Early fungicide applications reduce blackleg severity when disease pressure is high and the varieties lack resistance, a researcher told delegates at the International Rapeseed Conference in Saskatoon. But fungicide has little effect when varieties are resistant or moderately resistant, he said.
“We need to find kind of a sweet spot with fungicide use,” said Dr. Gary Peng, research scientist with Agriculture and Agri-Food Canada (AAFC) in Saskatoon.
AAFC researchers have studied blackleg management strategies in Western Canada. “But because those studies were done in the low disease years, generally the fungicide treatments did not provide an economical return,” said Peng.
But as blackleg has resurged, researchers have wondered whether fungicide could cut risk if there was a major outbreak, Peng said.
Peng and several colleagues examined the effect of several fungicides at five western Canadian sites. They left diseased canola residue at the sites to build up blackleg inoculum. To see what would happen in “the worst-case scenario,” they used Westar, a blackleg-susceptible variety, Peng said. Untreated plots were used as a check. Researchers also applied Headline to a moderately resistant and resistant variety at the two- to four-leaf stage as an additional check.
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Along with Headline, AAFC researchers gave several other fungicides a spin (see chart below). Once the canola was mature, they examined each plot for blackleg severity and incidence, and recorded seed yield. Ultimately, they pulled data from 17 site-years.
When researchers looked at all of the data, most of the treatments boosted seed yield of Westar and reduced blackleg. The exceptions were Tilt applied at the two- to four- leaf stage and Headline applied before bolting. This pattern held when researchers looked at data from trials with moderate to high disease severity.
“When variety resistance has been eroded and disease pressures are high, that’s probably the time you can consider (fungicide application),” said Peng. Early treatment is more effective, he added. “And the split treatment or double treatment is not really required.”
But the trend didn’t hold for trials with low disease severity — researchers saw no difference between treated and untreated plots in seed yield, disease severity, or disease frequency.
As for the resistant and moderately resistant cultivars, Headline often reduced blackleg resistance and severity. But it didn’t boost seed yield substantially, according to the study.
Peng recommended canola growers scout for blackleg at harvest to assess how much disease inoculum they’ll be dealing with in the future. He also suggested farmers use resistant cultivars and rotate crops before turning to fungicides.
In most cases, fungicide is not needed, Peng said. “It’s not what to spray or when to spray. It’s if you need to spray.”