Effect of Infection Timing by Four spp. on Soybean Damping-Off Symptoms with and Without Cold Stress.

spp. cause damping-off of soybean, especially when soil conditions at or shortly after planting are cool and wet. Soybean planting dates continue to shift to earlier dates, so germinating seed and seedlings are exposed to periods of cold stress at a time which favors infection by , and seedling disease occurs. The objective of this study was to assess infection timing and cold stress on soybean seedling disease severity caused by four spp. prevalent in Iowa, namely , , , and . Each species was used individually to inoculate soybean cultivar 'Sloan' using a rolled towel assay. Two temperature treatments (continuous 18°C [C18]; a 48-h cold stress period at 10°C [CS]) were applied. Soybean seedling age was divided into five growth stages (GS1 to GS5). Root rot severity and root length were assessed at 2, 4, 7, and 10 days after inoculation (DAI). At C18, root rot was greatest when soybean was inoculated with or at GS1 (seed imbibes water) and with or at GS1, GS2 (radicle elongation), and GS3 (hypocotyl emergence). After CS, soybean susceptibility to and was reduced compared to C18 for inoculation at all GSs except GS5 (unifoliate leaf emergence). Conversely, root rot by and was greater after CS compared to C18. Data from this study demonstrate that greater root rot, and consequently more damping-off, is likely if infection occurs at early germination stages before seedling emergence.