Research Highlights

Research Highlights
Temperature Affects Aggressiveness and Fungicide Sensitivity of Four Pythium Species that Cause Soybean and Corn Damping Off

by Alison Robertson, soybean plant pathologist, Iowa State University

Pre- and post-emergence damping- off, caused by various species of Pythium, can significantly reduce plant growth and crop yields due to seed and root rot and injury to the emerging stem.

Pythium species are oomycetes and require saturated soil conditions for the production and movement of zoospores that are attracted to roots, where infection occurs. Therefore, damping-off is influenced considerably by weather conditions in the spring, and is more prevalent when soil temperatures are cool and abundant rainfall has occurred. Pythium is primarily managed using fungicide seed treatments.

With checkoff funding provided by the North Central Soybean Research Program, Iowa Soybean Association, United Soybean Board, and the Iowa Corn Promotion Board, we undertook a project to increase our understanding of the biology of Pythium on soybean and corn.

We began by isolating Pythium species from diseased corn and soybean seedlings, and identified the four most prevalent Pythium species. These were Pythium torulosumP. sylvaticum, P. lutarium and P. oopapillum. We tested the pathogenicity of these four species on corn and soybean, and found that isolates recovered from soybean were pathogenic on corn and isolates recovered from corn were pathogenic on soybean in each test.

These observations confirm other reports of various Pythium species infecting both soybean and corn in Iowa and the Midwest, and indicate that a corn-soybean rotation will not reduce risk of Pythium damping off disease.

Percent root rot severity caused by Pythium species recovered from soybean or corn at three different temperatures:
Pythium lutarium illustrated in A-C; Pythium oopapillum in D-F;Pythium sylvaticum in  G-I; and Pythium torulosum in J-L

Temperature affects the aggressiveness of Pythiumspecies

We tested the pathogenicity of the four species of Pythium at three temperatures representing soil temperatures at planting in Iowa. 

We found that P. torulosum was significantly more aggressive at lower temperatures of 55°F (13°C), while P. sylvaticum was more aggressive at the higher temperatures of 64° and 73°F (18 and 23°C).  However, temperature did not affect the aggressiveness of the remaining two Pythium species:  P. lutarium and P. oopapillum.

These data indicate that different species have different niches with regards to temperature — suggesting that the soil temperature at planting will determine the degree to which specific Pythium species would cause greater severity to a germinating soybean or corn seed.

Therefore, in some pathosystems, adjusting planting date may reduce disease risk. However, our data showed that while adjusting planting date may be possible for certain species of Pythium, such as P. torulosum and P. sylvaticum, this practice would not be useful for other species, such as P. oopapillum and P. lutarium.

Fungicide sensitivity affected by temperature

We found that P. sylvaticum, which is more aggressive at warmer temperatures, was dramatically less sensitive to all fungicides evaluated at those warmer temperatures compared with cooler temperature. Likewise, P. torulosum, which causes greater disease severity at the cooler temperature, was significantly less sensitive to all fungicides at that cooler temperature than at the warmer temperatures.

We know from previous research that Pythium species on soybean, corn and other crops can vary in their sensitivity to fungicides. However, our research is novel in that we report that fungicide sensitivity of two of the most prevalent Pythium species in our survey, P. sylvaticum and P. torulosum, was affected by temperature.

Our observations of fungicide sensitivity-temperature interactions suggest that seed treatment “failures” in the field may be a result of fungicide insensitivity of certain pathogenic Pythium species at planting. Research investigating the interactions between pathogens causing damping-off on soybean and corn as well as their sensitivity to fungicide mixtures is in progress.

Risk management considerations and recommendations

The results of this study greatly expand our knowledge of damping-off of soybean and corn caused by the most prevalent species of Pythium in Iowa. Since we found that pathogen aggressiveness and fungicide sensitivity of some species can be significantly affected by temperature, this may compromise the effectiveness of traditional management practices such as rotation, planting date, or use of a seed treatment.

Fields with a severe history of damping-off and plant stand issues may need extra management tactics, which should include improving drainage, reducing compaction, avoid planting before heavy rains, and considering a seed treatment.

Published: Feb 15, 2018

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Oomycete Species Associated with Soybean Seedlings in North America Part 2
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