Soybean Seedling Issues: Seedling Disease Versus Herbicide Injury

Uneven soybean stand as a result of compounding conditions.

Over the past week we have seen numerous soybean field situations that have appeared to be seedling disease at first glance.  Uneven soybean stands appear to be quite common this season.  However, in some cases, entire blocks, on the order of 160 acres, have been planted and emerged only to die.  Rarely will a seedling disease cause large acreages to die.  In some cases fields were sprayed with a preemergence herbicide, and in one particular situation, six rows were not sprayed.  The six rows that didn’t receive the herbicide have emerged and appear healthy.

Seedling diseases, such as those caused by Pythium, are almost entirely dependent on environmental conditions.  Typically, seedling disease caused by species of Pythium require cool, wet weather, and can be more of an issue in lighter soils due to reduced fungal competition.  In addition, seedling disease can be a greater problem when water stands on a field for a period of time after planting or shortly after seedling emergence.  Generally speaking, Pythium is a fungus that prefers the environmental extremes.  For example, in some seasons we experience a disease known as non-lethal Pythium.  Non-lethal Pythium is an issue when temperatures warm up and high humidity, as a result of frequent rainfall occurs.  The same species of Pythium that causes seedling disease during cool soil conditions can also result in non-lethal Pythium when soil temperatures warm up.

This year in particular, planting situations that occurred prior to periods of heavy rainfall in excellent soil conditions, at the time, were subsequently stressed due to extremely cold nighttime temperatures.  Oftentimes, in these particular situations a fungicide seed treatment is blamed for not having performed correctly.  Seed treatments are effective products.  However, the fungicides applied to seed only provide a limited amount of protection.  Seed treatment fungicides are applied to protect the seed in the soil profile.  Once planted, the seed treatment performs by preventing seedling disease and allowing plants to emerge evenly and produce an even stand.  However, once the plant emerges from the soil the seed treatment is no longer effective at controlling the organisms that can cause diseases such as post-emergence damping-off (when the seed has emerged from the soil and will “damp-off”, or die, at the soil line).  In addition, in circumstances where excessive stress occurs, particularly as a result of cool temperatures, wet conditions, and any potential injury sustained from a preemergence herbicide, the likelihood of a seedling disease occurring is greatly enhanced.

Injured soybean seedlings observed following emergence.

Essentially, the above factors (herbicide injury, cold/wet weather, potential seedling disease organisms) have compounded one another.  No one cause can be blamed in every field situation.  However, in situations where the soybean emerges in the crook stage, and is weakened from cool, wet weather, rain can deposit herbicide onto the hypocotyl region and since the plant is already weakened it may have a more difficult time metabolizing the herbicide which can result in seedling death.  Moreover, attempting to culture a specific fungal organism from a dead seedling removed from the soil is a frustrating endeavor.  In those types of situations, regularly the organism that is believed to be the root cause of dead seedlings, in this case Pythium, is easily cultured from the dead seedling since non-pathogenic Pythium (or saprophytes) can feed on the stressed plant and make determining the initial cause of the dead seedling extremely difficult.

Symptoms of seedling diseases can be quite distinct.  In general, roots will appear unhealthy and typically are brown to black in color.  The emerging hypocotyl will swell to twice its normal size and in some cases will appear brown.  In situations where excessive humidity is present fungal growth can be found associated with dead seedlings.  However, caution should be taken since saprophytic fungi, or fungi that grow on last year’s crop debris can also be present.

More typical seedling disease symptoms as would be the result of infection from Pythium.

In the future, there are management practices that can reduce the likelihood of seedling injury (see: https://www.mississippi-crops.com/2013/04/26/chase-the-sprayer-with-the-planter/ ).  Preemergence herbicides are becoming more important due to herbicide resistant pigweed.  Applying herbicides 7 to 10 days prior to planting will reduce the likelihood of soybean injury as a result of some of the harsh chemistries used as preemergence herbicides.