How home gardeners can help protect Ontario’s commercial potato crops

By Sarah B. Hood - Published on May 17, 2016
Late blight on a tomato leaf. (Michael Celetti/Ontario Ministry of Agriculture, Food and Rural Affairs)



In last year’s Oscar-nominated film The Martian, stranded astronaut Mark Watney (Matt Damon) saved himself from certain starvation by turning a few potatoes into months’ worth of food; he knew that he could grow multiple plants by cutting up a single spud and planting the pieces. Luckily for him, all his seed potatoes were healthy ones. If any had been carrying blight, his whole crop would have been wiped out.

Here on earth, blight is a constant threat, and potato farmers must be vigilant to protect their crops against infection. Potatoes are Ontario’s largest fresh vegetable crop in terms of acres planted, and the annual harvest is valued at about $100 million. Ontarians eat, on average, 140 pounds – or 63 kilograms – of potatoes every year, making them an important staple food. Yet every year, enthusiastic amateur gardeners unknowingly help spread blight that can destroy unprotected potato fields within a few days.

The fungus known as Phytophthora infestans, which causes potato blight, appears regularly in Ontario and surrounding regions. It is the same disease that caused the Irish potato famine in the mid-1840s. Devastating the chief food source of the nation, it forced hundreds of thousands of people to flee Ireland to escape starvation. In 1847, some 38,000 Irish famine refugees arrived in Toronto, which at the time only had a population of about 20,000.

“It’s pretty devastating. We still battle late blight 170 years later,” says Michael Celetti, plant pathologist and horticulture crops program lead with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). “Fortunately, we have tools and knowledge to keep it somewhat at bay: that may include good crop rotation, sanitation and fungicides that work. We call that an integrated pest management system.”

OMAFRA keeps track of outbreaks in Ontario and neighbouring American states, and warns farmers when and where blight has been spotted so they can take measures to protect their crops. The ministry’s vegetable gardening information website recommends that home gardeners avoid growing potatoes entirely.

“Even though we call it ‘late blight’, it isn’t always that late, and the earlier it hits a region, the more devastating it is,” Celetti says. “When it comes in early, then growers — if they’re serious about harvesting a crop — will be using more pesticides and spraying them more frequently.” Besides the undesirability of using additional pesticides on a food crop, farmers always prefer to avoid the considerable financial cost of extra spraying.

But Phytophthora infestans is a crafty pathogen that’s very good at multiplying itself and moving to new locations. So good, in fact, that it has two different ways of reproducing: by cloning and by mating. “It’s not really a true fungus; we call it a water mould. It produces a spore that swims in water,” says Celetti. “That spore is looking for a tuber or something to infect.”

During spells of rainy weather, spores spill out of spore sacs and swim out onto wet leaves. From there, they’re well positioned to spread throughout a field, or worse: a passing thunderstorm can lift up blight spores and carry them as far as 200 kilometres from their original location to rain down on crops below. This is how even a hobby gardener in the middle of a city can spread blight to a remote potato field.

“When we plant potatoes, we call it a seed piece. If that seed piece has late blight on it, and you plant it, and it’s warm and moist, it will produce a spore sac. The challenge we see with home gardeners is that they may carry over last year’s tuber from their garden, not knowing it has late blight on it,” Celetti says.

Phytophthora infestans also infects tomatoes. A few years ago, a serious outbreak started with one greenhouse grower that was supplying seedlings for big box stores and garden centres.

“These seedlings got distributed through a particular box store throughout eastern North America, so they had a big epidemic that they could trace back to those seedlings. Now greenhouse growers are a little more diligent,” he says. “That’s why we want the home grower to really focus on growing certified seed and even inspecting their tomatoes when they grow them.”

Home gardens can also contract late blight from other sources.

“Late blight can hop fences, so if your neighbour has it, you’re going to get it too. If you get two or three days of wet weather like we did last year, if you’ve got the host and the pathogen, you’re going to get it,” Celetti says. A quick Google search will produce numerous illustrations of the telltale brown spots that appear on infected stems, leaves and fruit.

To protect their vegetable plots this summer, home gardeners might consider a blight-resistant tomato like Mountain Magic, space plants well apart, and avoid getting water on the leaves. Once infected, the plants cannot be saved, and will likely be dead within the week. Celetti advises cutting down the entire plant and burying it somewhere that will not be disturbed for several years to prevent the spores from spreading. Composting will not kill the spores, and in a setting such as a community garden may actually present the possibility for the pathogen to find a suitable mate, setting the stage for even more persistent strains of Phytophthora.

“The message should be that if you’re a homeowner, you need to buy certified [potato] seed from your local garden centre, and don’t plant table stock [eating potatoes],” Celetti says. Following this one rule can help keep blight out of Ontario’s commercial potato fields, and pesticides off our plates.

Sarah B. Hood is a freelance writer and author of We Sure Can! – a book on how jams and pickles are affecting the local food movement.

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