White mould in soybeans hits back

Vigilance is needed for a disease that’s still on the rise in many parts of Canada

Ask any grower about their soybean crop and the discussion might lean in a number of directions. It might turn to managing tough or resistant weed species in Eastern Canada, or the growing opportunities for the crop in Western Canada. If and when talk turns to diseases and pests, it’s more likely to concentrate on soybean cyst nematode (SCN) and aphids.

What about white mould? It’s one of the diseases that can have a huge impact on a field of soybeans, and once it has done its damage, a grower seldom if ever forgets the look of the disease and the losses that can result. The good news-bad news about white mould is that it still lags behind SCN as the largest yield robber in soybean production in Eastern Canada. Yet according to reports from the U.S., it is also on the rise.

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White mould occurs during flowering at temperatures in the mid- to upper-20 C range. Its impacts can range from 10 to 20 per cent yield loss in a moderate infection or as high as 10 to 20 bushels per acre in severe infections. If temperatures are above 28 C, however, the apothecia (the small, mushroom-like structures that grow on the soil surface) involved in sclerotinia stem rot cannot produce spores. Even if conditions turn wetter at flowering, higher temperatures prevent spore production.

Up until late August of 2018, white mould had not been a huge issue across much of Ontario and Quebec, mostly as a result of hot and dry conditions through most of July and early August. Temp­eratures above 28 C and a lack of rainfall kept white mould at bay, even in parts of eastern Ontario where the disease has become an almost perennial threat.

But during the second week of August, many parts of the province — east and west — began recording higher rainfall amounts, leaving some to question whether the eastern half of the country would escape severe white mould infection. Horst Bohner, soybean specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), concedes that with the heavier rains in mid-August, white mould became the topic of the day in many regions. He began getting more calls, particularly on how to manage it, and whether a second — and in some cases, a first — fungicide application is necessary so late in the season.

“There’s no question that white mould is much more prevalent in specific fields, and a large part of that has to do with crop rotation and disease history,” says Bohner. “But there are also regions that are hit more frequently and much harder with white mould and Eastern Ontario is one of those.”

More than one key

It’s a unique set of circumstances that exist to make Eastern Ontario the more prevalent location for white mould. The region tends to be a few degrees cooler than points in southern Ontario — sufficient to drop it into a more favourable temperature range for the development of the disease. Some years, Eastern Ontario will get more precipitation and in some locations, soil type also plays into the situation, particularly on soils with poor drainage. The history of their cropping practices can be another factor, particularly with peas a frequent addition to rotations of the past, since peas are also susceptible to white mould. During several decades, that might have led to a buildup of inoculum.

Put all of those factors together and white mould can be a continuous factor for growers in that part of the province. When Bohner tours through Eastern Ontario, talk invariably turns to white mould, regardless of any of the other topics he might be there to discuss.

For Albert Tenuta, the largest influences in the severity of white mould are moisture (relative humidity) and temperature. Calling it a “chronic, sporadic disease,” there are some years when it might only rank in the top five among soybean diseases; in severe years, it might jump to No. 2 or No. 3. Like Bohner, Tenuta reminds growers that a dry, hot summer tends to lessen white mould’s impact on soybean production.

“A season like this year where we had continuously 27, 28 or 30-plus degree temperatures (and higher) is not ideal for white mould infection,” says Tenuta, field crops pathologist for OMAFRA. “Overall, based on what we’ve seen in terms of environmental conditions (to the middle of August), the risk would be in a lower situation than a couple of years ago when it was cooler for most of the season and you had humidity and rainfall throughout.”

That said, 2018’s late-season precipitation patterns sparked hot spots in southern Ontario as well as the east. From Kincardine to Orangeville and through Shelburne, there was more consistent rainfall in early August, which may provide the optimum environment for white mould to flourish. It’s in that same strip that Tenuta is working with Dr. Dan Smith from the University of Wisconsin at Madison on a spore prediction model, based on the development of the disease’s apothecia.

The university’s Sporecaster app is available for download from the Apple Store or from Google Play. Keep in mind, the app only indicates whether conditions are right for the development of apothecia that produce the spores that cause white mould. It will not confirm whether the disease is going to develop or whether that field is at a higher risk, based on available data. It’s more targeting the development of apothecia and spores, and the risk for initial infection.

“The key with that is that it can help us with our fungicide timing,” says Tenuta. “That has always been one of the concerns around white mould… how to time a fungicide application to the best timing for a foliar fungicide application, and also which fields would be best suited for that application — which one is the highest risk.”

The 2018 growing season is the first year it’s been available, although it’s been under testing for the past two years.

Other factors involved in white mould’s infection include the history of the disease in a particular field. Echoing Bohner’s statements, Tenuta notes that a field with a long-standing incidence of white mould increases its susceptibility to infection. That can also have an impact in varietal selection, and Tenuta reminds growers there are no varieties that are resistant to white mould — although there are some that exhibit an innate tolerance to the disease.

Agronomics

Row spacing and plant populations are additional management considerations that Chris Olbach has noticed in various fields. As an area agronomist with Corteva Agriscience (the agriculture division of DowDuPont), Olbach’s experience with white mould has been something of a baptism-by-fire. Raised on the family farm in southern Ontario just outside of St. Marys, Olbach has learned more about the disease thanks to his new position covering Eastern Ontario.

“A lot of growers won’t grow a soybean if it doesn’t have a good white mould tolerance,” he says, referring to many farmers in that region. “We look for soybean varieties that have that as part of the package, more so than what I saw growing up in St. Marys, which is in the same maturity range.”

For the most part in 2018, Olbach hasn’t seen white mould to the same extent as years past, including fields where the soybean plants are chest-high. He’s been working with growers and other dealers, adjusting row spacing and plant populations, especially on some of the higher-yielding areas of certain fields.

“A lot of the growers have become accustomed to it and they’re actively out there trying to pick a variety that’s the best for white mould, and they’re trying to manage it,” says Olbach. “It gets back to the question of high-yielding soybeans, with a lot of growers who are trying to protect that yield — so they’re looking at 30-inch rows and lowering populations.”

Anything else?

As much as environmental conditions (moisture and temperature) and agronomics (row-width and populations) can lessen the impact and spread of white mould, Bohner notes that tillage plays a role in the severity of the disease too.

“No till has much less white mould than conventionally tilled soybeans — and that’s just a fact,” he says. “In side-by-sides some years, you can see it right down the line where it was tilled and not tilled.”

In terms of fungicide applications, Bohner is quick to point out that growers have more and better options when it comes to protecting their soybean fields. Farmers were powerless to do much against white mould 15 or 20 years ago, essentially surrendering to whatever level of infection arose and corresponding yield losses. Today, there are newer foliar fungicide products on the market that are better on the disease. There’s also a better understanding of when to spray, how to spray and maximize coverage, and the levels of suppression that are feasible.

In terms of using fungicides, a grower should be looking for higher humidity levels with cooler temperatures during that flowering period — and with a susceptible variety — then spraying is recommended.

“You should be making one fungicide application for sure, and twice if white mould is your primary concern,” says Bohner. “I’m a big fan of protecting as many flowers as possible, so if you’re spraying twice, put the first one on relatively early in the reproductive phase and then the second one on 10 days to two weeks later.”

This article was originally published in the October 2018 issue of the Soybean Guide.

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