Asian Soybean Rust

Both a threat and an opportunity for northern soybean growers – let’s not be unprepared

By Jason Hanson, Certified Crop Advisor
jlhanson@agriliance.com

How will Asian rust affect northern area planting intentions in 2005? Coming off a year like 2004, there will probably be fewer acres of soybeans in some areas of North Dakota due to the bad taste it left and in some cases a shortage of early enough maturities. Good prices for other broadleaf crops such as dry beans, flax, and ‘flowers will also steal a few acres away from soybeans.

But in areas that missed the frost and had a decent yield, the acreage trend might be the same, or even go up. In some discussions with retailers, the feeling is that the risk of Asian rust will have little effect on soybean planting intentions in our geography, at least not in 2005. Many northern growers, especially in the Red River Valley, see more reward than risk in putting soybeans in the ground this spring.  They feel opportunity in the air.

This could hold true, since all indications point to a sharp decrease in Delta soybean acres. In discussions with my counterparts in the south, they feel that many farmers there will switch a lot of their acres over to corn or cotton, that soybeans aren’t that big of a money maker for them anyway, and the extra cost of one or two fungicide applications won’t help.

Will our sense of taking advantage of a potentially profitable market situation lead us to take Asian soybean rust too lightly?  “Bah, this is a southern problem. Not going to be a problem here.  Rev up the planter – beans in the teens!”

This issue may very well be an opportunity for northern growers. But if conditions are right, we could also be vulnerable to it, just like spinning into the ditch from a patch of ice when we least expect it. So at the very least, we need to educate ourselves about this pathogen – it is education/information that is the key difference to managing scab/FHB today compared to the panic “what is this” situation we had with the scab epidemic in 1993.

So if you’re a northern grower who’s going to plant soybeans, have a management plan in mind for Asian rust, just in case.  Here are some interesting points on Asian soybean rust:

1. There are two species of soybean rust, but the one we are concerned about is Phakopsora pachyrizi, because of the aggressiveness of this pathogen under ideal conditions. This strain isn’t new; it was first reported in Japan in 1902 and was at first limited to tropical and subtropical areas of Asia, but then slowly spread to other soybean-growing areas from there.

2. In China, where rust has been observed for many years, the most serious problems occur south of 35-degrees latitude. This may put areas south of Iowa more at risk.

3. Rust spores can survive at temperatures between 59-85 degrees F. That means the spores likely can’t overwinter in our climate, although plant pathologists don’t overrule the possibility of the pathogen’s ability to eventually adapt to our environment.

4. The disease has moved up to 1,500 miles in one year, something that wasn’t expected by the scientific community.

5. Pay attention to wind currents that would favor wheat rust, as similar wind travel would be the most likely cause of infection of Asian rust.

Learn to recognize it
Accurate identification and early detection are a key. University of Missouri Extension plant pathologist Laura Sweets says that like other foliar soybean diseases, soybean rust begins with small yellow spots on the upper leaf surfaces.  “They all start off looking pretty much the same,” she says.  “Rust usually starts low on the plant, then moves up through the canopy.”  What distinguishes soybean rust from other diseases is the spore-producing pustules that develop on lower leaf surfaces, she says. “These pustules are like little volcanoes. When they erupt, they can produce billions of spores.”

Sweets said that while managing other foliage diseases can be accomplished through crop rotation, residue management and variety selection, these management strategies aren’t applicable to soybean rust. That’s because there are no resistant varieties, and managing residues and rotation also have no effect because rust only survives on living host material.

Consider planting monitor plots
Sweets says that planting monitoring plots can be one of the best early-warning systems to detect rust in time to treat it effectively. She says a grower could do this by planting a 20 ft.-by-20 ft. plot one month early, or with a shorter maturity hybrid than the one to be used in the commercial field , and scouting the plot or plots intensively.  If a monitoring plot becomes infected, destroy it once the disease is confirmed.

“By the time you see a yellow fleck on top of a leaf, it’s time for protective treatment,” she says. “If the disease is already producing spores, you’ve missed the boat.” Sweets has prepared a new bulletin on Asian rust, which can be found online at http://muextension.missouri .edu/explore/agguides/crops/g04442.htm .

Monitor southern fields and scout your own
You can bet that soybean groups and land grant universities will have a huge effort on monitoring the potential spread of soybean rust. Keep informed and bookmark web sites that track Asian rust.

