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Use caution with foliar fungicides

In Saskatchewan, provincial guidelines recommend spraying fungicides on durum wheat at the flag leaf stage for leaf spots and the flowering stage for Fusarium head blight (FHB), if warranted. But others are also recommending fungicide applications at earlier growth stages on a preventative basis. Yet little evidence existed, until recently, on whether this was a viable practice.

However tempting it is to throw some fungicide in with a herbicide application to save on application costs, Myriam Fernandez cautions it doesn’t help prevent disease and can even negatively impact quality.

“Our results suggest that under variable environmental conditions in Saskatchewan, not always conducive to the development of high disease levels in wheat, early preventative fungicide application on durum wheat should not be recommended as a strategy to improve productivity, even when followed by a second application,” Fernandez says.  

Fernandez is a research scientist with Agriculture and Agri-Food Canada (AAFC) at the Swift Current Research and Development Centre. Between 2004 and 2006, she led a study investigating single and double applications of foliar triazole fungicides at various growth stages, and the impact on FHB, deoxynivalenol (DON) concentration, dark kernel discoloration and grain traits in durum wheat. A second study was led by research scientist Bill May at AAFC’s Indian Head Research Farm between 2001 to 2003, which looked at the impact of single and double fungicide applications at flag leaf emergence and flowering stage on disease control and yield and quality of durum. Both studies were recently published in the Canadian Journal of Plant Science.

In Fernandez’s research, plots were established at the South East Research Farm in southeast Saskatchewan, and the trial ran for three years. The previous crop was canola in each year. AC Avonlea durum was seeded using a no-till plot drill. Standard agronomic practices were used.

Folicur was applied at the recommended rate in all years. Six fungicide treatments were conducted:

  • unsprayed;
  • at stem elongation (GS 31);
  • when flag leaf was half emerged (GS 41);
  • at early to mid-anthesis (flowering) (GS 62-65);
  • at stem elongation and mid-anthesis;
  • at flag leaf emergence and anthesis.

Leaf spotting disease, FHB incidence, Fusarium kernel infection, DON concentration, grain yield and quality parameters were measured. Percentage leaf spotting severity on the flag leaves was evaluated in 2004 and 2005, but not in 2006 because of poor disease development.

Fernandez says that in most cases, a fungicide application at stem elongation was not effective in reducing Fusarium diseases, nor in improving yield and grain characteristics. She explains that none of the early, single applications were consistently different from the unsprayed control. Fungicide application at flag leaf emergence was more effective in reducing disease levels later in the growing season or improving grain characteristics than an early application at stem elongation. An application at the flowering stage resulted in the most consistent reduction in Fusarium levels, leaf spotting and improvement in kernel size.

This is consistent with fungicide application timing for FHB control. Saskatchewan Agriculture recommends fungicide application when at least 75 per cent of the wheat heads on the main stem are fully emerged to when 50 per cent of the heads on the main stem are in flower.

The double fungicide applications at either stem elongation/flag leaf emergence and anthesis were no more effective than a single fungicide application at flowering, and would have resulted in increased fungicide and application costs.

None of the fungicide treatments resulted in a significant grain yield increase. “We can conclude that fungicide application, single or double, might be profitable only in the presence of higher disease pressure levels, with more suitable growing conditions for disease development and plant growth,” Fernandez says.

Grain downgrading might result from early and frequent fungicide application
The early fungicide applications also had a negative impact on dark kernel discoloration, a key quality parameter for durum wheat with tolerances for total smudge and black point at five per cent in No. 1 Canadian Western Amber Durum (CWAD) and 10 per cent for No. 2 CWAD. The discoloration would have resulted in downgrading for the early application treatments.

Fernandez says the results also indicated potential for a consequent increase in kernel discoloration like black point and red smudge after early fungicide treatment, which was associated with greater kernel size. This effect has also been reported with other fungicides from other wheat growing regions of the world.

The 2001-2003 study conducted in southeast Saskatchewan and southwest Manitoba led by May at Indian Head looked at the impact of single and double fungicide applications at flag leaf emergence and flowering stage on Fusarium-damaged kernels and other kernel diseases, leaf spotting, and resultant grain yield and quality of durum wheat.

Disease levels averaged over all site years were high enough to result in an 8.5 per cent yield increase from the application of fungicides. However, application at either flag leaf elongation or flowering stage also increased black point by 49 per cent, from 0.38 per cent to 0.56 per cent, and red smudge by 17 per cent, from 0.54 per cent to 0.63 per cent. In addition, double fungicide application further increased red smudge to 0.85 per cent, a 57 per cent increase compared to no fungicides being applied.

Effective August 2015, the Canadian Grain Commission changed the grading factors for CWAD. Red smudge is no longer a separate grading factor, but is still included under “smudge.” The maximum allowable level of smudge in CWAD is now 0.50 per cent for grade No. 1, and one per cent for grade No. 2 and grade No. 3. Prior to 2015, the tolerance level for red smudge in CWAD No. 1 was 0.30 per cent. In May’s research, the percentage smudge would have resulted in a downgrade to No. 2 CWAD.

May says two theories have been put forward to explain the association of red smudge and fungicides. The first is that an early fungicide treatment could result in an increase in kernel size that would facilitate the opening of the protective husk (glume), making it easier for fungi to penetrate and infect the grain. An alternative explanation is that the fungicide might alter the microbiological community on the spikes before or during kernel development, modifying the fungal interactions in that environment. More research is required under western Canadian conditions to determine the exact cause.

For foliar leaf disease control, Fernandez says the recommendation is still to apply a fungicide at the flag leaf stage, based on the level of disease infestation. This research found little benefit to applying fungicides for leaf spot diseases because the crop was not heavily infected. In other areas more conducive to disease, or years with high disease pressure, fungicide application at the flag leaf, or heading stage for leaf spotting disease could be profitable. The research also shows that current fungicide timing recommendations for FHB control at head emergence to 50 per cent flowering are still valid.

Fernandez cautions when applying any fungicide at any growth stage the potential development of fungicide resistance in wheat pathogens should always be considered, and unnecessary fungicide application may increase the risk of resistance developing.

May says faced with the recommendation of early fungicide application as a preventative measure regardless of disease pressure, farmers need to consider that early and frequent fungicide applications to durum wheat might reduce grain quality and result in downgrading and potential profit loss.

“I would expect that a fungicide application for control of FHB in durum wheat would provide a yield increase much more often than it would improve the grade of the harvested crop.” May says.

 


August 9, 2016
By Bruce Barker

Topics
The top kernel is classified no smudge In Saskatchewan