Soybean Rust: A New Disease Risk to Canada
Posted on February 17, 2005 at 14:03:51

Albert Tenuta, Field Crop Plant Pathologist, OMAF, Ridgetown

Two species of the Phakospora fungus cause soybean rust, Phakopsora pachyrhizi Syndow & Syndow (Asian species) and P. meibomiae (Arthur) Arthur (New World species). Of the two, Asian soybean rust (P. pachyrhizi) is economically significant (10-90% yield loss) and causes periodic epidemics in Australia, Asia, Africa and Hawaii. In 2001-02, Asian soybean rust was detected in South America initially in Paraguay, Brazil and in subsequent years Argentina, Bolivia and most recently Columbia. P. meibomiae is less virulent and although it does occur in Central America and South America, the fungus poses minimal risk. In contrast, Asian soybean rust is an invasive disease that threatens sustainable soybean production in both South America and North America.

As expected it was a matter of time until soybean rust was found in North America. The recent confirmation of Asian soybean rust in Louisiana by the USDA brings the issue surrounding this disease closer to Canadian soybean producers. There are a number of factors that could significantly impact Canadian producers and the soybean industry.

Currently there is no effective resistance to P. pachyrhizi in commercial soybean varieties grown in North America. The availability of rust resistant soybean varieties is still many years away (5 to 7 years). Foliar fungicides will be the primary method of control until resistant varieties are developed. Although producers and commercial operators have good experience applying herbicides, very few have a significant level of experience applying fungicides in commercial soybean fields. The cost associated with fungicide applications will increase the cost of soybean production in Canada, if required. Although presently there are no fungicides registered in Canada for soybean rust, a Emergency Use Registration Application was submitted this summer to the Pest Mangement Regulatory Agency for the following fungicides: pyraclostrobin (Headline), azoxystrobin (Quadris), propiconazole (Tilt) and tebuconazole (Folicur).

Rusts are a group of plant pathogens that require a living host to survive, reproduce and over-winter. It is this aspect of soybean rust biology that will most likely limit the fungus from over-wintering in Ontario and the North United States. It is unlikely that a winter host exists in Ontario. Although this may be seen as good news, we must keep in mind that rust fungi produce millions of spores that are well adapted for travel in the air.
Airborne spore movement as a result of the recent hurricane activity is suspected as the most likely cause of the Louisiana infections.

The USDA is monitoring and accessing the extent of these infections and how large an area is impacted. If this confirmation leds to a over-wintering soybean rust population in the southern U.S., soybean rust spores could migrate in a similar manner that wheat rust and common rust of corn are spread into Canada from these regions and Mexico. The impact and potential losses will vary from year to year and will depend on factors such as initial infection levels (mild versus severe), soybean crop stage, environmental conditions, and fungicide application timing.

It is important to remember that soybean rust has not been detected in Ontario or Canada and we anticipate the impact of the disease to be less in Canada then the southern United States. Our geographic distance from the pathogens potential over-wintering locations in the southern US and our environmental conditions being less favourable, lowers our risk to the disease. We have been anticipating soybean rust and proactive preparations have been on-going for its inevitable introduction into North America. These include "tools" to assist producers and ag. businesses in the scouting and identification of soybean rust as well as its management. In addition, confirmation at this time (November, 2004) allows for the education, continued development of management plans and delivery of these “tools” to combat this disease prior to next years growing season.