Identification, Prevention, and Treatment of Late Blight
We’re never short of challenges with tomato plants in the garden. Late blight is a disease affecting not only our ‘maters but also potatoes. It doesn’t appear everywhere every year, but when it does, it’s a big problem. Learn how to identify late blight preventatively to reduce the likelihood of this problem.
What Is Late Blight?
Late blight is also sometimes called tomato blight or potato blight. They’re all the same problem. It’s a fungal disease caused by an organism called Phytophthora infestans. The Phytophthora genus causes all sorts of problems for plants. It can attack both tomatoes and potatoes because they’re from the nightshade family, Solanaceae, and from the same genus, Solanum. It’s the disease that caused the Irish potato famine in the 1840s.
Tomato blight is caused by an oomycete, a fungus-like water mold. Gardeners and farmers worry about its rapid spread through the foliage and fruit, especially in warm, wet weather.
How Does Late Blight Infect and Harm Plants?
Like other fungus-like pathogens, P. infestans spreads by spores, which may be transferred by infected foliage or blown on the wind for long distances. Warm days and cool nights are perfect for late blight. If leaves remain wet for several hours, the spores can infect the foliage and establish themselves. A prolonged spell of hot, dry weather can halt the spread.
Of note, late blight is an airborne pathogen, unlike many other tomato diseases, which are soil-borne. Spores can spread by equipment, people, and wind, sometimes drifting for miles.
Once spores have infected foliage, they cause cell death. With the right conditions, an infected lesion can complete its life cycle and release new spores in as little as five days.
Signs of Late Blight in Tomatoes
Promptly identifying late blight in your tomato or potato patch can help you take action to slow it down. The first symptom is irregularly shaped spots that appear water-soaked. They often have a gray or light green ring around the edge of the lesion. Look at this guide from the University of Wisconsin for help identifying late blight lesions.
Along with spots on the leaves, brown lesions also appear on the stems. Eventually, fruits may also be infected, and large portions may appear golden yellow to dark brown.
As the disease progresses, the foliage will shrivel, and the entire plant will be defoliated.
Is It Late Blight or Another Problem?
Unfortunately for our favorite crop, many similar-looking tomato diseases exist, including early blight, Septoria leaf spot, Fusarium wilt, and Verticillium wilt.
Here’s some help identifying the look-alikes.
Early blight: Lesions (spots) first develop on lower leaves. They’re about ¼ inch in diameter, brown to black, with rings in the darkened area. Leaf tissue around the brown spot yellows, and the entire leaf may eventually turn yellow and die. The disease spreads into the upper leaves.
Septoria leaf spot: Much smaller spots on leaves than late blight. Spots are usually round, about 1/16 of an inch, and often light gray in the center.
Fusarium and Verticillium wilts: Wilting is the key indicator. Stems will show brown fungus inside the water-carrying tissues, and leaf yellowing is common. Fusarium may first wilt only one branch or side of the plant.
Prevention and Monitoring
Tomato blight, or late blight, isn’t as common as some of the other diseases and doesn’t appear every year. However, it is devastating and can cause the entire crop to be lost in days.
- Stake tomato plants upright to allow air circulation around the plant and through its canopy. Proper airflow will help to dry leaves, inhibiting spore germination.
- Water tomatoes from below if possible. While rain does fall from above, we often irrigate much more often, and doing so from above wets the foliage. Use a drip line, soaker hose, or a long wand on a garden hose to apply water at the base of the plant and not from overhead.
- Water in the morning, after the sun comes up, but before the heat of the day. This will ensure that your plants have water to last through the day and that the foliage will dry before the cooler night temperatures.
- Walk through your garden often, taking care to look for pests and issues. Catching problems early can help you mitigate the damage.
- Follow good garden hygiene. Clean and sanitize tools used on tomato plants.
Choose Resistant Plants and Early Maturities
Plant breeders have been working for years to develop more disease-resistant varieties of our most popular crops. Choosing a tomato variety resistant to the particular problem you’re experiencing is one of the first, most effective, and easiest ways to mitigate the issue. One popular seed vendor lists 21 varieties resistant to late blight. Choosing resistant varieties is especially important because most chemical treatments available to the home gardener aren’t effective in controlling late blight once it appears.
Resistance doesn’t mean immunity. These varieties will still be infected by spores and suffer some damage, but they’re more able to shrug it off or keep on doing their tomato thing even in the presence of the disease. Most late blight-resistant varieties are hybrids, meaning you’ll need to buy new seeds instead of saving them from previous years.
Choosing short-maturity tomatoes can also help the fight against late blight and even some other types of fungal tomato diseases. Late blight, as the name implies, typically arrives later in the growing season. As such, planting early-to-mature varieties means your crop might be almost ready for harvest by the time the pathogen arrives.
Treatments for Late Blight of Tomatoes
The most effective treatment for late blight is prevention. In addition to the cultural prevention actions listed above, follow these guidelines if you suspect late blight has infected your plants.
- Remove affected leaves as soon as you notice them. While this will not stop the disease altogether, it will prevent those lesions from releasing a new crop of spores a few days down the road.
- Use the fruit (tomatoes) from the infected plants as soon as possible. Eventually, the fruit will become infected and be lost.
- Infected plant material should be sent off to the incinerator or buried deeply enough so you won’t disturb it when preparing the garden next year. Don’t compost it.
- While rotation doesn’t prevent wind-borne spores from drifting into your garden, it can help with sanitation; you’re bound to miss picking up a few tomato leaves and stems in the fall, which can sometimes harbor spores. Remember, just moving over a few feet doesn’t do the trick. Provide as much space as you can between last year’s bed and this year’s location.
For more tips on pests and problems, see our complete Tomato Growing Guide and Potato Growing Guide.
Andy Wilcox
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