Pythium, Phytophthora, Fusarium and Rhizoctonia are soil borne fungal pathogens that infect soybeans as seedling blights. Soybean seedling diseases like these accounted for nearly 6 million bushels lost across the U.S. and Canada in 2023.¹
Pythium, Phytophthora, Fusarium and Rhizoctonia can be difficult to diagnose due to their similar symptoms and how quickly seedlings decompose after infection. Seedlings can also be infected by multiple blights at once, making scouting even more challenging.
Though lab testing seedlings is the only way to conclusively differentiate and diagnose Pythium, Phytophthora, Fusarium and Rhizoctonia, learning when and how to scout for these diseases can help.
In general, one of the first signs of these diseases is plant population loss or reduced stands. We’ve put together a list of additional traits for these three soybean blights to help you improve your scouting skills this season.
How to scout for soybean Pythium seedling blight
Learn when Pythium is most likely to occur and how it manifests in soybean fields.
Disease timing and ideal conditions for soybean Pythium
Soybean Pythium attacks and rots soybean seed and seedlings prior to emergence. This disease thrives in fields with high organic matter and wet, poorly drained conditions, and/or fields that are prone to flooding. The ideal temperature for Pythium infections ranges from 50 F to 60 F.
Visual symptoms of soybean Pythium
- Post-emergence damping off, which can occur under wet conditions
- Brownish, rotting tissue
- Poorly developed seedling roots
- Water-soaked lesions, ranging from tan to dark brown, that appear on hypocotyl or cotyledons and can cover the entire root
- Patchy patterns of infection in fields, most commonly seen in low-lying or compacted areas that are prone to flooding
How to scout for soybean Phytophthora root rot
Below, you’ll find out which conditions favor soybean Phytophthora root rot and which visual signs to scout for in soybean fields.
Disease timing and ideal conditions for soybean Phytophthora
Phytophthora can affect soybeans at any growth stage but is especially detrimental early on, when it can rot seeds before emergence and cause pre- and post-emergence damping-off.
Phytophthora is most prevalent in wet, poorly drained soils and areas prone to flooding.
This soybean seedling disease can infect soybeans at a wide range of temperatures, as long as the soil is wet. Infection can set in at temperatures as low as 50 F, though it typically develops most rapidly above 60 F.
Visual symptoms of soybean Phytophthora root rot
- Soft, tan-to-brown rotted plant tissue
- Water-soaked lesions on seedling stems and roots
- Yellowing and wilting of young seedling leaves
- Post-emergence damping off in seedlings (after V2) that appears as brown discoloration extending up the stem from the root (unlike Pythium damping off, which is found only at the hypocotyl and below²
- Irregular patterns of infection in fields, typically most prevalent in low spots and areas that experience flooding
How to scout for Rhizoctonia blight of soybeans
Learn when Rhizoctonia is most likely to occur in soybean fields, plus where to look and what to look for.
Disease timing and ideal conditions for soybean Rhizoctonia
Rhizoctonia blight can threaten soybean seeds and plants before and after emergence. Most common in seasons that begin wet and end dry, Rhizoctonia infections spread best in humid conditions when soybeans are growing in warm soils with poor drainage and the propensity to flood.
Visual symptoms of Rhizoctonia blight of soybeans
- Wilted plants at midday
- Rotted stems at soil line
- Firm brown or reddish-brown sunken lesions on the root or lower stem
- Irregular patches of infection across the field or affecting entire row sections
How to scout for Fusarium blight of soybeans
Fusarium root rot is another soil borne fungus that can cause damping off, stunted plants, and delayed emergence.
Visual symptoms of Fusarium root rot in soybeans
- Seedling may die or have stunted growth
- Roots have brown discoloration with dark to light brown lesions
- Stem vascular tissue can turn brown causing wilting of the plant
If you suspect your field is infected with Pythium, Phytophthora, Fusarium or Rhizoctonia (or a combination of the three), experts are available to help you accurately diagnose the threat. Reach out to a nearby extension office agent or a seed company professional like your regional BASF representative to learn more about diagnosis and lab testing to confirm which soybean seedling disease is affecting your crop.
You can reduce the incidence of soybean seedling blights by maintaining well drained soils, using quality seed, and using fungicide seed treatments such as Obvius® Plus.
________________________________________________
Endnotes
- Sikora, Ed, et al. “Soybean Disease Loss Estimates from the United States and Ontario, Canada — 2023.” Crop Protection Network, 14 Mar. 2024, cropprotectionnetwork.org/publications/soybean-disease-loss-estimates-from-the-united-states-and-ontario-canada-2023. Accessed 19 Jan. 2025.
- Giesler, Loren J. “Phytophthora Root and Stem Rot.” CropWatch | Nebraska, University of Nebraska–Lincoln, cropwatch.unl.edu/plant-disease/soybean/phytophthora-root-and-stem-rot/. Accessed 19 Jan. 2025.
