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Wednesday, September 18, 2013
While some earlier planted soybeans are starting to yellow and drop leaves in Western Illinois, it is also important to be on the lookout for soybean stem and root diseases. A seemingly unaffected field can actually be disguising plants that are suffering from stem or root diseases. Taking a look below the canopy can reveal plants that have already wilted and died. Disease symptoms indicate that yields may be affected as plants were under additional stress and seeds may have been competing with plant pathogens for the products of photosynthesis.
At the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC), one soybean field that was planted in mid-May, grew a taller and fuller canopy than many later planted soybeans. It was in this field where plants wilted from the stem disease called Sclerotinia stem rot or white mold were observed (Figure). Additional information about the white mold disease cycle, disease symptoms, signs of the pathogen, and disease management can be found here.
Brown stem rot (BSR), stem canker, and sudden death syndrome (SDS) are other stem and root diseases that have been observed in Western Illinois this year. These diseases can have similar foliar symptoms: yellowing (chlorosis) browning or tissue death (necrosis) between leaf veins. Knowing which diseases are present in a particular field can help producers to make disease management decisions in future growing years. The pictures shown above and the information below can help in distinguishing these diseases from one another.
Sudden death syndrome (SDS). SDS of soybean is caused by a fungus (Fusarium virguliforme) that survives in both plant residue and soil. Early planting and cool, wet soils during seed germination favor seedling infection. High pathogen populations can lead to root rot symptoms. Most often, however, infection goes undetected until much later in the growing season. The fungus resides in root tissue and produces toxins which can be transported to leaves. Symptoms in infected plants are most likely to occur after a significant rain event in soybean plants that are beginning to fill their pods (R3). Foliar symptoms can appear very suddenly and include yellowing and browning of leaf tissue between the major leaf veins (Figure). Yield losses due to SDS can be very severe, leading to 100 percent loss in highly susceptible varieties in some fields in certain years.
Brown stem rot (BSR). BSR is caused by a fungus called Cadophora gregata that survives in residue from previously infected plants. The name of this disease is very descriptive in that when plant with brown stem rot are split length-wise, the internal stem tissue (pith) will be brown. Depending upon the genes present in the fungus, foliar symptoms may occur as well, however many plants may be suffering from this disease without exhibiting external symptoms.
BSR is favored by cooler temperatures, which is why this disease tends to be more common in Wisconsin and far Northern Illinois than in central and southern Illinois. The cool temperatures this spring and summer favored the development of BSR in a field in Knox County Illinois (Figure). Plants in this field had both foliar and internal stem symptoms and many plants died (Figures).
Northern stem canker. Northern stem canker is caused by a fungus (Diaporthe phaseolorum var. caulivora) that survives in soybean residue and in infected seed. This disease, which has become more common in recent years, is favored by rainy weather early in the growing season (sound familiar?) as spores of the fungus are rain-splashed onto plants. Although plants are infected early on, symptoms tend to appear during late reproductive growth stages. Symptoms include reddish brown stem lesions, leaf discoloration, wilt, and plant death with leaves that tend to remain attached to the plant (Figure).
Distinguishing among diseases. A good way to distinguish among the different diseases is to dig up and split the plant length-wise from the middle of the stem through the taproot. In plants with SDS, root rot symptoms may be present and the pith (or center) of stems split length-wise remains white. While in those with BSR, the roots are clean and the pith is brown (Figure).
White mold and stem canker both cause external stem lesions and wilted plants. Stem lesions tend to be surrounded by healthy stem tissue above and below the lesions. White mold is characterized by bleached, white, stringy stem lesions (sometime with a white mold growing on it), while stem canker lesions are reddish brown (Figures).
Disease management: SDS. Although there is nothing that can be done in-season to combat SDS, if you have soybean fields that suffer from SDS in 2013, several actions can be taken to lower the risk of SDS-associated yield losses in future growing years. Variety selection can help to reduce disease risk. Soybean varieties vary in their susceptibility to sudden death syndrome. Each year Illinois university researchers evaluate hundreds of commercially available soybean varieties for their tolerance to SDS. This information can be found on the Varietal Information Program for Soybeans (VIPS) website.
The fungus that causes SDS can live inside swollen female soybean cyst nematodes (cysts) and can enter soybean roots through nematode-caused wounds. Consequently, working to reduce populations of the soybean cyst nematode (SCN) can also help to reduce yield losses due to SDS. Lastly, cultural practices that improve soil compaction and drainage can also decrease SDS disease severity.
Disease management: BSR. The pathogen that causes BSR infects only soybeans and needs intact soybean residue to survive. Management practices that favor residue degradation such as crop rotation and tillage can decrease pathogen survival. Managing the soybean cyst nematode (SCN) can reduce disease risk as BSR tends to be more severe in the presence of SCN. Soil pH above 6.5 and good soil potassium and phosphorus fertility can decrease BSR disease severity. Although soybean varieties are not evaluated for BSR resistance through the Illinois VIPS program, seed dealers likely have access to information regarding the susceptibility of locally adapted varieties to the BSR pathogen.
Disease management: Northern stem canker. Crop rotation is recommended in fields with severe northern stem canker symptoms in 2013. Burying soybean residue after harvest can help to degrade the plant tissue on which the pathogen survives. Planting high quality, certified seed treated with seed-applied fungicides can reduce disease. Maintaining good soil potassium fertility based on soil test results can reduce disease incidence. Soybean varieties are not evaluated for Northern stem canker resistance through the Illinois VIPS program, although commercial seed dealers likely have access to information regarding the susceptibility of locally adapted varieties to the stem canker pathogen.
A good resource for additional details about soybean stem, root, seed and foliar diseases can be found at the Plant Health Initiative website. Content of this website is provided by University researchers from 12 Midwestern states and is sponsored by the North Central Soybean Research Program through soybean check-off dollars.