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Start Scouting Soybeans For 'Damping Off'

Start Scouting Soybeans For 'Damping Off'

Scouting fields now for symptoms of "damping off" of soybean seedlings will help growers make better informed seed treatment decisions in the future. This disease problem can severely impact soybean stands.

Now is the time to scout soybean fields for signs of "damping off." By looking for symptoms of this disease problem this year, "the knowledge you gain can help you make better decisions in the future, regarding which soybean seed treatments would be better for you to use," says Alison Robertson, an Iowa State University Extension plant pathologist.

Damping off severely impacts soybean stands. Robertson, along with ISU Extension entomologist Erin Hodgson, provides the following insights, observations and recommendations.

Field was a farmer's nightmare, but a plant pathologist's dream

Damped off soybean seedlings

This past Friday, we drove to western Iowa to collect damped-off soybean seedlings for a research project (see Fields with damped off seedlings required for research project). The field we visited was a farmer's nightmare and, dare I say, a plant pathologist's "happy place." The soybean stand was severely affected. Both pre-emergence and post-emergence damping off  was prevalent. Seed, along with barely emerged soybean seedlings and seedlings at growth stages VC and V1 were rotted or yellowed and wilted.

A soft, watery-brown rot was common on the hypocotyl and roots of affected seedlings.

It is difficult to tell from the symptoms exactly what pathogen is the cause of this damping off, however, since this field was planted in early May and the soil has very high clay content and is prone to flooding, we suspect Pythium species. Isolations will be made in the lab to identify the causal organism.

Grower didn't use seed treatment fungicide on these beans

The grower who owns this field chose not to use a seed treatment fungicide. Based on the cropping history, soil type and disease history, this was probably a good candidate field for a seed treatment, however.

Seed treatment fungicides and insecticide on the seed significantly improved emergence in our early-planted soybean demonstration.

In July, we will be doing a workshop session on soybean seed treatments at the Crop Management Clinic at the ISU Field Extension Education Laboratory near Boone. In preparation for the clinic we planted microplot demonstrations. We inoculated half the microplots with Pythium species, and half with Fusarium virguliforme, the causal organism of SDS. Four commercial seed treatments, one seed treatment in development and an untreated check are show-cased. The plots were planted April 15, just as the cold, raining period set in.

Emergence occurred approximately 24 days after planting. We took seedling counts at 28 day after planting. In the Pythium-inoculated plots, 42% of the untreated seed emerged compared to 72% to 89% for the treated seeds. In the F. virguliforme-inoculated plots, emergence was 28% for untreated seed and 8% to 89% for treated seed.

Our conclusion: You don't need much more evidence that seed treatments pay for early planted beans particularly when it is cold and wet.

ISU specialists still looking for farmers' fields with damping off

Robertson adds: "I am still looking for soybean fields with damping off problems. I need to collect 50 symptomatic seedlings from the field. Please contact me by email alisonr@iastate.edu or call 515-294-6708 if you know of such a field."

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