By Anne E. Dorrance
PPO and Cold Injurt
Several calls last week with pictures of injured and/or diseased soybean seedlings. For most of these situations we have the following scenario: PPO herbicides (flumioxazin, sulfentrazone, saflufenacil) included as a component of the preplant burn down, fields planted 7 days later with fungicide treated seed, followed by 1 to 2 weeks of suboptimum growing conditions (between 40 to 50oF) for 2 weeks, and greater than 2” rain. These conditions are very conducive to both Pythium damping-off and PPO injury.
Some of the reports from the field were with seedlings that have already croaked. Wispy skeletons of soybean seedlings could be found on or below the surface. These are most likely from Pythium, it moves fast under these conditions. Other seedlings had black at the hypocotyl hook with a reddish brown on the underside of the cotyledon. These could be PPO injury.
There is still much to learn from this unusual weather pattern, but if the soybean plants are slow to get out of the ground, they are exposed to the herbicide/cold temperatures for a much longer period of time. In addition, with soybeans and cold soil temperatures, they are stressed and leak nutrients, signal compounds which attract seedling pathogens, if they are stressed may also be more vulnerable to PPO injury. First, wash up some of the seedlings and cut some open to look at the interior tissue. If it is healthy on the inside, there is a good chance that it will recover.
How to tell the difference between pathogen, flooding, and PPO injury is not going to be easy this year. As all 3 may be present in the field at the same time. The first for pathogen, is the tissue soft? The stem or roots give easily under pressure when you squeeze it between two fingers. The browning (from light brown to dark black) is consistent through the interior of the seedling. This will also continue to progress over the next week.
Flooding injury – the first thing you will notice is the smell from the anaerobic conditions that occurred in the field. For severe cases, there may also be algae on the surface of the soil. But from the seedlings, the color is gray in the outside of the root tissue and it can be easily separated from the root stele (center of the root) – giving a white “rat tail” appearance. If you can pull of the outer root tissue very easily – and the injury is limited to the wettest areas of the field.
PPO injury – contact necrosis on the emerging shoot, variable rate of emergence, possibly some growth distortion, and failure to emerge or death if severe. This is often considered to be caused by “splash up” of herbicide as the shoot emerges. Most frequent when PPO herbicides are applied at or after planting, especially in a tilled seedbed. In our research, we find that application at least a week before planting in no-till minimizes the risk of injury, partly due to subsequent rains that move at least some herbicide off the soil and residue surface. This year’s situation is different due to the extended cold and wet soil conditions after planting. The PPO herbicides are often a component of a premix with either cloransulam or chlorimuron. Chlorimuron can also cause growth inhibition, yellowing, and dark veination when rain and soil moisture is abundant, so injury that is attributed to herbicide may not be due solely to PPO herbicides.
As you compare different fields, some of you will note different colors on the stem of this new seedling, this is referred to as the hypocotyl region. The gene for flower color is also expressed here, so purple colored flower varieties will have a purple hypocotyl and white flowered varieties will have a green hypocotyl color.
For all of these situations, the warm weather this week will greatly help get these seedlings out of the ground and form new roots. Once plants are up and growing they should be able to make up for their extra-long (3 to 4 weeks) time underground during this stressful 2017 growing season. Aim to take stand counts at the end of this week – and follow the guidelines outlined by Laura Lindsey for the different regions in Ohio.
Injury: PPO Cold or Both