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Pasmo, lodging and fungicides

Researchers are assessing the effects of fungicides on lodging, pasmo, crop maturity, seed yield, seed weight and test weight of flax. Photo by Randy Kutcher.

A few years ago, Cecil Vera, a biologist with Agriculture and Agri-Food Canada (AAFC) at Melfort, Sask., was running one of the trials in a major flax project. He happened to notice that, in one particular year, lodging was dramatically reduced in his plots where a fungicide had been applied. That intriguing observation led him to start studying the relationship between lodging and pasmo, and the role of fungicide applications in dealing with both problems.

Lodging and pasmo are both serious issues in flax. Vera’s research shows when lodging occurs, it causes seed yield reductions averaging 32 per cent in flax, compared to 16 per cent in wheat. Disease surveys show pasmo is the most common flax disease on the Prairies – in some years, pasmo has been found in 100 per cent of surveyed flax fields in Saskatchewan. Pasmo incidence (proportion of plants infected) and severity (degree of infection on affected plants) vary quite a bit from year to year and place to place, depending on weather. Field observations suggest typical yield reductions from severe infestations tend to be around 10 to 20 per cent or more.

Pasmo and lodging are sometimes seen in association with each other, but Vera says there is some debate around the exact relationship between the two problems. “Some scientists, particularly plant breeders, believe lodging may cause plants to become infected with pasmo as they fall closer to the ground, which is a possibility. However, I believe it’s the other way around [that pasmo promotes lodging].”

He adds, “It could be both ways. I see pasmo and lodging as complex phenomena, with many factors involved, and sometimes, when not all the factors are present, these two events may not take place or, if they do, their expression may be less evident.”

Pasmo is caused by the fungus Septoria linicola, and is favoured by wet conditions, including rain and high humidity. The pathogen overwinters as little black bodies (pycnidia) on flax residues or as spores on flax seeds. Although infested seed can cause the disease, pasmo infestations usually start from infected residues. In the spring, the pycnidia release spores that infect the foliage.

“The initial symptoms are little brown flecks on the leaves. About late August, the stem will start to get a mottled appearance, with green or yellow healthy tissue and then dark brown patches where the fungus is infecting the stem,” Randy Kutcher, a plant pathologist at the University of Saskatchewan in Saskatoon, says.

Pasmo can cause defoliation, shrivelled seeds and boll drop, and severe infections can cause the plant to die. Pasmo can also predispose a plant to lodging because the infection weakens the stem. On the other hand, a lodged flax crop can trap moisture around the plants, providing ideal conditions for the disease.

Vera thinks the stronger association between lodging and yield losses in flax, as compared to wheat, could be because pasmo is playing a part in the yield losses attributed to lodging.

Initial investigation
To examine the association between pasmo, lodging and flax seed yield, Vera conducted a four-year project, from 2009 to 2012, at Melfort. The treatments included Headline EC (pyraclostrobin) application versus no fungicide application; and five nitrogen fertilizer rates (0, 33, 66, 100 and 133 per cent of the recommended rate).

Vera found that Headline application reduced pasmo severity and increased yield in the three years when pasmo occurred in the plots (2010, 2011 and 2012). The fungicide also prevented or reduced lodging in the two years when lodging occurred (2010 and 2012).

Headline’s effect on lodging was especially clear in 2010. Interestingly, although lodging was more severe in 2010, pasmo levels were lower that year, compared to 2012. According to Vera, these results indicate pasmo is just one of the causal factors involved in lodging. For example, he thinks unusually wet conditions in 2010 may have contributed to the more severe lodging. That year, the growing season was very rainy, and the field with the flax plots had a lot of water accumulation at certain times.

The project’s results also showed both the severity of pasmo and the amount of lodging increased as nitrogen fertilizer rates increased.

A deeper look
Vera is now leading a three-year project, which started in 2014, to answer some of the questions sparked by his initial study. So the project is taking a deeper look at the association between lodging and pasmo, and the effects of fungicides.

Vera is working with Kutcher, Ramona Mohr, an agronomist with AAFC in Brandon, and Jan Slaski, a plant physiologist with Alberta Innovates – Technology Futures (AITF) in Vegreville. The sites are at Melfort and Saskatoon, Sask., Brandon, Man. and Vegreville, Alta.

