Brown Stem Rot

Low Potassium Levels Impacting Soybeans in the Late Season

Andy Orr Agronomy

By: Cassidy Fletcher, Northeast Sales Agronomist and Technical Lead

This fall we saw soybean fields around the region show late season symptoms of what looked to resemble sudden death syndrome. With that disease isolated to only a few counties in New York, farmers quickly became concerned with what was going on in their soybean fields.

The upper leaves in the soybean canopy were the first to show what we believed to be the signs of low potassium levels with the edges of the leaves were turning a bright yellow. The leaf began to show chlorosis between the leaf veins and as this progressed, the leaves began to shrivel and up die off. It seemed the plants showing these signs matured and senesced faster than those which remained green and healthy. These symptoms seemed to appear later in soybean development, mostly during the pod fill stage and scattered throughout fields. These plants also had pretty heavy aphid pressure on the upper leaves and the pods that were on the upper nodes seemed to have aborted or had little to no pod fill.

Brown Stem Rot

After digging up some of the stems and roots, there was no discoloration of the root or pith to indicate Sudden Death Syndrome or Brown Stem Rot. On the roots, there were no Soybean Cyst Nematodes found, so there was very little evidence to suggest a fungus or disease-causing these symptoms. All stems seemed to remain white and healthy, even late into the season when leaves had completely fallen off and beans were beginning to dry down. To further investigate what we were seeing, whole plants were gathered from infected areas and sent to a diagnostic lab to determine if there was any disease present or if we were dealing with a potassium deficiency. The results came back and confirmed that there was no Sudden Death Syndrome or Brown Stem Rot present. Plant tissue samples were also sent out and determined that the plant was greatly lacking potassium, providing evidence that what we were seeing in the fields was most likely just a potassium deficiency. The aphid pressure along with the environmental pressure most likely enhanced the potassium deficiency and made the symptoms look similar to those of some soybean diseases.

Why are We Beginning to See This?

While it is more common to see potassium deficiency in the earlier growth stages of soybeans, seeing it later in the season during the late reproductive stages is becoming a newer occurrence. Potassium is mobile throughout the plant and therefore, the symptoms normally show up in the older leaves first. The reason that these deficiencies are showing up late in the season and in the upper canopy at that are not fully physiologically understood. It’s common that environmental factors such as droughty soil conditions, or compaction can lead to potassium deficiencies developing, but may not have had much of an influence where we saw it develop in this year. Universities have done very little research on potassium and its effects on soybeans late in the season, but they believe that with the ever-increasing soybean yields, plants may be translocating more potassium to the upper canopy than what we previously understood. There have been studies done that show that when potassium is lacking; pod fill, seed size, and even the total yield of soybean plants are affected. This is seen mainly in the nodes in which produce the most pods, which are the middle to upper nodes.

The full effect that potassium has on the soybean plant late in the season is still unknown. It may be that the upper nodes of the soybean plant have a higher pod set and grain production, which requires more potassium to be transferred up to the upper canopy. When potassium levels are lacking, these nodes are unable to receive the required levels needed and the plant may begin to show deficiency symptoms while starting to abort pods and show poor pod fill. This would correlate with what we were beginning to see this year, but it is hard to definitively say.Soybean plants affected

Another reason we may be seeing these issues is due to growers not fertilizing their soybeans like they do their corn fields. Soybeans, as well as any crop, need adequate nutrient levels to produce the higher yields that everyone is aiming for. It’s important not to overlook potassium levels or to focus on only one nutrient level in the soil. When nutrient levels aren’t adequate, crops begin to struggle and other issues may arise. In one study, it was shown that low potassium levels can increase the incidence of soybean diseases during conducive climatic conditions; this includes Sudden Death Syndrome, Cercospora Leaf Blight, Frogeye Leaf Spot, Downy Mildew, and Septoria Brown Spot. If we can keep nutrient and potassium levels optimum, or even above optimum, the disease pressure may be drastically reduced. Soybeans have a surprisingly high potassium removal rate. Fertilizing for yield and plant health requires an understanding of nutrient resources, expected production goals, and that regular soil samples are important.

With this growing season being so odd, it is tough to pinpoint exactly why we saw what we did in the field. Whether it was a nutrient deficiency or an unknown disease this year seems to create an opportunity for it. Feeding the crop to hit yield targets while maintaining soil levels is a better recipe for long-term success.