Fusarium head blight (FHB) and leaf spots are fungal diseases that can dramatically cut wheat yields and quality. For these reasons, there has been a heightened focus on spraying fungicides to mitigate associated crop losses. However, recent studies undertaken by Agriculture and Agri-Food Canada (AAFC) will cause farmers to question early and frequent applications of fungicides for control of wheat diseases.
Timing, Number of Applications
Between 2001 and 2006, AAFC researchers Myriam Fernandez and Bill May undertook two studies to examine how the timing and number of applications of the commonly recommended triazole fungicides affected durum wheat grown in southern Saskatchewan and Manitoba.
One of the studies, conducted in southeast Saskatchewan in 2004-2006 under relatively low disease pressure, showed that:
- Application at flag leaf emergence was more effective in reducing disease levels later on, or improving grain characteristics, than an early application at stem elongation.
- Application at the flowering stage resulted in the most consistent reduction in Fusarium levels, leaf spotting, and improvement in kernel size.
- Double fungicide applications (at stem elongation or flag leaf emergence, with a second at flowering stage) were not more effective in disease control than a single application at flowering.
Early, Frequent Applications Might Downgrade Grain
In their 2001-2003 study, the researchers looked at the impact of single and double fungicide applications at flag leaf emergence and flowering stage. They saw a distinct trade off, as kernel discolorations (downgrading factors that can reduce profits) black point and red smudge were increased.
While fungicide applications showed a yield increase, the timing and number of applications was shown to have a negative effect on quality. For example, during the period of the study a single application of fungicide resulted in a 4.3% yield increase, and a second resulted in an 8.5% yield increase.
But researchers saw significant impacts on grain quality: application at either flag leaf elongation or flowering stage showed a 47% increase in black point compared to no fungicides being applied, and by 76% for double applications. Furthermore, red smudge increased by 17% for single fungicide applications, and by 57% for double applications, compared to no fungicides being applied.
As an example of the negative affect on quality, these increased rates of red smudge would have downgraded the crop from being No. 1 Canadian Western Amber Durum by 2015 rating standards, which reinforces that fungicides should only be applied when necessary and at the right time.
The observation of increased kernel discoloration as a result of fungicide application agrees with results observed in previous AAFC studies, such as by Wang et al., as well as those done at the Indian Head Agricultural Research Foundation (IHARF).
Fungicide as Preventative Measure Questioned
None of the results support the recommendation that fungicides be applied to durum wheat crops on a 'preventative' basis, to increase grain yields.
"Fungicides should be applied when disease pressure warrants it, and the potential for grain and quality losses exists," says Bill May, crop management agronomist, for AAFC's Indian Head Research Farm. "Faced with the recommendation of early fungicide applications 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."
Fernandez also notes that the overuse of fungicides can result in the wheat pathogens developing increased resistance to the treatment.
More research on the relationship between kernel discoloration and fungicides needs to be conducted. In the meantime, farmers are encouraged to follow good crop management practices, such as growing wheat cultivars with resistance to the diseases prevalent in their region and rotating them with non-susceptible crop species.