Whether to spray for sclerotinia is always a challenging choice, but this year was harder than usual.
Dry soil gave little room for the disease to germinate early in the year through much of Manitoba, leaving producers to wonder if a spray pass was worth the expense, says Angela Brackenreed, of the Canola Council of Canada. At the same time, a spate of rain just as many fields started to flower had producers suddenly weighing the merit of a late application.
“Initially, in a lot of regions in Manitoba that were quite severely dry and we didn’t have great canopy closure, I probably felt more confident than ever that not spraying in those specific cases was probably the best choice. But then we had rains come when a lot of fields were 20 to 30 per cent bloom or just at the beginning of flowering,” the agronomist said.
The province considers a field with 15 flowers per main stem to be at 20 per cent flower, while plants starting to drop petals are considered 30 per cent flowered and a half-flowered crop will count 20 flowers on the main stem, with more coming on side branches.
An ordinary spray year might target crops between 20 to 30 per cent bloom. This year however, with early conditions unfriendly to sclerotinia, producers might have stretched that window up to 50 per cent flower.
“You’ll drive by a field and you’ll see a field being sprayed and you’ll go, ‘Well, Jeez, that looks really late,’ but unless you know the conditions leading up to and what’s going on in the field, some of those conditions can make it worthwhile spraying,” Lionel Kaskiw, farm production adviser with Manitoba Agriculture, said during a July 4 Crop Talk webinar.
Late application reflects the time lag as fields previously too dry germinate sclerotinia sclerotia suddenly get enough moisture to start their life cycle.
According to Kaskiw, soil must be wet for seven to 12 days before the tee-shaped, mushroom-like apothecia appear, while the Canola Council of Canada sets the timeline at 14 to 21 days of constant soil moisture for sclerotia to germinate, sprout apothecia and start producing spores.
“You need moisture throughout that entire process and moisture for once the spore is on the flowers and the flowers drop for that disease cycle to continue,” Brackenreed said. “What I’ve been telling guys is to think about where they are in the flowering stage and where they were at when that moisture came and think about what flowering stage would they be at 14 to 21 days later and then to consider that typically those later infections aren’t as big of a yield robber as the earlier infections.”
Kaskiw noted that some farmers might be better off targeting those later-season infections normally considered less dangerous to yield, but that this year might be the best opportunity for sclerotinia to infect a crop.
“You’ve got a little bit of a window there this year, mainly because of the dry conditions that affected germination in the spring and now, after the rains, we’re getting almost two different stages of canola growing, so you can almost make your judgment call as to what stage you want to go after,” he said.
“You could be maybe doing the one application just to go in there and get them at 50 per cent, and then that way you’re covering some of the first crop and getting some of the second crop as well.”
The canola council also urged farmers to look past rainfall when evaluating field moisture, arguing that high humidity and a closed canopy can still provide a perfect microclimate for the disease, even if there has not been much rain.
The canopies have not been as dense this year, Brackenreed repeated, but was still urging farmers not to take any of those field conditions for granted.
The “wet pants test,” which gauges how wet conditions are under the canopy by how wet the farmer’s pants are after walking through the field, is still something that Brackenreed and the rest of the Canola Council of Canada endorse.
“I think there’s less of a conducive microclimate for sclerotinia than most years. But, we should tell folks to get out in the field morning to afternoon to see if you’re coming out with your pants wet still at noon. If that’s the case, then you know you’ve got a risk there,” she said.
The conditions that lend themselves to a late application are also not uniform across the province. Some regions of Manitoba had more consistent moisture and some areas sat at well over adequate moisture or even excess moisture going into flowering, making for more friendly conditions for sclerotinia.
“When moisture is there, there is a sclerotinia risk,” Brackenreed said.