Do you have patches of poorly-growing soybeans?

Last week took me on a trip to northern Minnesota soybean fields with problems in need of a diagnosis. In a field of otherwise phenomenal looking soybeans, there was a very large patch of soybeans that were visibly paler, shorter and thinner (nowhere near closing rows) from the road.

First things first.

Upon entering the field, I made a beeline for the dividing line between the gorgeous, healthy soybeans and the sickly soybeans. It is this area of a field of plants that can provide the quickest clues as to the cause of the problem. I gently dug up the plants to inspect the roots, as pulling plants from even moist ground may help to obscure the cause of the problem.

There was evidence of the plant encountering a compacted layer of soil. This could be easily observed by all of the taproots taking a sharp turn to the right or left and not growing straight down. However, the taproots prevailed, breaking through the compacted layer of soil to again grow downward (Figure 1).

Figure 1. Notice the ‘crook’ in this soybean’s taproot indicating that the plant encountered a compacted layer in the soil early on. 

SCN = Poor nodulation.

Both fields’ tough-looking soybeans had evident cysts of the most yield-limiting pathogen of soybean – soybean cyst nematode (SCN). SCN colonizes parasitizes soybean roots,  taking for themselves the nutrients and water that the plant would otherwise use for growth and development. Significant yield losses can occur without a plant exhibiting diagnostic symptoms above ground, meaning that as we could see the results of the infestation from the road, the egg counts in these fields are likely to be quite high.

The roots of soybean plants that have been infected by SCN are compromised and so they do not function in the same way as roots that are not infected. As a result, oftentimes when there is an SCN problem in a field of soybeans, there is also a nodulation problem (Figure 2). Nodules are swollen portions of the roots of legume plants in which bacteria capable of fixing atmospheric nitrogen into a form that the plants can use live. As there was no evidence inside or outside the roots of root-rotting pathogens or the symptoms they cause, the poor nodulation likely resulted in nitrogen deficiency symptoms.

Figure 2. Plants on the left, which appeared to be growing normally above-ground, show good nodulation, while the plant on the right, that appeared pale-green and stunted above-ground have no visible root nodules.

Collecting soil samples to determine the SCN population density is the next step for both farmers in their respective fields as this will determine how many years they will need to rotate away from a soybean or dry bean crop (the two main SCN hosts grown in northern Minnesota) to a non-host crop or risk significant yield loss due to SCN. Additional information about sampling for SCN egg counts or to determine which forms of varietal resistance to deploy will be shared closer to harvest (after which time sampling is recommended).

Two-spotted spider mites.

Leaves of the plants in the area in which soybeans were growing poorly were also suffering from a two-sotted spider mite infestation which had progressed to the top of the plant. The University of Minnesota has good information about how best to scout and determine when the infestation has reached the treatment threshold and an insecticide or miticide application is warranted.