CARMAN, Man. — Bent over at the waist and gazing intently toward the ground, Vikram Bisht used his hands and arms to spread apart a dense canopy of plants as he searched for signs of early blight in a potato field in south-central Manitoba.
Fortunately for the owner of the field, but unfortunately for Bisht, he found no examples of disease on the potato leaves.
“This field is clean,” muttered Bisht, a plant pathologist who works for Manitoba Agriculture in Carman.
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Bisht was hoping to find and show off a leaf with early blight because the pathologist wants to educate the public and home gardeners on early blight and late blight.
In 2010, several Manitobans inadv e r t e nt l y bought tomato plants infected with late blight. The infected plants wound up in home gardens across Manitoba, which produced sufficient late blight innoculum in the local environment to infect other gardens and commercial potato fields.
“Last year, I suppose, because the infected tomatoes were distributed all over, you had a big disease spread,” said Bisht. Before joining Manitoba Agriculture last year he worked as a plant pathologist in Maine and China, where he specialized in potato diseases.
Brian Hunt, Manitoba Agriculture greenhouse and alternative crop specialist, explained that local greenhouses distributed the infected tomato plants to retailers in 2010.
“A lot of the greenhouses germinate their own seed and grow them out. Those are usually the safest,” he said. “The ones that we have the most concern about are the (seedlings) that are brought into the province from other jurisdictions…. They (greenhouses) get the small seedling plugs, then they grow them out in pods and deliver them to retail.”
The infected greenhouse tomatoes, combined with other sources of innoculum and wet growing conditions, led to late blight detections in multiple commercial potato fields in 2010.
To prevent a repeat of last year, Bisht initiated a campaign to educate the public, greenhouses and employees at big box stores on the signs of the disease.
The pathologist and other provincial employees met with a number of greenhouse operators in the province and also spoke with managers at Rona, Walmart, Home Depot and other stores with garden centres.
“The message has gotten out to many, many people…. But we (pathologists) took many years of training to recognize (disease), so it is difficult for the home gardener who sees (late blight) once in 10 years.”
Out of the potato field and back in his office in Carman, Bisht pulled leaf samples out of Ziploc bags and pointed at photographs to illustrate the difference between early blight and late blight.
Despite its name, early blight usually develops later in the growing season. The first symptoms are small spots on leaves. As they grow, the dry brown or black spots will rarely cross leaf veins, giving the spots an angular or oval shape.
One of the distinguishing features of early blight are concentric rings in the spot that look like tree rings.
However, if a leaf on the bottom of a tomato or potato plant appears rotten or black it’s usually because the leaf has been in contact with the wet ground, not because the plant is infected with blight, Bisht noted.
With late blight, which is much more destructive than early blight because the disease can destroy an entire field of potatoes, the lesion may cross a vein in the leaf and the spot may be damp.
When the humidity is high the disease will produce white, mildewy spores, which appear on the underside of leaves.
“Especially early in the morning you would see some of the sporulation…. That’s one of the key symptoms,” Bisht said.
If a gardener suspects late blight, Bisht said the grower should send a photo to a plant pathologist or a local ag representative. If confirmed, the infected plants shouldn’t be dumped into the garbage or thrown onto the compost pile, because the fungus will continue generating spores.
If the wind and weather conditions are right, Bisht said, late blight spores can travel 10 kilometres, 20 km or more, spreading the disease to commercial growers and home gardens.
The best way to dispose of infected plants, Bisht said, is to double bag the plants and leave them out in the sun, so the heat kills the infected material.
Ways to manage late blight
- Buy certified seed potatoes
- Select tomato seedlings that appear healthy for the home garden
- Monitor tomato and potato plants in the garden
- Apply fungicide spray if late blight is present
- Remove and destroy plants that show symptoms of the disease
(Source: Manitoba Agriculture)
