“If you’re looking at your field and thinking, what the heck’s going on and you know it’s not fertility, it’s not moisture or topography, but you’re seeing kind of dead patches late in the season. Chances are it’s probably PED.” said Griffin Bailey, Little Potato Company’s regional agronomist for Eastern Canada.

WHY IT MATTERS: PED causes tuber size reduction, resulting in yield losses of 10 to 15 per cent under moderate pressure and 30 to 50 per cent under severe pressure.

Verticillium alone causes leaves to curl and change colour, leading to plant decline. But when the nematode is also present, the problem worsens significantly, Bailey told Ontario Potato Conference attendees in Guelph on March 5.

“The verticillium has to work pretty hard to infect the root on its own. If the nematode is there, feeding and creating an open wound, it’s kind of like opening the door for the verticillium,” he explained. “And infection happens a lot faster, a lot more readily, and the damage can be a lot more severe.”

Infection starts at the root and moves up, with symptoms appearing after flowering, causing the lower leaves to yellow, brown and die. A key indicator is that one side of the leaf, or the whole plant, wilts and browns while the other half remains green and healthy. The plant can also appear to wilt during the day and recover at night, or show flagging, where the plant remains upright despite being dead.

Non-visual symptoms that occur beforehand include reduced photosynthesis and transpiration, and increased leaf surface temperature.

In 2021, Griffin launched a study of Ontario’s potato-producing land, including 19 commercial fields in long-term potato production, to quantify root lesion nematode populations and verticillium soil inoculum pressure by using soil samples. Additionally, a grower questionnaire examined historical management practices — main crops grown, cover crop use, organic amendments, and pesticide use — to assess potential impact.

‘We do need alternatives’

Fortunately, Ontario’s nematode populations aren’t very high; however, several fields are at high risk of PED due to verticillium populations.

Bailey explained the threshold for nematodes to cause damage is over 1,000; if verticillium is present, that threshold drops to one.

In 2022, he focused on four fields due to field variability. He used small plots, about two rows of vines by 10 metres instead of 50 to 100 acres. Even in these small areas, he found significant variability within fields, despite their close proximity and similar management.

“It’s really, really hard to put your finger on what might be happening on your farm. You can sample all day long, every year, and maybe not totally understand the exact level that you have,” Bailey said.

Chemical fumigation for PED is expensive, highly regulated, and kills both good and bad pathogens, providing growers with a costly double-edged sword.

“It’s a good tool to have, certainly, but we do need alternatives,” he explained. “There’s a lot of good research on cover cropping, organic amenities and biochemical options.”

At the Simcoe Research Station in Norfolk County, Bailey ran several trial treatments over three years, testing biofumigants, along with potato trials using grower Chuck Emre’s biofumigant system:

• Continuous potato (CP) — no crop rotation, use of cover crops or fumigant intervention.
• Continuous fumigated potato (CPCF) — no crop rotation or cover crops, plus annual chemical fumigant intervention.
• Continuous potato with rye and oilseed radish (CPRR) — no crop rotation, use of PED-supressing cover crops, no fumigant intervention.
• Charles Emre mix with initial fumigation (CEMF) — Rotation with full-season cover crops (Caliente 199 mustard and arugula, followed by pearl millet), winter cover crops of fall rye and oilseed radish, and biofumigation in the second year, all following an initial chemical fumigation prior to treatment application.
• Charles Emre mix (CEM) — adaptation of CEMF without the initial chemical fumigation prior to the treatment application.
• Soil building with pearl millet (SBFM) — tested soil-building principles following a one-time chemical fumigation by rotating potato with a summer cover crop of pearl millet, use of winter cover crops and biofumigation in the second year.
• Soil building with hairy vetch (SBVF) — tested soil building principles following a one-time chemical fumigation by rotating potato with a summer cover crop of hairy vetch, use of winter cover crops and biofumigation in the second year.
• Corn rotation (CR) — conventional rotation, potato being rotated with field corn and use of winter cover crop.

Soil samples were collected each spring and fall, and each plot was irrigated and used the Superior potato variety.

Bailey reported in 2021, before the trials began, verticillium numbers were “pretty low” but jumped across all treatments after potatoes were planted.

He noted that verticillium inoculum was reduced and potato yields increased following the initial chloropicrin application; however, repeated applications provided no benefit.

“Our numbers kind of went up and went down as the season went on, but none of our treatments made any difference after that first year,” he said.

Given that it was virgin potato land, the first year of fumigation caused a “fertilizer effect,” with a yield boost, a higher proportion of larger potatoes, and a lower proportion of smaller potatoes. However, there was no discernible benefit from fumigation or cover crop on yield in the second year.

