Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Two issues that are increasing in prevalence in soybeans are Sudden Death Syndrome (SDS) and Soybean Vein Necrosis Virus (SVNV). Both of these diseases are fairly new to the area and have been becoming increasingly prevalent in soybean growing regions in the United States. It is important to properly diagnose these diseases as they are very different and have different implications on future planting and disease management practices.
SDS
SDS is caused by a fungus (Fusarium vurguliforme- yes another Fusarium causing problems in our crops). The fungus is fairly unique as it is blue in color, making it fairly easy to diagnose if it is present on symptomatic plants. This disease starts in the soil, where the fungus overwinters either in residue, as thick walled resting spores, or even in cysts of the Soybean Cyst Nematode. SDS infects soybeans early in the growing season, often within the first 1-2 weeks of emergence. Cool, wet weather favors infection by the fungus. After the fungus infects the plant roots The SDS pathogen remains fairly inactive in the lower roots and stem of the plant until after flowering. Wet and warm conditions during the reproductive phases of soybean growth cause the fungus within infected plants to produce toxins which move up the plant and enter the leaves. The toxin builds up in foliage, causing necrosis between the leaf veins and defoliation. The internal tissue of the lower stem and roots will be brown when compared to healthy plants and if you are lucky, blue fungal growth may be observed at the soil line or on the roots. The blue fungus is diagnostic for the disease.
As far as management- avoiding early planting dates, selecting a variety with good tolerance to SDS, and avoiding compaction are key factors to consider when planting soybeans into fields with a history of SDS. SDS is not likely to be an issue in double crop soybeans.
Sudden Death Syndrome in soybeans. A) When viewed from afar looks like patches of plants with brown, dry upper leaves. B) When you approach the plant you may notice the foliage has odd yellow and brown patches between the veins. C) If you pull out a symptomatic you may observe blue fungal growth. This is the SDS pathogen. When you cut open the lower stem the internal tissue will be brown. D) Affected leaves will have yellow/brown interveinal discoloration and the veins remain green. When the leaves fall off the petioles often remain intact. Often the upper foliage appears the most affected by the toxin. Photos by N. Kleczewski
Soybean Vein Necrosis Virus (SVNV)
SVNV is a newly characterized virus that we really know very little about. What do we know? It is the most prevalent virus in soybeans. It is vectored by thrips. It seems to have a broad host range that includes some of our common weeds such as Ivyleaf Morning Glory. We don’t have a handle on yield impacts, but we do know that it can cause defoliation of the upper canopy in susceptible varieties and losses of 2-4 bushels/A are not unheard of. This year we have been conducting field surveys funded by the Delaware Soybean Board to determine the prevalence of SVNV, to gather information on factors that may influence its incidence, and potentially obtain some yield loss data. Without going too far into the current data, we have made over 100 site visits on dozens of fields across Delaware, and SVNV is present in nearly every field surveyed, to some degree. Therefore it’s likely that you are seeing it in your fields. For more information on SVNV check out a blog article I wrote on the subject last year: http://extension.udel.edu/fieldcropdisease/tag/svnv/.
Due to the symptomology it is easy to attribute SVNV to other common diseases such as Septoria brown spot, Bacterial blight or pustule, Downy mildew, and even SDS. Symptoms of SVNV may occur anywhere in the canopy but typically you will see it on select leaves in the canopy. Symptoms start out as a light discoloration in parts of the leaf followed by a chlorosis that may or may not follow leaf veins and contains “pock holes” from thrips feeding. Lesions eventually turn brown/red with the pock holes turning black. We currently do not have any management for SVNV nor do we quite understand the pathogen and how it works. Just know that if you have SVNV, fungicides will not impact symptoms.
Soybean Vein Necrosis Virus (SVNV). Note that often lesions are found on or around veins, where thrips often feed. Lesions start out as a pale discoloration that turns yellow then brown/red. One or several leaves may be affected on a plant. Photos by N Kleczewski