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Agronomy Update

Missouri Soybean Fields Showing Signs of PPO-Herbicide Injury

Published on Thursday, May 19, 2016

Over the past week I have been called to numerous fields to evaluate emerging soybeans that are weak, injured by feeding or herbicides, or were impacted by seedling disease, primarily Pythium (root rot/damping off). I wanted to take a minute to discuss PPO-herbicide injury.

Soil-applied chemicals from the “14” family (PPO inhibitors) are utilized more frequently for residual control of resistant weeds in soybeans. When soil-applied near or at planting some chemistries such as sulfentrazone, saflufenacil, and flumioxazin, can cause injury. This is due to a high concentration of the chemical coming close to, or in contact with, the soybean seedling tissue. 

These chemistries were applied on a lot of our soybean acres, sometimes right after planting and before emergence. Soybean fields planted between April 22 and April 25, and during the first week of May, preceded or coincided with a significant air and soil temperature drop and heavy precipitation in the northeast claypan area of Missouri. Some of the rainfall came in the form of a surface-packing rain that sealed up the soil surface in areas where we have a high percentage of silt in the topsoil.

In order for soybeans to grow and metabolize herbicides that have been applied for residual weed control, they require warm soil temperatures, adequate (but not excessive) soil moisture, and good soil conditions. Many of our fields have been experiencing cold soil temperatures, soil saturation and crusting for the last two weeks. These conditions make it very challenging for the young soybean plant to emerge quickly or get critical tissue areas past chemical hot zones.

Below are a few pictures from the University of Illinois, The Bulletin, posted by Aaron Hager on May 20, 2014, depicting the injury we have also been observing.

   

Most of the time soybeans will grow out of this injury if it is superficial (on the tissue) or if it only impacts the cotyledons and not the neck of the hypocotyl. If the wet conditions have caused injury to the neck of the hypocotyl (where the arch of the neck is just sitting in the chemical hot-zone right at the soil surface) and the soybean growth has slowed or stopped, the chances of its survival are not very good. When the soybean neck becomes brittle, it can snap off in high wind or rain and become an infection point for soil-borne fungal disease. I have observed many fields where a Pythium infection is beginning to or has already pinched off the neck or root tip on herbicide-injured soybean plants.

When assessing herbicide-injured soybean stands for replant or interplant potential, you need to determine how many plants/A. you think will survive. This would be plants that are either non-injured or you expect can grow out of the injury. If your estimated surviving stand is 110,000 to 130,000 plants/A. or greater, I'd recommend keeping the stand. If you estimate your viable stand is between 60,000 and 120,000 plants/A. and your soybeans are still at the V1 growth stage, I recommend interplanting however many seeds/A. required to finish at 130,000 plants/A.  If you are at less than 60,000 surviving plants/A., I recommend a full replant. There are additional factors to consider when thinking about replant so please make sure to consult your Beck’s representative before making any decisions. 

Lastly, the conditions we are experiencing favor Pythium infection even where no herbicide injury has occurred. The best way to distinguish between Pythium and PPO injury are:

  1. If no PPO chemistry was applied.

  2. PPO herbicide injured seedlings usually still have some roots and "tug" back when you try to pull them out of the soil.

  3. Pythium-infected seedlings pull right out of the soil (no "tug") and the root tip looks smooth and pinched off.

  4. Pythium infected tissue looks more "rotted ", soft, and systemic whereas PPO-injured tissue looks more "burnt", brittle, and superficial.

  5. Pythium-infected neck looks more wilted and decayed rather than burnt and brittle.

As always, feel free to call if you need help making a field assessment or management decision. Warmer temperatures and better growing conditions will go a long way to promoting stand viability and recovery.


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