Brown stem rot (BSR) of soybean, caused by the fungus Phialophora gregata, has remained a yield-limiting disease in Iowa. The main reasons attributed to this are shorter crop rotations and reduced tillage practices, since the pathogen survives in soybean residue. The most efficient way to manage BSR is with the use of resistant varieties. As growers incorporate better management practices for soybean, they may be disappointed in the yield response if a factor such as BSR is limiting yield, since the yield loss from the pathogen is greatest in high yielding environments. Implementing a multifaceted approach to disease management is important in maintaining the genetic yield potential of soybean and reducing yield losses to plant pathogens such as P. gregata.
Brown stem rot was first reported on soybean in Illinois in 1944. Today, it has become widely established throughout the north central United States and around the world where soybean are grown. It is considered economically important in the Midwest since soybean’s monetary and environmental impact has intensified soybean acreage, reduced tillage practices, and implemented shorter crop rotations. Estimated yield loss from BSR has ranged from 9 to 44% for individual fields depending on environmental conditions, the variety used, and the fungal strain.
Phialophora gregata is a residue-borne pathogen that over winters saprophytically (without a living host) in soybean debris that was colonized parasitically, via the main and lateral roots, during the growing season. Phialophora gregata does not produce any long-term survival structure and does not reproduce sexually. However, P. gregata can reproduce asexually within soybean tissue and can continue to reproduce throughout the winter; influencing inoculum levels in the spring. Severity of BSR and yield reduction has been related to fungal population levels in soil residue since survival is most closely associated with the rate of soybean decomposition.
Disease development is greatest between 59-81ºF. Increasing temperatures decrease disease incidence and severity. As air temperatures approach 81ºF vascular browning is reduced, and little or no disease develops above 90ºF. Leaf symptom development is greatest when air temperatures are high during growth stages R3-R4 and the first foliar symptoms will therefore often first show up at R5. Severity and incidence of BSR is greatest when soil moisture is near field capacity, which is often optimal for crop development. Symptoms of BSR are exacerbated when adequate water is available through early reproductive stages, but is followed by hot and dry conditions.
Phialophora gregata causes a progressive browning of vascular and pith tissues. Internal browning is evident in infected plants when stems are split longitudinally, especially at and between nodes near the soil line (Figure 1). Infected plants do not show foliar symptoms before R3 where sudden interveinal chlorosis and necrosis occurs (Figure 2), followed by curling and leaf wilting. Although both stem and foliar symptoms are caused by P. gregata, discoloration of pith tissue is a characteristic symptom of BSR, with the height of internal stem discoloration as a measurement of severity. Discoloration of vascular tissue should not be considered diagnostic, since other pathogens, can cause the same symptom under similar environmental conditions. Discoloration of the pith tissues however, has only been observed with BSR. Sudden Death Syndrome (SDS) or stem canker can also be confused with BSR since these diseases show similar leaf symptoms.
One of the many challenges with BSR is that symptoms can vary depending on the genotype of P. gregata present. Two forms of the pathogen have been identified genetically which correspond to the severity of symptoms they produce. Genotype A causes significantly more severe foliar symptoms and internal stem discoloration, while genotype B causes internal stem discoloration and only minor foliar symptoms. However, foliar symptoms may fail to develop if seasonal precipitation is below normal. Genotype A is further associated with greater yield losses in susceptible soybean varieties than genotype B, though yield loss associated with infection by genotype B alone has not yet been determined. Foliar symptoms are influenced by soybean genotype, environmental conditions, and pathogen genotype. The use of foliar symptoms alone and as a supplement to stem symptoms can be used to differentiate between soybean genotypes resistant or susceptible to P. gregata.
Making an informed decision in regards to soybean variety, crop rotation, and tillage regime, based on your field’s history, can influence the incidence and severity of BSR. An integrated approach implementing some of the suggestions below can help you effectively manage this disease.
Resistance. Soybean varieties resistant to BSR are the most economical means to reduce yield loss associated with the disease. Although resistance was reported in 1968, progress in improving resistance of soybean varieties has been slow due to the misconception that BSR was inconsequential, inadequate inoculation and evaluation methods to accurately characterize soybean accessions for resistance, lack of phenotypically characterized sources of resistance, and a limited understanding of the genetics of both resistance and pathogen variability for virulence. Today, we have a greater understanding of both host and pathogen, and superior varieties are available to the farmers to manage BSR.
Crop rotation. Continuous planting of soybean will result in inoculant build-up on crop debris which results in greater incidence and severity of BSR when environmental conditions favor disease development. If you have a history of BSR in your field it is recommended that a nonhost crop be grown for 3 or more years to reduce P. gregata population densities. Non-host crops of P. gregata include corn, small grains, and forage legumes.
Producers with a history of BSR in their field should choose soybean varieties with a shorter relative maturity. These earlier varieties may escape the yield reducing effects of BSR in comparison to varieties with later maturity or varieties that are planted later in the season. Earlier planting also takes advantage of reducing the disease in your field. By choosing that early maturity group and planting earlier, you are decreasing the number of days soybean is grown at cooler temperatures (64-75ºF) in August and September, and reducing the severity of BSR.
Research from the University of Wisconsin has found that the incidence and severity of BSR is greatest at low soil pH (below 6.3). As long as you follow ISU extension recommendations on soil pH for corn and soybean then it should not be an issue in Iowa.
Research shows that soil pH may be used to predict the yield-robbing potential of soybean cyst nematode (SCN) and BSR and to guide soybean growers in management decisions to minimize yield loss caused by both pathogens. The following publication from the University of Wisconsin and Iowa State University describes these situations. Soil pH Influences Soybean Disease Potential (pdf)
More information on BSR can be found at the University of Wisconsin Soybean Health web page