, Late blight: What does it look like?, showed you how to identify it. Here, we show you how to deal with it.

Prevention is the best way to manage late blight. And knowing the life cycle of the fungus helps you figure out ways to prevent it. If you can disrupt the fungus life cycle, you can protect your plants.

The fungus invades plant tissue and kills the cells.

The late blight fungus (Phytophthora infestans), like many other fungi, 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.

The cottony white mold is composed of millions of spores which spread the disease.

Each spot on leaves and stems produces 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 gardeners. 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).

The key to protecting your plants from late blight is to kill the spores and disrupt the life cycle of the fungus. If you can prevent the spores from germinating and infecting your plants, you can control this disease.

To keep healthy plants free of this disease, protect them with products certified for use on organic food crops. Neem oil, copper, and sulfur, for example, are chemicals that create conditions on the surface of leaves, stems, and fruit that prevent late blight spores from germinating. Read the labels to learn how to use each product. Check your tomato plants (and their relatives) once a week for symptoms. This disease spreads quickly, so be vigilant.

Sadly, no cure exists for this disease once your plant becomes infected. No magic bullet exists; not even modern synthetic fungicides can cure a plant with late blight. However, there are still some things you can do if your plants get sick.

For slightly infected plants – those with only a few lesions (spots) on leaves or stems – you must protect the remaining healthy leaves.
First, spray Neem oil, copper, or sulfur, and allow the plants’ foliage to dry.
Second, sanitize by removing infected tissue from the plants, the ground, and your entire garden.
Third, 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.

For plants seriously ill with late blight you have to pull the plug.
First, pull up the infected plants, roots and all.
Second, put them in a plastic bag and seal the bag tightly.
Third, put the bag in the sun for a couple of days. The sunlight and the heat will help kill the spores.
Finally, 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. Do not put the infected material into a municipal composting system, because you will spread the disease to others.

Late blight fungus can live in the soil and may even overwinter. Next year put unrelated plants where your tomatoes and their relatives were this past summer. Plant some other crop, such as corn, cabbage, or squash, in that location and move your tomatoes to a completely new spot. You may have had a lousy tomato crop this year, but you can  always hope for a great season next year.

David Deardorff is a botanist and plant pathologist. Kathryn Wadsworth is a writer and naturalist. Visit their website. They are the authors of “What’s Wrong With My Plant? And How Do I Fix It?” from Timber Press.