Soybean Rust Infosheet

Table of Contents

1. Introduction
2. Scouting
3. Symptoms
4. Disease Cycle
5. Management
6. Related Links

Introduction

Asian soybean rust (Pakospora pachyrhizi) is a new and invasive fungal disease of soybean in North America. The threat to Ontario and Canadian soybean production from soybean rust has increased substantially with the recent confirmation of the aggressive Asian strain of the disease near Baton Rouge, Louisiana (November 10, 2004). Subsequent, USDA surveys have found other fields in Louisiana as well as Mississippi (Nov. 16, 2004) and Florida (Nov. 17, 2004), Alabama (Nov. 19), Georgia (Nov. 19), Arkansas (Nov. 22), Tennessee (Nov. 29), South Carolina and Missouri (Nov. 30) with soybean rust. The USDA will continue to survey high-risk areas in the Gulf Coast states. Prediction models cautioned that the Gulf Coast states would be at increased risk as a result of the unusually active hurricane season.

The Asian strain has also been found in Australia, China, Korea, India, Japan, Nepal, Taiwan, the Phillippines, central and southern Africa (Mozambique, Nigeria, Rwanda, Uganda, Zimbabwe, South Africa) Hawaii and South America (Brazil, Paraguay, Bolivia, Argentina, and Columbia). Soybean rust is a very destructive disease and yield losses of 10 to 90% percent have been reported from these areas.

Now that soybean rust has been found in North America, what can we expect? Since the warm, humid conditions of the Mississippi delta and Southeastern U.S. are ideal for soybean rust development, crop
losses of 40 to 50 percent are expected in these regions. The Ontario situation would be similar to the North Central U.S. where potential losses could range from 10 to 20% in most years and possibly as high as 40% depending on the environmental conditions during the season and management practices.

There are a number of factors that will significantly influence the disease's impact in Ontario. 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. The cost associated with fungicide applications will increase the cost of soybean production in Canada, if required. Scouting and early detection are critical to managing this disease.

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Scouting

When scouting for soybean rust, focus on:

1. Early planted fields with early maturing varieties
2. Low lying or protected fields with prolonged dew periods
3. Fields with early canopy closure
4. Lower leaves since the disease often becomes apparent in the lower canopy
5. Use a 10X or 20X hand lens to detect pustules (urediniospores) on underside of the leaf
   

Symptoms

The disease begins as small (2-3mm) yellow (chlorotic) spots that are irregular in shape and as the disease develops these lesions turn brown or reddish in appearance. Bumps are visible in these lesions when examined closely. These bumps are where the spores are produced and they are called "uredia". Uredia usually are usually produced close to the veins, and the rust-like appearance is given by the formation of the spores (urediospores) from the uredia. Although, uredia are more abundant on the underside of the leaf but they can also be found on petioles, pods, stems and the upper leaf surface.

Soybean rust causes premature defoliation leading to yield losses, fewer seeds per pod, decreased number of filled pods per plant, and early maturity. Soybean plants are susceptible to rust at any stage, but disease severity depends on the developmental stage of the plant at the time of infection.

Photogrpahs courtesy of the USDA-ARS, Fort Detrick, Maryland

Be careful since soybean rust especially, in the early stages of the disease, can be confused with other common foliar diseases that occur in Ontario. These include Septoria Brown Spot, Bacterial Pustule, Downy Mildew, Frogeye Leaf Spot and Bacterial Blight.

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Disease Cycle

Asian soybean rust unlike most other rust diseases has a wide host range (32 legumes species) but is of economic importance primarily in soybean production (Table 1). Soybean rust like other rust pathogens requires a living host to survive, reproduce and over-winter. It is unlikely that a winter host (such as the kudzu) exists in Ontario or the Northern United States since potential hosts lack green tissue during the winter months. Like other rusts, soybean rust can produce millions of spores (urediniospores) that are well adapted for travel by air.

The pathogen can be spread over great distances by wind-blown urediniospores. Airborne spore movement as a result of the recent hurricane activity is suspected as the most likely cause of the Louisiana, Mississippi and Florida infections.

Photos courtesy of USDA-ARS, Fort Detrick, Maryland

Environmental conditions that favor disease development include an extended period of leaf wetness (6 hours) which allows for urediniospore germination between temperatures of 15 and 28° C (59 - 82° F). These requirements do occur in Ontario. Hot (> 29° C), dry conditions limit or slow disease development. Spores are produced 10 days after infection, and are released after three weeks. Rusts are polycyclic diseases that continually infect and produce spores under favourable environmental conditions.

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Management

Early diagnosis and treatment are critical for soybean rust management. 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 a way (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. Penetration through the canopy to the lower infected leaves in a uniform and consistent manner is critical to disease control and thereby, maximizing yields. 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, prior to soybean rust confirmation to the Pest Mangement Regulatory Agency for the following fungicides: pyraclostrobin (Headline), azoxystrobin (Quadris), propiconazole (Tilt) and tebuconazole (Folicur).

The CFIA, USDA along with OMAF are 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.

Although soybean rust has been shown to infect these plants the amount of injury varies depending on the plant species, growth stage, environmental conditions and disease pressure.

Related Links

• Soybean Rust: A New Disease Risk to Canada

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