September is the time to scout for brown stem rot (BSR) and other late-summer diseases. Normally, late planting reduces the level of soybean root diseases seen in the fall, which appears to be true this year. So far, one of the major soybean root diseases in the state, BSR, has not been a severe problem in our research plots, located in northern and central Iowa. As far as white mold, which is prevalent in parts of Iowa, see a separate article in this newsletter for more details.
Brown stem rot (BSR)
This fungal disease appears late in the growing season. One typical symptom is internal stem browning. Some strains of BSR fungus also can cause defoliation in late summer. These strains produce toxins that lead to interveinal necrosis in which tissue between the veins turns brown and dries rapidly, while tissue adjacent to the veins remains green a few days longer. Foliar symptoms of BSR can be misidentified as sudden death syndrome.
Stem symptoms of brown stem rot.
Don't be surprised if you find BSR because this fungus is present in at least 95 percent of Iowa soybean fields. The importance is the severity of the disease. Severe disease causes premature yellowing of leaves, sometimes followed by lodging and defoliation.
Leaf symptoms of brown stem rot.
Recent studies at ISU and other universities show that incidence of BSR normally is higher in reduced tillage fields than in conventional tillage fields. This is because the BSR fungus survives in soybean residues, and the risk of disease increases as the amount of infected residue increases. Therefore, no-till fields should receive more attention in scouting. Management of this disease is relative easy because of the availability of resistant varieties. For management information, please see ISU Extension publication, Brown Stem Rot, Pm-890.
This fungal disease becomes visible in late summer. Normally, it does not show severe symptoms on leaves except in the South, but last year as well as this year we have seen defoliation by this disease in Iowa. Foliar symptoms include reddish veins, rolled leaves, cankers on petioles, and premature defoliation. Stems of infected plants are covered with superficial purple lesions (see photograph). Since this disease is seedborne, do not use seeds from infected plants. The pathogen also survives as mycelium in infected crop debris. Use crop rotation and cultivate patches where the disease was found to prevent disease build-up.
Stem lesions from anthracnose.
Two diseases that have been discussed in the last two months need our continued attention: sudden death syndrome (SDS) and Rhizoctonia root rot. I have visited areas where SDS has been prevalent the past two years, and this disease is appearing at least two weeks later and infection is light. The later the disease occurs, the less is the damage. A detailed description of this disease can be found in the last ICM newsletter (August 19, pages 152-153). We also continue to receive plants with root rot symptoms, often caused by Rhizoctonia. Frequently we see a combination of symptoms with root rot and soybean cyst nematode.
This article originally appeared on pages 162-163 of the IC-476(23) -- September 3, 1996 issue.