About Soybean Sudden Death Syndrome
Soybean sudden death syndrome (SDS) is caused by a soilborne fungus called Fusarium virguliforme. This fungus survives by overwintering in diseased plant debris or in soil. The fungi’s chlamydospores are able to survive in harsh conditions including freezing and desiccation. In the spring, rising soil temperatures support germination of the chlamydospores. Any spores near soybean roots will attempt to infect the plant. Aerial symptoms of SDS appear at around mid-July, but roots can become infected within one week after crops emerge. In wet soil the fungus will produce toxins in the roots, which are then transmitted to the leaves of the plant. These toxins are responsible for the foliar symptoms of this disease. As plants become defoliated, infected leaves will allow spores to rest in the plant residue and soil, leaving them to overwinter for next season.
Soybean Sudden Death Syndrome Identification and Habitat
Initial symptoms of SDS include browning and defoliation of leaves in the upper canopy. Early on, leaves will experience mosaic and mottling. As the disease develops, leaf tissue around veins will turn yellow, then browns and falls off. Following this, death of leaflets occurs, leaving petioles attached. When symptoms occur, they may be limited to a number of small areas in the field. Usually these are wet and compact areas where spores overwintered from last season. In the following weeks these areas may grow larger, spreading disease to surrounding plants. On a more diagnostic note, examining the interior of the lower stem of a symptomatic plant can distinguish SDS from other related diseases. The lower stem and taproot of an infected host will show a light brown discoloration, making the disease different from a disease like brown stem rot for example.
SDS is favored by cool to moderate air temperatures (50-80°F) and extended wet weather. Below 60°F, the fungi are able to germinate quickly and effectively. Long periods of wet weather are important for the pathogen to be able to germinate and be dispersed to nearby plants. High density fields with poor drainage are highly susceptible to fungal growth and rapid spread of disease from plant to plant.
Soybean sudden death syndrome Management and Control Methods
Choosing to plant soybean crops earlier in the spring, before extended rain periods occur. As long as the early spring conditions are favorable, soybean crops can experience rapid growth, reducing the risk of SDS infection. Furthermore, practicing tillage can be useful because it improves water permeability and soil compaction. However, there is variability in eliminating SDS using tillage, depending on the type of soil being maintained. Avoid planting highly vulnerable seed strains to limit yield losses. Throughout the season, removing dead plant materials from the soil can help limit spread of the disease and reduce the chances of spores overwintering for next season.
Foliar fungicide is not effective at controlling SDS because the fungi infect plants through their roots. For this disease, cultural controls are the best way to reduce incidences of infection.