The First Indications Of An Epidemic Year For Sudden Death Syndrome?

 

A couple of weeks ago during my morning commute to work between Galesburg and Monmouth a patch of soybeans about 20 feet from the highway was yellow enough to be seen while driving more than 55 mph. Last Thursday, I found a spot to park along the side of the highway to try and figure out what may be causing these symptoms.

 

Narrowing down a disease diagnosis. At eye-level (about 6 feet up from the level at which I would have been viewing the plants from my car) the patch of discolored soybeans appeared to be much larger than anticipated and the discoloration had turned from a yellow to a brown-ish color on some of the leaves (Figures). Upon closer inspection, the yellow and brown discoloration between the main leaf veins were symptoms that suggested diseased stem or root tissue.

 

Looking at the stem: inside and out. Further examination of both the external and internal stem tissue helped to narrow down a diagnosis. The tissue on the outside of the stem appeared to be the typical green color that one would anticipate seeing at this point in the growing season. Some diseases that can sometimes result in symptoms similar to those observed in this field are identified by brown discoloration at leaf nodes or at the base of the stem from the soil line. One disease that is associated with these leaf symptoms can be identified by looking at the inner-most stem tissue (pith). Plants suffering from brown stem rot have pith tissue that is brown in color, particularly toward the bottom half of the stem and near leaf nodes, while plants that have sudden death syndrome (SDS) have healthy pith tissue (Figure). Splitting stems in this field indicated that plants were likely suffering from SDS.

 

Below-ground symptoms and signs. I didn't have my shovel with me and the dry soil conditions were not cooperating to release soybean roots and so an examination of the taproot was not possible. However, sometimes there can be symptoms inside the root and signs of the pathogen on the outside of the root. Brown discoloration inside and bluish fungal growth on the outside of the taproot can help to solidify the SDS diagnosis.

 

Weather conditions that favor SDS. Weather during the 2016 growing season has been favorable for the development of SDS: cool, moist soils after planting and frequent rains ever. Iowa State University researcher Dr. Daren Mueller found that widespread SDS epidemics tend to occur in those growing seasons with above-normal rainfall totals in June and July. At the Northwestern Illinois Ag R&D Center, 5.46 of rain fell in June and 7.52 inches fell in July – 1.1 and 3.31 inches more than the 30 year averages for June and July, respectively.

 

Fusarium virguliforme infects roots of soybean seedlings very early in the growing season but foliar SDS symptoms don't typically appear until plants reach reproductive growth stages. Foliar symptoms begin with a yellowing of the tissue between leaf veins. This tissue then dies, becoming brown in color with only the leaf veins remaining green. Leaves eventually fall off, while petioles remain attached to the main stem. The earlier that symptoms develop and leaf drop occurs, the greater the potential for yield loss.

 

Although the most conspicuous symptoms of SDS occur in leaves, the fungus itself remains in the roots and in the stem nearest the soil line. Foliar symptoms are caused by toxins produced by the pathogen. These toxins are carried along with water to leaves through the xylem tissue. The SDS disease cycle has important implications as far as management is concerned. Infection and colonization have long since taken place and there are no mid-season management tools with which to manage this disease. Management decisions must be made before the growing season begins.

 

Variety selection is key. The best way to manage SDS is to plant varieties with high levels of disease resistance. Soybean varieties vary considerably in their level of genetic resistance. Seed companies typically provide SDS resistance ratings. Over the last several years, in an effort to provide impartial SDS resistance ratings to producers, teams led by Drs. Jason Bond of Southern Illinois University and Silvia Cianzio of Iowa State University evaluated hundreds of soybean varieties (MGs 0 to V) from many seed companies. Results from the 2015 trials are available here.

 

The University of Illinois's Northwestern Illinois Ag R&D Center is participating in the SDS commercial variety trials this year. We are evaluating 350 varieties from maturity groups 2.0 to 3.9. Results of these trials will be compiled and released this fall in time for producers to use while making their 2017 seed purchases.

 

Research has also shown that SDS may be more severe in fields that also have high populations of the soybean cyst nematode (SCN). Monitoring SCN populations and planting SCN-resistant soybean varieties can also be important components to managing SDS.

 

Soybean seed treatments. The newest tools available for managing this disease are fungicidal seed treatments labeled specifically for SDS. University-based plant pathologist throughout the Midwest conducted several uniform SDS seed treatment trials. In these trials, the active ingredient in ILeVO (fluopyram, marketed by Bayer Crop Science) showed efficacy against SDS. Research has shown that while some disease will always be present, the largest yield responses with ILeVO occur when disease pressure is highest.

 

As the season progresses and we near harvest, check out the Northwestern Illinois Research Center's Website and Blog for results from the 2016 SDS Commercial Variety Test Results, our 2016 SDS seed treatment x cover crop trial and other research trials at the center.