Soybean Cyst Nematode and Rhizoctonia Root Rot in Soybeans

I have had a lot of questions concerning Soybean cyst nematode and rhizoctonia in soybeans recently.  In terms of SCN, remember the primary SCN symptom is often no above ground symptoms at all.  Growers are advised over the next few weeks to scout for SCN by carefully digging-up plants, gently remove the soil from the roots and examine them for the “pearl white to yellow” cysts.  This goes for growers who are using SCN resistant varieties as well since SCN field populations can adapt to these varieties resulting in increased cyst reproduction and yield losses. You can not afford to ignore SCN and what is happening in the ground!

As we have seen this year, Rhizoctonia root rot continues to be a challenge in soybeans as well. It’s an issue across the whole province, not just in southern Ontario and basically every soybean field has some degree of Rhizoctonia, but in some fields the disease becomes more problematic than in others. Factors that contribute to Rhizoctonia are: (1) more soybeans in the rotation; (2) the pathogen’s ability to survive for years in the soil (rotation on its own has minimal impact on decreasing Rhizoctonia populations); (3) early planting and (4) more crop residue. These factors can all influence why we’re seeing more Rhizoctonia.

Rhizoctonia has distinctive symptoms that help identify it. You’ll see a sunken, brick red canker right on the stem at the soil line.  This canker might go an inch or so above the soil line but rarely much higher than that. However, it can work its way down and prune off a lot of the roots. It will be a much darker red colour than say Fusarium root rot. With Fusarium, you’ll often see more of browning discoloration of the roots. If you cut into the roots you’ll often see more discolourization inside with Fusarium than with Rhizoctonia,. Both diseases cause a leathery texture to the roots. In comparison, phytophthora or pythium causes the roots to become more water-soaked or softer and they’ll disintegrate. So if the roots are leathery, then it’s Rhizoctonia or Fusarium, and if the stem has that brick red canker at the soil line, it’s more than likely Rhizoctonia.

There is some promise for developing soybean varieties with tolerance to Rhizoctonia, but we’re not there yet. So seed treatments are the main method for managing Rhizoctonia in soybeans and fortunately more active ingredients such as Sedaxane which was recently approved by the PMRA are being developed specially targeting Rhizocontia.  Although we are making progress in Rhizoctonia management we have some work still ahead of us!