Soybean Fungal Disease Outbreak Reported

In the agricultural world, soybean rust (SBR) is a significant concern for farmers, particularly in regions with favourable climatic conditions. This microcyclic rust, caused by the pathogen Phakopsora pachyrhizi, is an obligate parasite that cannot survive outside of living host tissue.

SBR thrives in wet conditions and temperatures between 60 to 78 degrees Fahrenheit, making the southern United States an ideal environment for its spores to germinate. The disease is most often found in the lower canopy at or after the flowering stage and can rapidly progress upward into the upper canopy.

To combat SBR, two classes of fungicides - strobilurin and triazole - are currently recommended. If SBR is identified in Ohio prior to the R5 growth stage, a fungicide application would be recommended to prevent potential loss in yield. However, research shows that fields with the greatest yield loss did not receive a fungicide application or were treated too late.

The Ohio Department of Agriculture is the organization responsible for monitoring the potential risk of soybean rust in the state. They, along with land grant universities and the United Soybean Board, maintain a network of sentinel plots across the United States. These plots are scouted weekly for SBR and act as an early warning system for growers in that region. Observational data collected from these plots is uploaded to a central site, creating real-time maps showing rust activity across the country.

Symptoms of SBR first appear as small tan to brown and brick red spots on leaves. Within four days after infection, these spots can be visible, and pustules will appear within 10 days. Urediniospores, the only spore stage known to infect host plants, can be seen within these pustules on the underside of leaves using a 20x hand lens. They are pale yellow-brown to salmon colored and have some ornamentation on the surface.

Pustules produce spores for approximately three weeks. After this period, teliospores, black spores produced in old lesions, may appear. However, their contribution to the disease cycle is currently unknown.

Historically, hot and dry summers in the southern United States have slowed disease progress until cooler conditions prevail later in the growing season. However, an incident in 2012 in Alabama serves as a stark reminder of the potential impact of SBR. An area of over 200 hectares of soybean reported losses of over 60 percent due to SBR.

From 2005 to 2016, soybean producers in Ohio have not had to consider managing SBR. However, careful and diligent scouting of soybean fields and kudzo patches in the southern states is important for determining the threat of SBR for Ohio and other Midwestern states.

Monitoring systems have been used to detect the presence of SBR inoculum in soybean growing regions in the south. With a better understanding of this disease and the implementation of effective monitoring and control measures, farmers can take proactive steps to protect their soybean crops from the threat of soybean rust.