By Alison Robertson, Gary Munkvold and Leonor Leandro, Department of Plant Pathology and Microbiology
From 2011 through 2013, surveys of soybean seedling disease were done across the North Central Region. The surveys were part of larger research studies to improve our knowledge of stand loss in soybean due to seedling disease. This research is ongoing and is funded by USDA-NIFA, the United Soybean Board, the North Central Soybean Research Program and the Iowa Soybean Association.
Several species of Pythium and Fusarium cause seedling blight of soybean.
Approximately 20 species of Pythium were recovered from diseased soybean seedlings in Iowa. Across the North Central region, more than 50 species of Pythium were recovered. At least 12 species of Fusarium were recovered from diseased seedlings in the same area. The four most prevalent species were F. oxysporum, F. graminearum, F. solani and F. acuminatum.
Both corn and soybean are susceptible to the prevalent Pythium and Fusarium species.
Growth chamber studies indicated that species of Pythium recovered from soybean were pathogenic on both corn and soybeans (Figure 1). The severity of soybean root rot caused varied amongst Fusarium species (Figure 2). In a separate study, Pythium species recovered from corn were also pathogenic on both crops. Similarly, many Fusarium species are able to infect both corn and soybean. Consequently, crop rotation may not be an effective management option.
Figure 1. Effect of temperature on pathogenicity of four species of Pythium recovered from soybean on corn (purple bars) and soybean (blue bars) at 73 degrees F and 55 degrees F.
Figure 2. Root rot severity caused by different Fusarium species. FG = F. graminearum, FS = F. solani, FA= F. acuminatum, FV = F. virguliforme, FSP = F. sporotrichioides, FP = F. proliferatum, FO = F. oxysporum, FSE = F. semitectum.
Some Pythium like it cool; others like it warm.
Research at Iowa State University and Michigan State University showed that some species of Pythium are pathogenic at cooler temperatures (~55 degrees F), while others are pathogenic at warmer temperatures (64 and 73 degrees F) (Figure 1). Thus, the prevalent species causing seedling blight likely depends on the soil temperature soon after planting.
The sensitivity of Pythium and Fusarium to seed treatment fungicides varies.
The sensitivity of the most prevalent species of Pythium and Fusarium to active ingredients found in commercial seed treatments was tested in lab and growth chamber assays. Some species were sensitive to the fungicide at rates similar to those found on seed, but the same rate of active ingredient had little effect on other species. For example, fludioxonil has excellent activity against F. graminearum, but other species were less sensitive to this fungicide in the laboratory. This suggests that a combination of fungicides with complementary activity against the spectrum of Fusarium spp. would be most effective.
In general, seed treatments that contain a mix of fungicides with activity against Oomycetes, Fusarium spp. and Rhizoctonia are more likely to reduce stand loss from seedling disease.
New publication available
A new publication "Scouting for Pythium root rot in soybean," CSI0024, is now available as a PDF file from the ISU Extension and Outreach Online Store.
Are you interested in learning more about seedling diseases of corn and soybean caused by Pythium? Two webinars are available in the Plant Management Network's 'Focus on Soybean' and 'Focus on Corn' resources. Join Dr. Jim Kurle, assistant professor of plant pathology at the University of Minnesota, to hear about "Oomycete Diseases of Soybean and Current Management", and Dr. Martin Chilvers, assistant professor of plant pathology at Michigan State University, for his presentation "Seedling Diseases of Corn."
Alison Robertson is an associate professor of plant pathology with research and extension responsibilities in field crop diseases. Robertson may be reached at (515) 294-6708 or by email at firstname.lastname@example.org. Gary Munkvold is a professor of plant pathology and seed science endowed chair in the Iowa State University Seed Science Center with research and teaching responsibilities in seed pathology. He can be reached at (515) 294-7560 or by email at email@example.com. Leonor Leandro is an associate professor in the Department of Plant Pathology and Microbiology. He can be reached at (515) 294-8855 or by email at firstname.lastname@example.org.