Taproot decline of soybean (TRD), is an emerging fungal disease affecting soybean production in the southern US TRD has been reported in several states, including Alabama, Arkansas, Louisiana, Mississippi, Missouri and Tennessee. To date, TRD been reported to cause annual yield losses ranging from 0.2% to 1.5%, with the greatest losses in Louisiana (1.18 million bushels) and Arkansas (0.57 million bushels) 1, 2. The emergence of the pathogen is currently under investigation, but historical specimens found in the region support the idea that the fungus has been present for decades, but its pathogenicity is only recently being characterized 3, 4. The symptoms of the disease can be confused with many other maladies of soybeans, and this may be the reason why it eluded researchers for so long.

Symptoms of TRD are generally most notable during the beginning of pod fill to full seed growth stages (R3-R6), during which small clusters of infected plants exhibiting foliar yellowing (interveinal chlorosis) and death (necrosis) are observed (Figure 1). Dead plants are usually found in the center of the cluster (Figure 1A). Infected plants are frequently in contact with leftover crop debris from the previous year, which often serves as the source of infection. Infected plants are also commonly surrounded by plants that appear healthy and do not exhibit symptoms. When infected plants are roughly pulled from the soil, stems often break at the crown. However, when infected plants are carefully removed from the soil, roots exhibit rot and are typically embedded with black fungal tissue (stroma) along the primary root and less frequently on lateral roots (Figure 2). When roots are split longitudinally, white cottony fungal growth (mycelia) can often be evident within the pith. The foliar symptoms of TRD can be confused with many other soybean maladies, including chemical phytotoxicity, which is typically from the demethylation inhibitors (DMI, triazole); red crown rot; decline due to root-knot nematodes; southern blight; stem canker; and sudden death syndrome. The fungus produces reproductive structures (stromata, Figure 3A-C), sometimes referred to as “dead man’s fingers,” on infected soybean roots and debris. These reproductive structures are usually white at the tip but are sometimes pinkish when young and turn black at maturity. Stromata usually develop on infected soybeans, soybean debris and additional plant debris; these structures can be quite small and easy to overlook (up to 1 inch in length). In general, the appearance of reproductive structures occurs following periods of frequent rainfall that coincide with the soybean crop lapping the row middles. In conjunction with the aforementioned foliar, root and stem indicators, stromata can be used to help identify the presence of TRD.