Blackleg and Clubroot in Canola

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Field with heavy clubroot incidence (note reduced flowering in the foreground). (Photo courtesy S.E. Strelkov, UofA)

Blackleg and clubroot are both serious diseases that are growing in severity across Alberta, but with proper and diligent management by all farmers, they can be effectively controlled.

Blackleg

Blackleg is a fungal canker or dry rot that results in stem girdling and lodging. The disease has been present in canola fields since the 1980s.

Today, the availability and use of canola cultivars with resistance to blackleg has helped to avoid significant damage, notes Michael Harding, a research scientist with Alberta Agriculture and Forestry (AAF). However, it is still very common to see blackleg in canola crops.

When blackleg-infected seed is sown, the seedlings that emerge may be infected with lesions on the seedling leaves or stems.
(Photo courtesy Michael Harding, AAF)

Harding and his colleagues have undertaken recent surveys for blackleg (and stem rot) on Alberta canola. In 2016, they found that of 480 canola fields, 432 of them had blackleg symptoms. Indeed, Harding states “the prevalence of blackleg in Alberta has been measured at 55 to 99 per cent in the six surveys conducted over the past eight years. Prevalence was slightly lower in 2017 compared with 2016, as it was a relatively dry year in comparison.”

Long-term survey trends show the pathogen to be present throughout the province, and Harding does not believe any area or farm should consider itself “blackleg free.” Some fields experience little to no loss due to blackleg while others may have significant disease pressure, and he says economic loss experienced by individual farms depends on their location in the province, local weather and field history, as well as cropping and disease management practices.

“Blackleg is always a risk for canola producers and blackleg management practices should be proactive,” Harding says. “Crop rotation (one host crop every four years) is a very effective way to keep disease pressure from building. The pathogen does not survive in soil without a host. So, once the canola residues are decomposed, there is little to no risk of economically-damaging blackleg pressure originating within that field.”

Harding also notes that genetic resistance in the MR- and R-rated canola cultivars is keeping disease severity very low in most fields, as was seen in the survey data. However, Ralph Lange Team Lead Crop Pathology and Molecular Biology at InnoTech Alberta, notes there are now yearly cases of severe loss in cultivars labelled “resistant,” a significant change from the 1990s and 2000s that indicates the pathogen is adapting.

Lange says there are about eight different blackleg strains in Western Canada, and in Alberta, about 80 per cent of all isolates belong to just three strains.

“We continue to have good resistance genes available, and what’s changed is that we now need to actively manage the crop resistance genes we present to blackleg fungus populations,” he explains. “So, frequent and accurate scouting with excellent record keeping is essential for determining if the genes we’re presenting are working or not. Then, producers need to eliminate the non-functioning resistance genes when selecting which canola cultivar to plant (at least one functioning resistance gene).” This is now much easier, Lange notes, because seed companies are starting to reveal which genes are in which cultivar.

Another tool for blackleg management is fungicides. Harding notes while all certified canola seed is cleaned and treated to make it essentially blackleg-free (although infection can still occur due to spores being released from infected stubble), in high-risk situations during the growing season, foliar fungicides may be applied at the one-to-three leaf stage.

Going forward, Harding says the risk of resistance-building in the pathogen is very real when crop rotation recommendations are ignored, especially in wetter years when blackleg has a better chance to infect and cause disease.

“If genetic resistance were to erode due to selection of virulent pathotypes of the fungus, it would have a devastating impact in areas where genetic resistance was no longer effective,” he notes. “While we are not currently seeing widespread changes in blackleg severity, it has been seen in some individual fields. This is a warning sign that we need to think carefully about crop rotation practices and resistance stewardship in order to stay ahead of blackleg.”

Clubroot

In canola, this soil-borne fungus-like disease causes swellings to form on the roots, ultimately stunting the plant and even causing premature plant death. Infection and severity are supported by warm, moist, acidic soil.

University of Alberta scientists and staff from Alberta Agriculture and Forestry currently conduct yearly clubroot surveys, which began in 2003 when clubroot was first identified in the province. The 2016 survey found 289 new clubroot-infested fields and the 2017 survey another 301.

“What we’ve found is that clubroot is spreading fairly rapidly for a soil-borne plant pathogen, and this seems to be due mainly to the movement of infested soil and machinery,” explains Stephen Strelkov, professor in the faculty of Agricultural, Life and Environment Sciences at the University of Alberta. “We’ve also found significant numbers of spores in wind-blown dust from infested fields which could contribute to local spread.”

