Keep an eye out for late season soybean diseases

This time of year can be tough to get out into fields very far, but scouting is still very important. With parts of Iowa re­­ceiving heavy rains this spring and consistent rainfalls throughout the summer, soybean diseases are popping up in fields throughout the state.

The main culprits seem to be sudden death syndrome (SDS) and brown stem rot (BSR). Both of these diseases can be devastating to soybean yields when they set in, but the good news is there are options to minimize disease impact through good planning for next year.

Sudden Death Syndrome

SDS is caused by Fusarium virguliforme (formerly F. Solani f.dp. glycines). This fungus survives on infected plant residue and infects the soybean root system early in the growing season. It is not until later in the season, most often after flowering, when the foliar symptoms typically appear.

The fungus on the root system releases a toxin that causes the interveinal necrotic spots to appear and, ultimately, causes the leaflets to drop from the plant.

Wet and cool soil conditions favor infection of the pathogen, and late season rain increases foliar symptom development. Soybean Cyst Nematode (SCN), as well as wet compacted areas of the field, also increase the likelihood of infection. When SDS symptoms appear, there is no treatment to prevent the further spread and destruction of the disease. To manage SDS, focus on selecting varieties that are resistant, work to reduce soil compaction and plant soybeans that have good resistance to SCN. New seed treatments are also available that reduce the impact of SCN and SDS.

Brown Stem Rot (BSR)

Brown Stem Rot is causes by the fungus Phialophora gregata. This pathogen typically infects soybeans early on in the growing season, but external symptoms typically will not show up until pod fill. Wet and cool conditions favor infection and disease development. This pathogen causes leaf symptoms that are very similar to SDS: interveinal chlorosis and necrosis, leaf curling and, ultimately, leaves falling off from the petioles.

To distinguish between SDS and BSR, split the stem open. Plants with BSR will exhibit internal browning of the vascular tissue and pith, whereas the SDS infected plants will not. Variety selection and crop rotation are the best manage practices to utilize to control BSR. In the event of a questionable disease, do not hesitate to contact your local FS crop specialist for diagnosis and recommendations.