Blog

Growers are learning that planting earlier soybeans can bump yields, but the last several years in Illinois have been wet and cold and that comes with a cost: the risk of infection from Sudden death syndrome (SDS) and other early infecting fungal diseases. 

 

We see soybeans start to yellow too early and want to automatically assume we have SDS in our fields, but that’s not always the case. There are a few other soybean diseases that look very similar to SDS and we should properly identify what is occurring in our fields. Some diseases that could cause early yellowing: 

Causes and Symptomology of SDS

Let’s start with what SDS is and how it infects the plant. SDS is caused by a soil inhabiting fungus, Fusarium solani (virguliforme). It infects the soybean plant very early in life, as early as emergence, but doesn’t show its symptoms until much later when it releases a toxin that damages the leaves. We have seen this disease grow over the past several years because soybeans have been planted in unfavorable conditions:

Symptoms start to show mid to late summer when saturated soils allow toxins to be produced by the fungus in the roots of the plants which are then translocated to the leaves. This easily recognizable disease is still contained in the roots despite the symptomology in the above portions of the plant. Soybean cyst nematodes (SCN) can aid in the infection of SDS. The damage to the roots helps create infection points that are more easily accessed by the SDS fungus. 

 

Causes and Symptomology of BSR

Brown stem rot is caused by Phialophora gregata, a fungus that survives in soybean residue. Like SDS the BSR fungus infects roots earlier in the season but shows symptoms later. Infection rate is higher with excessive moisture and SCN can also increase infection rate. 

 

SDS vs. BSR

It is often challenging to distinguish between SDS and BSR, but there are some key things to note. Digging up the roots is the best way to properly identify the two diseases in the field. Upon digging, a blue discoloration on the outside of the taproots may indicate an SDS fungal colony of spores. You should not see these blue-ish spores with BSR. Next, split the stem and examine the color of the cortex and pith of the lower 8” of the plant. A grayish-brown cortex with a white, healthy looking pith will signify that SDS has infected the plant. A dark and rotted pith would be sign of disease from BSR. 

 

New Disease on the Block

There is also a new disease that has shown up this year, making the game of “what’s wrong with my soybean field” even more challenging. Red crown rot (RCR) is caused by the fungus Calonectria ilicicola. RCR will overwinter in the soil and can essentially lay dormant for years without a host. Like the other two diseases discussed, you can find this more prevalent in low-lying, saturated areas of a field. This disease can be distinguished by color on the lower portion of the plant, a rusty-red spore-bearing structure will be noticed on plants infected with RCR. Infected plants are often easily pulled from the soil because of the pest’s damage and how they decay the root system. 

 

Once the disease is noticed, there is unfortunately not much one can do to reverse the effects of any yield loss or plant damage. The best management practice for these types of disease is to plant into ideal conditions and manage drainage for fields that tend to be more saturated. Seed treatments are an effective way to help control these diseases as well, especially when planting into a field that has showed history of these diseases. 

 

Before you diagnose a field showing symptoms in the leaves similar to SDS, make sure to do some investigative digging, literally, to help determine the proper pest. 

 

ISA is funding a research project at the University of Illinois to learn more about red crown rot and its impact on soybeans in Illinois. Researchers are looking for samples of RCR, so if you find it in your fields learn how to send a sample to researchers here: http://cropdisease.cropsciences.illinois.edu/?p=1220.