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Managing Phytophthora Root Rot

One of, if not the most, destructive soybean diseases in Western Canada is Phytophthora Root Rot (PRR). This fungal disease attacks the roots and stems of your soybean plants and can happen at any stage in the plant’s life – so let’s talk about what you can do about it.

The disease is in the “water moulds” group, right next to downy mildews, Pythium, and Aphanomyces. The difference, though, is PRR is more common in warm soils and Pythium is more common in cold soils. PRR forms brown lesions on the soybean plant stems, which restricts the flow of water and nutrients to the plant. This eventually causes your plants to shrivel, wilt, and die.

Before managing PRR, your plants must have it. BrettYoung Regional Account Manager for Eastern Manitoba, Michael Moore, recommends scouting for it if:

  • Your field has a history of PRR infection or neighbouring fields that drain into yours do
  • There are areas in your crop where beans haven’t emerged uniformly
  • Plants are showing visible symptoms (yellow to brown leaves, brown lesions on stem)
  • Your field has poor drainage or low spots
  • Your field has been receiving steady rainfall

 Moore said you can start scouting for the disease as soon as your soybean seeds start to germinate.

“If there are areas within rows where no plants are emerging, you can gently dig down to investigate,” he said.

It’s important to remember PRR can affect your soybean plants any time after they start bringing in water, so some seeds may rot before they even have the chance to germinate. Other seeds may germinate but die off before emerging with a noticeable pinch at the top of the stem. This, commonly referred to as damping off, can make it hard to distinguish between Pythium and PRR.

“If you noticed damping off in your field in the spring, you should be on high alert for PRR,” said Moore. “Especially if you receive steady rainfall.”

Stem rot is another symptom of PRR and is easier to identify as it will show a clear brown lesion vertically on the stem, moving from the ground up. The top of the plant will remain green at the beginning, but will eventually succumb to the PRR and wilt, turn yellow, and die.

PRR is most common in soils with high clay content and lots of moisture, but it can show up anywhere. It’s common in Manitoba, present in Saskatchewan, and rare in Alberta as soybeans aren’t a common crop there.

“Once you have PRR in a field, it’s very tough to get rid of,” said Moore. “PRR spores can stay dormant in your field for up to seven years. Those spores will germinate and break dormancy in the right conditions, eventually attacking your soybeans plants again.”

So, what can you do? You can manage the infection with:

  • Crop rotation, ideally planting soybeans no more than every fourth year
  • Soil sampling to determine which strain of PRR is present in your field so you can plant a variety with adequate resistance
  • Field drainage
  • Fungicide on-seed treatments
  • Cleaning field equipment and vehicles before changing fields

Moore said preventing PRR is like managing it. Grow PRR-resistant soybean varieties, put your crops on rotation, drain your high moisture fields, use fungicide seed treatments, and clean equipment going in and out of different fields.

“PRR can be minor or catastrophic to your yield,” said Moore. “It really depends on the level of infection present in the field, the variety seeded, and environmental conditions.”

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