If you’re growing peas for the first time this year, of if you haven’t grown them in a while, you’ll want to watch for these two diseases in your fields: ascochyta blight and aphanomyces root rot.
The disease that is perhaps most concerning, most visible and most likely to cause yield loss is ascochyta, or mycosphaerella blight, said Holly Derksen, former field crop pathologist at Manitoba Agriculture. (Derksen has since moved on to Arysta LifeScience where she works as a technical support special.)
Although ascochyta likes moisture, Derksen says the disease also occurs in dry years. “It may not be at economical levels in a dry year, but it always shows up,” she said.
Ascochyta shows up in the leaves as centric lesions. When it starts on the leaves, Derksen says it’s an indication that growers need to get out there and scout before it moves to the stem. Peas already have lodging issues, which can be exacerbated by disease issues.
“Once it moves to the stem, it girdles the stem, preventing water and nutrients through the plant,” she said.
Timing is important with ascochyta. Symptoms can be show up in the lower leaves, but may not move into the canopy.
“If you start to see symptoms, but then the weather turns dry, they’ll probably just stay on the leaves in the lower canopy, and again, won’t be a huge yield issue,” said Derksen.
If you do need to spray, do so at early bloom, and then again seven to 10 days later, if conditions are conducive to disease. But if disease symptoms show up after that early bloom, she recommends watching the weather forecast closely before taking action.
“If disease only moves in at late bloom, you might be able to see quite a few symptoms, but it won’t necessarily affect the yield,” she said. “Because once that seed pod is getting in there and the peas are forming, there’s not a lot that’s going to change your yield after that point.”
Aphanomyces root rot
While ascochyta is the disease that most concerns farmers, aphanomyces root rot should be on your radar.
Currently, aphanomyces is not widespread in Manitoba, but growers do need to be concerned about it in the long term, said Derksen.
Aphanomyces first showed up in Canada around 2012. “It’s one of those fun diseases that produces those long-lived spores in the soil,” said Derksen sarcastically. “Once you have it and it’s at a level where it’s causing economic damage, there isn’t really a rotation that keeps peas in the rotation that we can recommend that will manage the problem.”
Aphanomyces is an aggressive disease, a water mould that moves easily through the soil, which makes it a bigger concern in wet years. Soybean growers familiar with phytophthora root rot will already know how to manage for aphanomyces, since the way one manages the two diseases is very similar.
No matter which disease is present, aboveground, root rot symptoms look similar. The reality is that it’s unlikely you have just one pathogen when aphanomyces is present. If only aphanomyces is present, the roots will have a caramel colour.
“But chances are you have both aphanomyces and fusarium,” said Derksen, who recommends having soil tested for the presence of pathogens. If aphanomyces is present, avoid planting peas in those fields and limit soil movement. As far as rotation goes, wait six to eight years before adding peas back to the rotation.
Currently there is just one active ingredient available that targets causal agents, and that’s ethaboxam. Researchers are looking for other solutions.
While peas struggle in wet years, Derksen thinks they are a good fit for Manitoba growers.
“Peas do not do well in wet years, which is why they kind of fell off the radar for a while in Manitoba,” said Derksen. “In dry years, they definitely thrive.”