Farm Horizons, Feb. 2005

Asian soybean rust is in US

By Myron Oftedahl
CCA Hutchinson Co-op, Lester Prairie

November 10, 2004 will probably go down in history as being pretty quiet for the general population, but for the American soybean producer, it will have a huge impact.

This was the day that any soybean producer, along with anyone else in the agriculture industry feared.

Nov. 10 was the day that Asian soybean rust was positively identified in Louisiana.

Since then, it has been found in a total of nine states, as of Dec. 29, 2004. This is believed to be compliments of hurricane Ivan.

Soybean rust is a fungal disease that is windborne and has the potential to cause major yield losses. There is presently no known varietal resistance. High humidity, moderate temperatures, and at least six hours of leaf wetness are the ideal conditions for rust development.

It is believed that rust cannot survive over winter in the northern soybean growing regions, so it must blow in from the southern areas, most likely the areas along the Gulf of Mexico.

So what to do? Should you forget about raising soybeans? I don’t think that we need to panic.

Now is the time to educate yourself. Don’t rely on the talk at the coffeeshop; but read the articles, attend the meetings, look at web sites, so that you are prepared for it and you know what to expect if it should arrive in our area.

Soybean rust is going to look like other soybean diseases in its early stages. It will begin in the lower canopy as small gray lesions on the underside of lower leaves.

The lesions will turn tan or reddish-brown and form a blister, which is usually on the underside of the leaf.

This blister will then release rust spores which can go on to infect more soybeans. Soybean rust is a fairly quick disease which can take a healthy field of soybeans to a field of stems in about four weeks.

This is true in Brazil, and other more tropical areas, but it is believed that rust will progress slower in the northern climates.

Regular field scouting is the most important thing that you can do at this point.

Walk your soybean fields at least once a week. If you have areas along a tree line or heavy canopy, areas that will tend to hold humidity, watch these closely.

If you see something out of the ordinary, call someone – myself, your agronomist, or your extension agent – to look at it, and try to determine what it is.

A cooperative effort has been set up through the University of Minnesota to screen leaf samples that are sent in for diagnosis. There are fungicide treatments that can be used to protect plants from being infected by soybean rust, and there are other fungicide treaments that can be used to stop the disease once it has started.

It is important to note that the recomendation for fungicide treatment will be to rotate fungicide chemical families to prevent resistance from developing.

The North American continent is the last continent for soybean rust to be identified, so there is a lot that is known about it.

The task will be to educate ourselves and transfer what has been learned in other countries to how will it affect us and how to deal with it here.

Several web sites are available for information:, sponsored by Syngenta;, sponsored by BASF; sponsored by the U of MN; sponsored by MN Dep’t of Agriculture; sponsored by USDA; sponsored by the National IPM Center.

I hope that this begins your education process. If you have Internet access, go to some of these web sites and look at the color photos.

Have a prosperous 2005.

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