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Soybean Seedling Disease: What Have We Learned?

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. The severity of soybean root rot caused varied amongst Fusarium species. 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.

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). 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.

Source: Iowa State University Extension and Outreach 

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