Arkansas, USA
July 1, 2015
Source: University of Arkansas Research & Extension
by Terry Spurlock, Extension Plant Pathologist
Figure 2. A stunted soybean plant with yellow chlorotic leaves typical of the “mystery root disease”.
I have visited a number of fields in Drew, Desha, and Chicot Counties this year and last year that have a root rot disease. Plants appear stunted and the leaves are yellow between the veins. Some plants are dead or exhibit yellow foliar symptoms with necrotic “flashing” (Fig. 2). When the plants are removed from the bed, if the taproot is extracted and still attached to the plant, it is dead or dying, soft, and black (Fig. 3). Sometimes much of the taproot is absent. The distribution of the disease varies from sporadic to a few consecutive diseased plants every 10-15 yards of row (approximately). This disease has been observed on soybeans in Louisiana and Mississippi and the symptoms in each state are very similar. Yield loss in Arkansas fields in 2014 was calculated to be 30% on affected plants. However, due to the sporadic nature of the disease in most fields, the yield lost in the entire field due to this disease appeared to be negligible.
Figure 3. A soybean taproot infected with the “mystery root disease” with black growth evident and in obvious decline.
Experimentation is ongoing to determine the causal organism, characterize the disease, and determine management options. A cooperative effort is being made between all three states to better understand this disease. Because this is a disease of the roots, fungicide sprays are not recommended and would likely not be effective at this time. This disease is likely being incorrectly identified as sudden death syndrome caused by a fungus, Fusarium virguliforme. While the foliar symptoms may appear similar to some, the flashing of SDS typically occurs later in the year during late reproductive stages of development. The root symptoms are also different as the black growth on the taproot is not commonly associated with SDS. Additionally, another disease of soybean, which has been found in Arkansas, is black root rot caused by Thielaviopsis basicola. This particular disease was found in 2008 in a field in Phillips County and described in the scientific literature in 2010. The description of that report can be found at http://www.apsnet.org/publications/plantdisease/2010/September/Pages/94_9_1168.1.aspx . The same fungal pathogen also causes black root rot in cotton. Black root rot is found in cooler wet soils, typically earlier in the year soon after the plant emerges. Numerous plant and soil assays have been conducted in Arkansas and Mississippi soybeans with little to no evidence of association by T. basicola with the “mystery root disease”. - See more at: http://www.arkansas-crops.com/2015/07/01/disease-sorghum-soybeans/#sthash.wkWwh7q8.dpuf
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