Improved soil health helps

In the third year, potato yields were slightly higher with the Charles Emre mix and soil-building methods, indicating that improving soil health worked as well as, or better than, other methods for managing PED and supporting higher yields.

Bailey joked that if someone wanted a difficult career, studying nematodes was a good choice, since root-lesion nematode populations were not influenced by the agronomic practices tested in the study.

“I will say that the (nematode) numbers are quite low, so maybe under severe population, you might see a little bit more of a difference,” he said.

The 2023 season was conducive to PED, with signs beginning in the first week of July and reaching severe pressure by the end of August.

“We had a pretty wet spring with a lot of moisture, and then a very hot, dry setting,” he explained. “Those are perfect conditions for PED.”

By the end of the trial, a significant difference in disease severity emerged, with continuous potato showing higher severity than the Charles Emre mix and soil-building treatments.

“Just rotating your crops, extending the rotation length, is a really good way to manage this disease in the long term,” Bailey said.

When testing the soil fungal-bacterial ratio (fungal DNA relative to bacterial DNA), Bailey found it was higher in crop-diverse treatments than in continuous potato treatments. Treatments with the highest disease severity had the lowest ratio, suggesting that soil health can directly improve crop resilience to stress and disease.

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Here’s what to watch out for as your winter cereals emerge from domancy.

WHY IT MATTERS: Getting winter wheat up and growing in the spring is critical to eventual yield.

Agronomists say winter wheat in what were snow-heavy parts of Ontario might face higher pressure from snow mould and excess moisture.

Snow mould in winter cereals can occur when a crop is covered in snow before the ground adequately freezes, says Joanna Follings, cereals specialist with the Ontario Ministry of Agriculture, Food and Agribusiness.

The snow provides insulation for fungal pathogens that can survive in soil and on residue, allowing them to grow on plants throughout the winter. After snow melts, damp and cool weather is conducive for continued growth of the pathogens, further damaging the plants.

“Growers might be noticing right now some patches that might look brown, or think they’re dead. In a very severe infection the growing point can be damaged and the plants can eventually die. However, the plants should grow out of it if the growing point isn’t affected,” says Follings, speaking March 11.

“Identify for snow mould and keep an eye on them. We need to see how these plants grow as temperatures warm to see how things are.”

Follings adds some growers have expressed concern about the return of cold temperatures after snowmelt. Her field observations thus far suggest growers don’t need to be overly concerned because warm-temperature periods have been limited.

“It’s not like it was 10 degrees (C) all winter, and suddenly we’re flipping back,” she says.

Wheat that’s cold and wet, however, is a concern as heavy rains have followed significant snowmelt. While sitting water is a danger, the moisture appears to be absorbing into soils and moving well, with field edges, headlands and other such areas forming the majority of acres with standing water.

“As we always say, they hate wet feet. Wheat can withstand it for a couple days. But if we’re looking at seven to 10 days, that’s when we start to see an impact or damage on some plants, and thinner stands. If growers have the ability to get that water away in some way, that’s good. The biggest thing is to be patient,” she says.

“We went through something similar last year where all the snow melted and waterlogged, and we fared fine. The biggest thing is to give the wheat a chance to green up and begin growing … One more thing we’re seeing more and more is the benefit to early-applied sulphur once wheat has greened up. If conditions allow, that will also help with robust growth and help those plants get going if they have been stressed.”

Jonathan Zettler, Wellington County agronomist and operator of Fieldwalker Agronomy Ltd., confirms ponding is likely to be his area’s biggest issue in early spring.

“There’s flooding in spots I haven’t seen in a while, and a lot of roads that have been closed in spots,” Zettler says. “Last year we had a fair amount of winterkill. It’s hard to know for sure but it was probably related to snow mould in triticale and certain winter wheat varieties.”

“The main one I’ve been battling lately is powdery mildew. If the wheat does survive between flooding and ponding, growers may want to scout for that.”

Challenges for barley trials

For Quentin Martin, operator of Cribit Seeds near Waterloo, excess water has raised concerns about the company’s Ontario Cereal Crops Committee winter barley trials.

“They were positioned to avoid spring water pooling,” Martin says, speaking March 13. “On Sunday I crawled up to take a photo and there was water right up to the plots. It stayed on Monday, but was gone on Tuesday. This indicated the frost wasn’t very deep, and the snow cover insulated against the cold. Then along the edge of the field is a cedar hedge, which produced quite a snowbank. At the beginning of the week that area looked dead, but today it seems to be greening up.”

A -10 C forecast for early in the third week of March, however, spurs Martin to add “we’re not out of the woods yet.”

Zettler also reiterates it’s still early to make definitive assessments, as the remaining snow disappeared less than a week ago.

“The water does seem to be leaving because the ground I don’t think was very frozen. Hopefully it can leave fast enough for the wheat to survive.”

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