Start Scouting Beans for Damping Off

Alison Robertson, Department of Plant Pathology; and Erin Hodgson, Department of Entomology
 

Damping off severely impacts stand

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 pathologist's 'happy place'. Stand was severely affected (Figure 1). Both pre-emergence and post-emergence damping off (Figure 2) was prevalent. Seed, barely emerged 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 (Figure 3).

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.

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.

Figure 1. Soybean stand severely impacted by damping off
 


Figure 2. Damped off soybean seedlings

Figure 3. Soft brown watery rot of hypocotyl of a damped off soybean seedlings

 

Seed treatment fungicides and insecticide significantly improve emergence in early planted soybean demo

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 (dap). We took seedling counts at 28 dap. In the Pythium-inoculated plots, 42 percent of the untreated seed emerged (Figure 4) compared to 72 percent to 89 percent (Figure 5) for the treated seeds. In the F. virguliforme-inoculated plots, emergence was 28 percent for untreated seed and 8 percent to 89 percent for treated seed. You don't need much more evidence that seed treatments pay for early planted beans particularly when it is cold and wet.

Figure 4. Untreated soybean seedlings planted into a microplot inoculated with Pythium spp.
 


Figure 5. Treated soybean seedlings planted into a microplot inoculated with Pythium spp.

Still looking for fields with damping off

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 a field.

Alison Robertson is an assistant professor in the Department of Plant Pathology with extension and research responsibilities; contact at alisonr@iastate.edu or phone 515-294-6708.  Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at ewh@iastate.edu or phone 515-294-2847.