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Understanding tomato late blight

November 30, 1999  By Hugh McElhone

Late blight can be a devastating disease in tomato as well as potato crops.

Late blight can be a devastating disease in tomato as well as potato crops.

Historically, it was partially responsible for the Irish potato famine in the 1840s, in which 1.5 million people starved. Records from that time in Ireland report that it spread 50 miles per week, says Michael Celetti, a plant pathologist and horticulture crop program lead with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) based in Guelph, Ont.


To better fight the disease, Celetti says researchers and producers alike must better understand it.

Despite its appearance, late blight is not a true fungus but is caused by Phytophthora infestans, which is a member of the water mould family. It produces asexual spore sacks on the underside of the leaf.

During moderate to warm, moist conditions, the spore sacks germinate and infect the leaf and fruit tissue. Under cool, moist conditions, zoospores are released that can swim along the film of water on the leaf to infect other areas. These little swimmers can also be transferred to other areas of the plant, or from one plant to another, by the splashing done by rain or machinery.

These spore sacks look like a white fuzz on the underside of diseased leaf spots, and are the airborne stage of late blight. According to research, these spore sacks will sail 160 kilometres in four hours where there is a 40-kilometre tailwind.

“This is why late blight is truly a community disease. It can destroy your crop in a week,” says Celetti.

Late blight has two mating types called A1 and A2. Celetti says the A1 mating type was the only type found in North America until the late 1980s. During the early 1990s, however, a more aggressive A2 mating-type strain appeared on tomatoes and potatoes in North America and is now the predominant one here. When the A1 and A2 mating types come together in a field, they can mate and produce a persistent oospore.

Producers need to be wary of the weeds in their fields and the ornamental plants in their gardens, he cautions. Late blight has been found in bittersweet nightshade and hairy nightshade. Some new strains have also recently been discovered infecting petunias.

“Late blight can be introduced into a field by infected tomato transplants,” he says. Home gardeners might also bring it home with infected tomato transplants or with the infected potato tubers they plant in the spring and hope to harvest in early fall.

“These are all sources of infection for your spring-planted tomato crop,” he says. “Always ensure clean plants from your nursery.”

Celetti explains the early signs and symptoms of late blight include olive-grey water-soaked lesions that often, but not always, appear on the edge of the leaf. These then turn to purplish-black, often surrounded by a yellow halo on a crispy leaf. Later on, the tomato stem and petiole appear chocolate brown with lesions that later girdle stems and petioles both.

During the tomato harvest, Celetti advises producers to watch for water-soaked greasy patches on the top and side of the fruit. These could be signs of late blight infection. Such signs suggest the lesions are trying to expand on harvested fruit, which will result in a dry tomato rot.

“Fruit from infected fields that appear to be disease-free will often develop symptoms later in storage. Eventually, soft rot bacteria will invade diseased fruit and cause it to become soft and mushy,” he says.

For tomato growers, 2009 was a problematic year. Nursery greenhouses in the U.S. shipped infected tomato plant seedlings across the Eastern seaboard to the garden centres of a large retail chain. Fortunately, no infected plants were shipped to Ontario. Unfortunately, the weather conditions of 2009 were conducive to a huge disease spread.

The first late blight sighting was at Grand Bend on July 8, 2009, followed the next day with a report from Alliston, some 200 kilometres away. Other reports followed mid-month from tomato growers in Chatham, Kent and Essex. By the end of July, a potato farmer in Norflok County reported his entire crop destroyed.

For 2010, Celetti said infected plants were again shipped to garden centres in the U.S. northeast. While none were shipped to Ontario, the airborne spores arrived later with the first infection reported in potatoes, July 26. Fortunately, a hot, dry August slowed its spread until the wet September came along.

“So what can we learn from the 2009 and 2010 seasons?” Celetti asks. “We again learned the importance of timely, frequent sprays.”

He adds that the need to register more fungicides is always present. There are few effective products for organic tomatoes, which he suspects might be part of the late blight problem.

For the future, Celetti says disease resistance is being bred in potato and tomato cultivars, although it could take some time for the tomato cultivars to be commercially available and accepted by the industry.

New fungicides are also in development. “They appear to be very efficacious against late blight,” says Celetti.

For the 2011 tomato growing season, Celetti offered some tips, such as spreading the 2010 crop debris from tomatoes and potatoes over the field and allowing it to freeze. Just in case persistent oospores did survive, do a deep field cultivation of the plant debris in the spring. Also in the spring, before planting, bury potato cull piles away from any fields that will be planted with potato or tomato crops.

“You should also select fields that are away from disease sources, such as the fields that were infected in 2010,” Celetti says.

Follow a three- or four-year crop rotation. Inspect tomato transplants and plant only disease-free seedlings.
Celetti would like fungicide spraying to begin before row closure, or when wet weather is predicted, or when fruit first appears on the vine.

“Maintain a fungicide spray schedule of five to seven days when cool and wet, or every 10 days when it is warm and dry,” he says.

Scouting during the season is important and should be done at least twice a week. If late blight is found in a small, isolated area, remove the diseased plants and place them in a plastic bag. Celetti would also like producers to stay in contact with OMAFRA for updates.

“We’re here to help keep Ontario tomato growers informed about the disease progress each season,” he says.

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