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3/17/2008 - 3/23/2008

Foliar Fungicides Publication Available

Daren Mueller, Extension Plant Pathologist

If the risk of soybean rust development is high and the crop is before growth stage R6, fungicides may be needed. For fungicides to be most effective, they need to be applied as close to the time soybean rust initially infects plants in a field.

In most cases, this means that fungicide decisions will have to be made at a time when the risk is high, but before the time when rust is detectable in the field. Detecting low levels of soybean rust (<10 percent) typically requires incubating leaves and observing them under laboratory conditions by a trained diagnostician.

A bulletin titled Using Foliar Fungicides to Manage Soybean Rust was originally published in 2005 and recently was updated and expanded. This book reviews the factors involved in making fungicide spray decisions and basic fungicide information, including mode of action, application and use strategies. An updated list of available products also is posted in Appendix B.

This book was prepared by soybean pathologists working in many regions of North America. This full-color publication contains numerous illustrations, photographs, maps, tables, and charts, as well as a glossary of terms and a list of sources for additional information.

Daren Mueller is an extension specialist in the Department of Plant Pathology.

Be Careful with Susceptible Soybean Variety in Rotation to Manage SCN

By Greg Tylka, Department of Plant Pathology

The soybean cyst nematode (SCN) can be managed effectively by growing SCN-resistant soybean varieties. There currently are more that 700 SCN-resistant soybean varieties available in maturity groups 1, 2 and 3 for Iowa growers. Information on the varieties is available in ISU Extension publication Soybean Cyst Nematode-Resistant Soybean Varieties for Iowa (PM 1649, pdf).

Although not 100 percent effective at preventing reproduction of the nematode, SCN-resistant soybean varieties usually prevent increases in SCN population densities and can even decrease the nematode’s numbers throughout a growing season. But since some nematode reproduction occurs on resistant varieties, there is the potential for an SCN population to become “resistant to the resistance” as resistant varieties are repeatedly grown.

Soybean varieties that are resistant to SCN possess resistance genes from one of four sources of resistance (which are breeding lines). To reduce the chance of a SCN population bring selected for that can readily reproduce on resistant varieties, Iowa State University recommends growers use varieties with different sources of resistance in different years. However, almost all SCN-resistant varieties available for Iowa growers have the PI 88788 source of resistance (“PI” stands for plant introduction). So rotating varieties with different sources of SCN resistance is difficult, if not impossible.

Iowa SCN Resistant Varieties Table

Number of maturity group O, I, II, and III SCN-resistant soybean varieties available to Iowa soybean growers, 1991 – 2007. Data were not compiled in 1992 or 2005. The red portion of each bar represents the number of SCN-resistant soybean varieties with resistance from a specific source other than PI88788.

Another tactic that Iowa State University recommends to consider using to slow the development of an SCN population that reproduces well on resistant varieties is growing a susceptible (non-resistant) variety periodically after resistant varieties have been grown a few times. Iowa State University cautions that SCN population densities must be low (2,000 eggs per 100 cc soil or less) before a susceptible variety should be grown in an SCN-infested field. And a good, representative soil sample should be taken from a field prior to determine the SCN population density before considering growing a susceptible variety.

SCN causes much greater damage and seems to reproduce at a greater rate in hot, dry growing seasons than in years with adequate to excess rainfall. So if a severe drought is anticipated, growers might opt not to grow a SCN-susceptible variety in an SCN-infested field, even if SCN population densities are low. 

The Iowa State University management recommendations for SCN (IPM 63, pdf) are available online.

Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

This article was published originally on 3/24/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.