Seedling blight or Protoporphyrinogen Oxidase (PPO) injury?

Jason Little
Director of Sales

With the recent rainfall and cooler temperatures, early planted soybeans showing signs of stunting could be a common sight throughout the Midwest.  Large areas of the soybean fields in Central Illinois planted the third week of April are looking pale and stunted, and lesions are present.  To find out the cause, you’ll need to take a walk and dig up plants in the affected areas.
 
The symptoms of a seedling blight and PPO injury appear to be very similar.  However, to determine the cause of the injury, we need to understand the differences of the symptoms of a seedling blight vs. a PPO injury, gather some weather information, and ask a few questions.  The questions are: what is causing it, and what needs to be done now?

Seedling blight and PPO injury can both present as lesions on the cotyledons and girdling of the hypocotyl.  One of the significant differences to look for would be the presence of lesions below the soil surface.  PPO chemistries inhibit the synthesis of chlorophyll and rarely cause injury below the soil surface—lesions below the soil surface would indicate a seedling blight.  

PPO chemistries such as flumioxazin (Valor, Gangster), sulfentrazone (Authority, Spartan), and saflufenacil (Sharpen) all carry the risk of crop injury. Under normal growing conditions, however, the risk of crop injury is low and acceptable.  Certain varieties of soybeans are more susceptible to PPO injury, and certain chemistries are more prone to causing injury.  It is important to note, that while PPO injury will reduce the height and biomass of the plant later in the season, this does not always result in reduced yield.

The same weather conditions that favor seedling blight also favor PPO injury.  Prolonged cool, wet soils after planting slow the growth rate of the developing plants.  The moisture also brings the chemistry into a concentrated solution.  The slowed growth rate increases the risk of a seedling blight and reduces the plant’s ability to metabolize the herbicide chemistry quickly.  

The Field Forecast tool in your Morning Farm Report account shows rainfall accumulation and the minimum and maximum daily temperatures for each of your fields.  Look for persistent temperatures in the 50’s and low 60’s, along with wet soils.With this information, you can determine which fields experienced the weather conditions that may show injury.

Before you head out to the field to start scouting, some background information will be very helpful to have.  The first question to answer would be what herbicide chemistry was applied to the field and when?  Planting date and herbicide application records are easily retrieved from the Field Story tool in your Morning Farm Report account.  If a PPO Inhibitor (Group 14) was not applied, seedling blight is the most probable cause of the injury you are seeing.

Now it’s time to dig up some plants.  Since cool, wet conditions favor both seedling blights and PPO injury, it’s important to look at multiple plants in each affected area of the field.  It is very probable that both conditions could be present in the same field.  Be careful digging up the injured plants, they are fragile.  It’s important to try and keep the plant intact and thoroughly examine it.  If lesions are present at or below the soil surface, seedling blight is most likely the cause.  If the injuries are found only above the soil line, you are then most likely dealing with PPO injury.

Will it be necessary to replant in the damaged areas?  The answer to that question depends on a few different things: aesthetics, replant policy of the seed company, upcoming weather, and final stand, but the most important is economics.

Most universities have recommendations regarding when yield is reduced from low plant populations.  Most suggest that a final plant population of 70-80,000 or higher in 15 or 30 inch rows should not have a significant yield loss. If your final stands are below that threshold, you need to make the decision based on income from a potentially reduced yield of a later planting date, offset by a need for additional weed control costs that might be necessary.   

Spot planting may be a possibility if only one or two smaller sections of the field are seriously affected.  Overplanting, by running the planter units immediately adjacent to poorly emerged rows, is not recommended unless the stand is extremely sparse. You would want to till those areas first.