The researchers are assessing the effects of fungicides on lodging, pasmo, crop maturity, and seed yield, weight, test weight, oil content and protein content of CDC Bethune flax. In particular, they would like to determine if the fungicide’s effect on lodging, as shown in Vera’s initial project, was because of Headline’s particular mode of action or if other types of fungicides might also provide similar benefits. Vera notes, “Farmers have observed that Headline may result in higher yields even in the absence of disease, which may indicate Headline has other properties, not just fungicidal properties, that are affecting yield.”

So the researchers are comparing three fungicide products: Headline EC (pyraclostrobin, Group 11), Xemium (fluxapyroxad, Group 7), and Priaxor (a combination of pyraclostrobin and fluxapyroxad). Until recently, Headline was the only fungicide registered to control pasmo in flax on the Prairies. However, Priaxor was registered in 2014 and is available in Western Canada for the 2015 growing season for use on flax, as well as canola, pulses, corn and soybeans.

The researchers are also comparing three fungicide timing options: “early,” which is the recommended application time at about seven days after flower initiation; “late,” which is about seven days after the early application time; and early plus late.

In addition, the project includes a proactive study on fungicide resistance in the pasmo pathogen. The researchers are collecting samples of plant tissues with the disease from the project’s four sites each year. Starting this spring, Trisha Islam, Kutcher’s new graduate student, will be culturing genetically uniform isolates from the samples. Then she’ll test the isolates to determine their sensitivity to different concentrations of Headline and Xemium.

“In Western Canada, with so little fungicide being applied 20 years ago or even 10 years ago, fungicide resistance wasn’t as big a concern. But we know from Europe and other countries that fungicide resistance is quite common,” Kutcher explains. “[With fungicide use increasing on the Prairies,] we need to start looking at the issue.

“I don’t expect we’ll find fungicide resistance at this point, although it is possible,” he adds. “What we want to do is set a baseline so we know the normal level of sensitivity of the pathogen to these fungicides right now. Then in the future, if farmers start to find the products are no longer working the way they used to, we’ll be able to determine if it is because the pathogen has become insensitive to the fungicide.”

Headline’s active ingredient belongs to the QoI, or strobilurin, family of fungicide chemicals. QoI resistance is found in strains of various pathogens in various countries; some Canadian examples include the pathogen that causes ascochyta blight in chickpea, the apple scab pathogen, and the pathogen that causes early blight in potatoes.

“Headline is very effective on a lot of different diseases in a lot of crops. That tends to increase the risk of resistance if a grower is using that one product repeatedly on many crops and diseases,” Kutcher notes.

“[Fungicide application on flax] is becoming quite routine for some growers, especially because the last few years have been pretty wet, particularly in June and July. So pasmo, for many growers, has been above what was typically seen 10 or 15 years ago.”

The first year of the project produced some interesting preliminary results. “In general, fungicide application decreased disease infection and increased seed yield, seed weight, test weight and oil content, but delayed maturity of flax at some locations,” Vera says.

None of the three application times was clearly superior to the others. He explains, “This means that dual early plus late application may not always be superior to a single (early or late) application. Disease infection was low to medium at most sites and absent at Vegreville, and conditions favourable to the expression of lodging were also lacking at all sites, which prevented the opportunity to study the association of pasmo and lodging in 2014.”

Management tips
Vera’s studies and other research point to several practices that help in dealing with pasmo and lodging.

“The use of a fungicide, such as Headline EC, has been shown to control disease and, in some cases, severe lodging in flax. Headline has also been observed to increase seed yield, even in the absence of disease. Other products, such as Xemium and Priaxor, may prove to be as effective,” he says.

Previous research by Slaski and Vera has shown that early seeding (mid-May) helps prevent lodging, perhaps because early seeded flax produces shorter and more robust plants. This research also showed that flax seeding rates greater than the recommended 40 pounds per acre (45 kilograms per hectare) increased the severity of lodging.

“Farmers in North Dakota have been advised not to over-fertilize flax,” Vera notes. His own research shows high nitrogen rates are associated with increased lodging and pasmo, which both reduce yields.

To reduce the risk of fungicide resistance, growers can use the same types of strategies that are used to reduce the risk of herbicide resistance. Kutcher says, “Growers should try to use all of the [integrated pest management] practices they can to prevent crop disease, not just rely on fungicides. And when they use fungicides, they should try to rotate fungicides from different groups when available, or use products with more than one active. If they use the same family of fungicides over and over, they will select for resistant strains of the pathogen.”