There is a continued spread eastward, he adds, with several new infestations recently found near the Saskatchewan border.

“Part of why it often takes a few years for growers to ‘up their game’ when dealing with clubroot is because the impact on yield is often very slight,” notes Dan Orchard, agronomy specialist with the Canola Council of Canada. “It’s almost always found in a patch at the field entrance, and the overall field yield isn’t really affected. But if not managed, that patch will become much, much larger and potentially cause total loss of the entire crop.”

Severe clubroot pulled from infested soil (note wilting of plants in the background).
(Photo courtesy S.E. Strelkov, UofA)

At least 12 new strains of clubroot have been identified in Alberta since 2013, and they are all capable of overcoming the resistance in many clubroot-resistant canola varieties.

“In 2016, these strains were confirmed in over 60 fields in Alberta, and in 2017, we identified another 42 fields with potential resistance issues,” Strelkov notes. “These new strains have likely emerged as a result of cropping of clubroot-resistant canola in short rotation in fields with moderate to severe clubroot infestations.”

Orchard notes while best management strategies make a big difference, they are difficult to deploy. “This would include equipment sanitation, which growers have suggested could be hours and hours per piece of equipment for each field,” he says. “Not cleaning equipment is a risk growers seem to be accepting, although I believe many or most of them make sure equipment from unknown regions or potential clubroot regions is clean before entering their lands, which is a great practice to follow.”

He adds there is evidence around the world and preliminary evidence in Alberta suggesting pH plays a major role in clubroot spread and severity.

“Liming fields could reduce clubroot impact, but it’s another excellent management strategy that’s easier said than done,” Orchard says. “I’m convinced, however, that over the next few years and with the help of new technology, the industry will produce better lime recommendations, better pH mapping, better application techniques, and just a better understanding of lime and the benefits/challenges.”

While he believes genetic resistance is currently the most significant factor in keeping this disease at bay, the fact that new clubroot strains are quickly appearing means growers need to deploy a multi-pronged approach.

“The recipe for success would seem to be liming badly-infested patches and seeding them to a perennial grass until spore loads are manageable, coupled with planting resistant canola varieties and rotating sources of resistance on top of crop rotation.”

Strelkov agrees that with the new strains appearing, it’s unwise to use resistant canola varieties as a sole management strategy. He stresses longer rotations are important, and adds while “sanitation often is not viewed as practical, even steps such as trying to remove large chunks of soil from machinery or working infested fields last can be helpful.”

Planning for Fusarium in 2018

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Fusarium head blight. (Photo courtesy AAF)

It will be easy for growers to let their guard down this year when it comes to managing for Fusarium head blight (FHB). The bone-dry summer in 2017 meant there was little issue with FHB or any other disease.

However, going into the 2018 season, growers need to stay vigilant against the yield- and quality-robbing cereal disease.

“There’s little one can do at this point in time to eliminate the risk for Fusarium if the pathogen is present,” says Brian Beres, research scientist with Agriculture and Agri-Food Canada in Lethbridge. “We know the inoculum is there, and while you can’t control the amount of pathogen in the soil this year, nor can you predict the weather, you can control your agronomics and give your crop a fighting chance against the disease.”

Fusarium graminearum is the most common cause of FHB in Alberta, and is particularly well established in the southern areas of the province. It affects yield and grade as it produces the mycotoxin deoxynivalenol (DON), which makes the grain a poor grade for the feed, malting milling and biofuels industries. High humidity and warm weather in late June and July favour spore development on infected residue, and those spores can spread through the wind or through moisture splashed onto the plant.

“After what appears to have been a year of low Fusarium head blight damage, we hope it is not out of sight, out of mind,” says Michael Harding, research scientist, plant pathology with Alberta Agriculture and Forestry’s Crop Diversification Centre South in Brooks. “The inoculum overwinters in the soil and on residue, and if we have a wetter season this summer, it could still be a very bad year for FHB.”

Going into the growing season, the first thing growers can do to manage for Fusarium is carefully choose their variety based on their local risk. While no cereal varieties are truly resistant to Fusarium, some tolerate the disease better than others. Durum varieties can be the most difficult to choose as they are all susceptible to moderately susceptible to the disease.

