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Fall field scouting can highlight diseases

Verticillium and Goss’s wilt are both easily spotted near or after harvest

Field scouting doesn’t stop with the combine, but it does become more specific, according to Dr. Vikram Bisht, pathologist with Manitoba Agriculture.

“Usually, to scout for soil-borne pathogens is not an easy thing because you have to do a lot of laboratory work, but if you have the pathogens which survive in the crop residue or are easily survivable in the soil, those are the ones that we need to look at,” he said.

Goss’s wilt is one of those pathogens in corn, according to provincial field crop pathologist, Holly Derksen. The bacterial infection is controlled by fungicide and overwinters in the crop’s abundant residue. The wilt commonly appears on field edges and causes shiny grey or tan leaf discolouration and lesions. Lesion edges may appear water soaked or freckled.

Growers may notice prematurely ripe patches, one of the disease’s common signs, although Derksen noted that it might be difficult to tell Goss’s wilt from natural ripening.

Since 2009, the wilt has spread to all corn-growing areas in Manitoba.

Verticillium

Farmers may also want to take a walk through their canola stubble, according to Derksen. Verticillium wilt in canola, a relatively new pathogen, may be misdiagnosed early in the season and may actually be easier to spot in fall, she said.

“That’s one thing we often do see shows up later,” she said. “It can be hard to differentiate from other issues in canola until the crop is actually drying down or when you look at the stubble on the field.”

Producers are often told to look for yellow or brown vertical stripes on the stem when scouting for the soil-borne disease. When infected plants mature, however, stems will peel, revealing black pepper-like spots in the stem’s inner layers. Those spots will continue to develop in stubble after harvest, according to Manitoba Agriculture’s Verticillium In Canola fact sheet, while infected plants will turn grey black after being cut.

The disease should not be confused with blackleg, which may show similar black speckles on the surface, rather than within the stem, Derksen said.

With no registered fungicides or resistant canola varieties, there are few control options for farmers fighting verticillium. Like diseases such as clubroot, spores can survive up to 10-15 years, although viable fungi will drop without host plants. The province recommends longer rotations and biosecurity, including equipment sanitation and controlled traffic, to stop the spread and limit impact. Finding the disease early can prevent its spread.

Manitoba had North America’s first case of verticillium in canola, identified in 2014. It has since spread through the province, although the frequency of infection is unknown, Derksen said.

Another verticillium species is endemic in potatoes, Bisht added.

Like its canola counterpart, the fungus will manifest late season in potatoes and, once in the soil, will be a long-term fight for producers.

Damage, however, will depend largely on crop stress, according to Bisht.

“The problem becomes more serious if the crop is facing water shortage or there’s too much heat and, in same cases, (if) the salinity is high, but the pathogen is a weak pathogen on potatoes,” he said. “If the crop is growing really well, it does not show up as much. If there’s stress on the plants, then it shows up and it is a big problem.”

Unlike verticillium in canola, the pathogen will cause early plant death and smaller potatoes, but no other outside sign. Inside the stem, infected plants will brown in the root zone, Bisht said.

Clubroot

Clubroot, another soil-based concern for canola, may also be scouted in fall, and Derksen recommends using a trowel to find it.

“As the crop ripens, the galls actually begin to decay — so pulling up the plant, you might actually break off the galls and not actually see them, whereas if you dig them up with a trowel, you might be able to keep them more intact and then you’ll have a better idea as to what you’re dealing with,” she said.

Producers are most likely to notice stunted patches that may indicate clubroot during harvest, Derksen said, particularly in the less travelled field centre.

Identifying clubroot early is critical to stopping its spread. The pathogen has become infamous for its ability to jump fields, often transported by contaminated soil on farm equipment.

The Canola Council of Canada says there is no economical way to remove the disease once it is established and spores may survive for decades without a host crop. Even if canola is never grown on that field again, farmers will be forced to adopt heightened, time-eating biosecurity in order to avoid cross-contaminating uninfected fields.

Eight fields in five municipalities have shown physical clubroot symptoms in Manitoba and 270 have tested positive for spores, although Manitoba Agriculture notes that spore counts generally need to break 100,000 spores per gram of soil before symptoms appear.

Other pests

Fall scouting prior to harvest can also cover sclerotinia stem rot, fusarium head or stalk blight in corn or white mould in pulses, canola or sunflower, Bisht said.

Meanwhile, potato producers should pay attention to their cull piles.

“A lot of people will throw away their rotten potatoes and put them in a big pile, cull piles they call it,” Bisht said. “That is another place you would want to scout because, next year, close to those cull piles — if they are not removed — would be a good source for the leaf blight if the potatoes were infected or it would be a source of soft rot disease.”

The province advises farmers to draw a disease map marking general area and severity of any infection about a week before harvest. Those maps can outline problem spots the next time that crop is grown in that field, Manitoba Agriculture says.

About the author

Reporter

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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