Farmers can be a little too ready to pull out the sprayer and apply fungicides and may be doing more harm than good.
When there’s little or no leaf disease present in a field, those early applications are an expense for no benefit and could do more harm by encouraging other diseases such as black point, says Myriam Fernandez, a cereal pathologist with Agriculture and Agri-Food Canada (AAFC).
“You are wasting your time, you are wasting money and you may be causing further problems along the way,” Fernandez said in an interview June 14, from her base at the AAFC Swift Current Research and Development Centre. She was speaking about research she and AAFC agronomist Bill May conducted between 2001 and 2006 that AAFC recently highlighted in an online publication.
“The effect of the fungicide does not carry over to later growth stages. So if you apply it early on I’m sorry, you’re going to have to apply it later again (if the infection warrants it).”
However, that doesn’t mean there aren’t times when an early fungicide application to battle leaf spots is warranted, Fernandez added. There’s no formal leaf spot threshold, but spraying should be considered if more than five per cent of the penultimate leaves are infected, she said.
The penultimate leaf is the last before the flag leaf emerges.
“If you only have the odd spot then it is not worth spraying because all (wheat) seedlings are going to have some spots because that is just the nature of the beast,” Fernandez said.
“If you have a lot of leaf spotting — it is happening this year in a lot of places — at the seedling stage and later on, then yes, go ahead (and spray).”
Their research was done on durum wheat, but Fernandez said the same principle likely applies to spring and winter wheat.
“You have companies pushing for early application at the seedling stage and a little bit later,” Fernandez said. “The message we want to get across is, based on our studies, it doesn’t work. There have been studies done in the U.S. and other places too. It doesn’t work. And one of the reasons it doesn’t work… is economics. I haven’t seen a single study honestly that shows double applications of fungicides and early applications, et cetera, result in an economic benefit because it doesn’t.”
Farmers shouldn’t rely just on fungicides, but also use other agronomic tools to reduce the disease threats, including growing disease-resistant varieties and rotating crops, Fernandez said.
Using fungicides at the wrong time or when unnecessary just speeds up the selection of fungicide-resistant pathogens, she warned.
“We know what kind of trouble we are in right now with a lot of herbicide resistance,” Fernandez added.
Monitoring crops is important too. If leaf spot diseases are at low levels, farmers might be able to wait until they spray at the early-flowering stage to suppress fusarium head blight, a fungal disease that cuts yield and quality.
“Any application earlier than flag leaf we showed that it could be detrimental and it is not worth it,” Fernandez said. “But also people need to remember the leaf that contributes most to yield is the flag leaf. So what you need to do is protect the flag leaf therefore you need to apply it at flag-leaf emergence.”
Fernandez and May looked at the impact of single and double fungicide applications at flag-leaf emergence and the flowering stage. They found applying fungicides boosted yield, but also caused more black point and red smudge, resulting in lower grades.
A single and double fungicide application increased yields by 4.3 and 8.5 per cent, respectively. However, an application at either flag-leaf elongation or flowering showed a 47 per cent increase in black point versus no fungicide use. The incidence jumped by 76 per cent for double applications.
Red smudge increased by 17 and 57 per cent with a single and double fungicide application, respectively, “which reinforces that fungicides should only be applied when necessary and at the right time,” AAFC said in a news release.
“The observation of increased kernel discolouration as a result of fungicide application agrees with results observed in previous AAFC studies… as well as those done at the Indian Head Agricultural Research Foundation (IHARF).”
Fernandez has some theories why yields and kernel diseases increased with early and frequent fungicide use. Applying a fungicide results in bigger kernels, hence more yield. Bigger kernels push out the glumes around the kernel.
“When you do that it’s fair game for any pathogen in the air to infect the crop so it is less protected in a way,” she said.
“The fungicide could also kill the good guys — the organisms on the glume surface that are protecting the kernel.”
Fungicides are an effective tool for controlling leaf spots in wheat, but farmers need to know when to use them, Fernandez said.
“We are not telling people ‘do not apply the fungicide,’ but be aware of what the consequences are,” she said. “Just because you bought the fungicide cheap or you have a fancy sprayer you want to use that’s not a reason. And then there is the issue of fungicide resistance that we are all worried about.”