Once a variety is chosen, Beres says uniformity is the main theme to allow a crop to battle FHB. A uniform crop will flower at the same time with a shortened flowering duration, ideally before Fusarium spores begin invading the crop. To achieve uniformity, growers need to turn up their seeding rate and then seed early.

“Two beneficial things happen with a higher seeding rate,” says Beres. “First, you achieve uniform growth stages throughout the field; and second, you have increased main stem production and fewer tillers, both which makes foliar fungicide timing easier and improves fungicide efficacy greatly.”

The ideal winter wheat seeding rate of 450 seeds/sq. metre has been established through previous research Beres and his team conducted. Combined with timely planting further enables winter wheat the opportunity to “escape” FHB infection as it can complete flowering ahead of the onset of spore inoculation. For spring wheat, research was also conducted to gauge what response growers get from a high seeding rate.

“Traditionally, spring cereal growers have seeded around 200 to 250 seeds per square metre. However, we’ve observed spring wheat such as durum and high-yielding CWRS varieties respond to rates as high as 450 seeds per square metre,” says Beres. “While the yield response may stop at 300 to 350 seeds per square metre in some environments and with some varieties, the Fusarium risk may be lower because you have a much more uniform crop that also happens to be much more competitive with weeds.”

Once the resistance package is established and the decision is made to seed at a higher rate, growers should consider lodging risk. A crop seeded at a high rate is at greater risk for lodging later in the season if lodging resistance is overlooked when choosing a variety. However, taller wheat can also be a benefit when battling FHB in low to moderate disease severity with low spore dispersal, as the disease spores that splash up from the ground during wet periods may not reach the (head) vulnerable areas of the spike.

Before the seed is even placed in the ground, cereal growers should consider a seed treatment.

“Seed treatments won’t directly tackle FHB, which happens later in the season,” says Harding. “But it will battle seed- and soil-borne pathogens as well as abiotic factors which could weaken your crop and its stand establishment. You don’t know what’s in the soil. Treated seed is exposed to less stress early in the season so it has a better chance for success after emergence.”

While you want to have your field come into maturity at the same time, high risk FHB areas should consider seeding different fields at different times. “You really don’t want to plant all of your wheat during the same week,” says Harding. “If they are all coming into anthesis at the same time, your entire season is at risk. But if fields mature at different times, you are spreading out the risk of a total loss.”

Rotation is another way to reduce the disease pathogen in the soil, but given Alberta’s short growing season and cropping options, effective rotations can be challenging.

“Ideally you would only have one crop in four years that is a host for Fusarium,” says Beres. “But that’s impractical as many of the crops that comprise today’s cropping systems are hosts for the disease. Even if the field is rested in chemical fallow, the undisturbed stubble, tissue and plant roots can be colonized with Fusarium.”

Harding says even though ideal rotations are difficult, inoculum levels can be reduced by managing that rotation. “Durum and corn are the best hosts for the disease, so if you are growing corn, which is likely to have more Fusarium residue, don’t grow durum right away. Allow the residue to break down on a less susceptible crop. You should work out your field selection in the fall and winter so that you can minimize the buildup of the pathogen.”

If the plant is more uniform and flowering more even, the timing window to apply a fungicide is also easier. More heads will come out of the boot at the same time, and the more heads available to the fungicide, the better its efficacy. Recent research shows that waiting until as many heads are visible will reduce Fusarium and DON at harvest. A fungicide will remain effective four to six days after flowering, or seven to 10 days following head emergence.

You also have to take caution as to how you apply the fungicide,” says Beres. “You need to consider ground speed, keep your boom levels low around 30 centimetres above the canopy for better coverage, and make sure your nozzle is configured so that the angle and pattern covers as much of the plant as possible utilizing a coarse spray pattern. You’ll get as much out of an input that you put into how you apply it.”

Harvest management can be tricky once Fusarium has infected a field. Many growers will increase the fan speed on their combine and blow the lighter, infected seed out the back end. However, then the stubble has been seeded with infected material.

“The opinions around the utility of tillage for FHB management in the science community are somewhat mixed,” says Beres. “However, studies conducted on the Canadian Prairies show no increase in FHB under no-till, and there is only agreement that F. avenaceum has been controlled with tillage, not F. graminearum.”

Optimizing Fungicide Applications on Wheat and Barley

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Septoria (speckled leaf blotch) of barley.

Leaf spot fungal diseases can decrease cereal yield by up to 20 per cent or more, as well as diminish kernel weight, and in some cases reduce grade. This, according to Neil Whatley, crop specialist, Alberta Ag Info Centre.

“When disease risk levels are moderate to high, protection of the upper two leaves of a developing wheat plant with a timely in-crop fungicide application prevents significant losses. With barley, it’s essential to protect the upper three leaves,” he says.

Disease risk level increases when weather conditions are favourable for disease development, cereal crops are frequently grown in the same field, the chosen cereal variety is susceptible to leaf diseases, yield potential is good and crop price is high.

Tan spot and septoria leaf blotch are the most common leaf spot fungal diseases in wheat, says Whatley.

“While scald and net blotch most commonly affect barley, fungal diseases like spot blotch affect all cereal crops. These leaf spot pathogens survive on infected straw residue and stubble from cereal crops grown during the previous two to three years in a particular field. A prolonged period of rainfall, fog or heavy dew combined with moderate air temperatures in June and July raises the risk that these pathogens will produce spores and re-infect young, developing cereal plants.”

Although leaf spot diseases can be present during early plant growth, scouting for cereal leaf diseases is especially important prior to, during, and after flag leaf emergence. “The appearance of moderate levels of disease in the lower canopy indicates there is a risk to the upper canopy leaves,” says Whatley. “Under conditions favourable for disease development, leaf spot disease symptoms appear as tiny water soaked or brown or tan spots or lesions on the leaf surfaces of seedlings and tillering cereal plants. Lesions produce spores that act as the disease transfer mechanism. Under prolonged humid weather conditions, the spots become more visible as they expand and blend together. If the weather turns dry in June and July, risk level diminishes as the leaf spot pathogens remain confined to the lower leaves, causing little overall harm. However, as upper leaves emerge on the developing cereal plants, wind or the splashing motion of rain drops transfer spores from the lower canopy, and possibly from old infested crop residue, to the flag and penultimate (the leaf just below the flag) leaves, increasing the risk that key leaves for yield and grain filling may be compromised. A decision must then be made whether or not to protect the upper leaves with a foliar fungicide application.”

Spores that splash or are blown by wind onto the upper leaves will germinate under favourable conditions allowing the pathogens to infect leaf tissues and cause necrotic lesions on leaf surfaces, potentially resulting in significant loss of green leaf area.

“Preventing the loss of green leaf area on the flag and penultimate leaves is the main concern as optimal sunshine on these leaves contributes to over 50 per cent of the cereal crop’s eventual yield. The goal is to apply a foliar fungicide when the flag leaf has just fully emerged if disease risk level is moderate to high,” says Whatley. “As a rule of thumb, 20 per cent per cent disease coverage of the area of the flag leaf results in a 10 per cent yield decrease, so preventing this amount of disease is usually desirable if yield potential is good and grain prices are fair to high. If a fungicide application is made and weather conditions favourable for disease development persist, a second fungicide application may be necessary just after head emergence, and especially where Fusarium head blight is also a concern.”

Spraying too early or spraying too late results in poor disease control. “While the idea of mixing a half rate of fungicide with a late herbicide application may seem convenient, given that the flag and penultimate leaves are not fully formed, this crop management practice doesn’t directly protect the upper canopy leaves and is generally not economical for leaf spot diseases like tan spot, septoria, scald, net blotch and spot blotch,” notes Whatley. “In contrast, if stripe rust is observed early in the growing season, a fungicide application may be needed before flag leaf emergence and then again later at head emergence if risk is high and the variety is susceptible. It’s preferable to prevent leaf spot diseases with a full rate of fungicide at the flag leaf stage or just after head emergence. However, spraying too late, after the disease is well established on the flag and penultimate leaves, is also not economical because significant green leaf area has already been lost and the fungicide doesn’t cure infected leaf tissue, but instead protects healthy leaf tissue.”

Whether fungicide choice is a systemic or a contact mode of action, the fungicide should be applied directly to the leaves that are important for grain filling, i.e. the flag and penultimate leaves.

“Translocation movement of a systemic fungicide is limited to within an individual leaf and does not occur between leaves,” says Whatley. “Leaf spot pathogens can become resistant to a specific fungicide, so if disease pressure is high enough to spray more than once in the same field during the growing season, or during successive years in the same field, rotate fungicide modes of action. Adequate water volumes ensure optimal leaf coverage and, therefore, optimal disease control, so ten gallons of water per acre is generally applied with the fungicide.”