During the growing season, Sweets advises to scout soybean fields often, especially if rust has already been reported in southern states. Initially, target your early-planted fields, fields planted to early-maturing varieties, low-lying or protected fields with prolonged moisture in the canopy, and fields with early canopy closure. 

When scouting, carefully check leaves low in the canopy, she advises.  Turn leaves over and examine lower leaf surfaces. A magnifying glass might be handy when examining leaves, as well as pictures from an Asian rust bulletin to help identify the disease.  If you’re uncertain about identifying a disease, send a sample to a plant disease diagnostic lab.

Treatment and control measures – If a producer does find soybean rust, there are several fungicides labeled for soybeans. Some help prevent infection, others are curative. Rotate chemistry/modes of action to slow development of resistance to present fungicides. Treatment timing is critical. Some important points to note in managing soybean rust:

• In Brazil early on, the advice was to check for symptoms, then apply fungicide. Now the advice is: treat in advance of symptoms.
• Also in Brazil, 80% of the crop was lost when fungicides were not applied. When fungicides are applied on a timely basis, the losses were in the 20% range. Soybean rust attacks very rapidly and if fungicides are applied too late, you can’t control it.
• Again, timing is critical. Applying a fungicide too late has shown limited benefit, especially for strobilurin fungicides.
• Soybean rust attacks begin on the lower leaves at about flowering. Spores need about a minimum of six hours of leaf wetness to germinate and infect.  Maximum infections occur within 10-12 hours of free moisture.  Temperatures from 59-82°F are ideal for infection.  New spores can be produced in about 9 days after initial infection.
• It is best to use strobilurin fungicides as a protectant as they prevent sporulation, therefore they should be used before the disease shows up. Fungicides in this class are Quadris and Headline. Then it is best to follow with a triazole fungicide.
• Research in South Africa indicates that triazole fungicides were the best choice when soybean rust is present at the time of spraying. Fungicides in this class are Folicur, Tilt, Propimax and Laredo. Then it would be best to follow the second spray with either a strobilurin if rust pressure is light or another triazole or combination product if pressure is heavy.
• Combination products are a good choice in managing resistance. Some of these products are Stratego, Quilt and Pristine.
• Fungicide coverage of the soybean plant is critical to getting the best control with any of the fungicides. Initial infections are on the lower portion of the soybean plant which is harder to cover with a fungicidal spray.
• According to Sweets, the crucial time to protect plants is from the R1 stage to the R5 stage, “but if you hear about rust in the area, scout, and don't wait for R1” before applying a preventative treatment.
• Rates for soybean rust are higher than what you might be used to when using them on small grains. For example, labeled rates for soybean rust are 4-8 oz per acre for Tilt and 6-12 oz for Headline. Those are about twice the common used rates for small grains.
• Research done by Agriliance LLC has shown that the addition of certain adjuvants has increased canopy penetration and plant coverage when a fungicide was applied with either InterLock™ at 2 - 4 ounces/acre + Preference® .25% v/v or Placement® at  5 ounces/acre + Preference® .25 % v/v. Work with Asian soybean rust in South Africa has also shown increased fungicide efficacy with either InterLock™ + Preference® or Placement® + Preference® with the rust fungicides used there. The combination is important, since the degree of control with individual products was much less.

Consider working with an agronomist
Identifying this fungus is going to be difficult, as it can look like many other diseases like brown spot, bacterial blight, downy mildew and pod mottle virus. As pointed out before, timely scouting, timely treatment, and using the right product are also very important. If you don’t already, you may wish to work with a professional agronomist to scout your soybean fields and recommend treatment options (or if rust isn’t a problem and you don’t have to spend money on fungicides). Professional scouting is the best way to watch this thing, and there are dividends in working with an agronomist to monitor other problems as well, ie weeds, other diseases, insects, etc.

Start a plan now with your supplier/applicator/seed expert

If soybean rust would show up in your area on July 30, do you think there are enough ground or air applicators to handle the demand? Can you cover your own acres – in a timely fashion –  with your own rig at 20 gallons per acre or more of water? Could you do it twice if needed? Will there be enough fungicide product available if and when you need it? 

Communicating with your dealer now is better than expecting the product on the floor when needed. Talking things through with your dealer or seed advisor now can be very helpful for all parties. The same goes for your applicator, even if that is you. Now is the time to research and think through application considerations, such as boom height, nozzles, and travel speed. 

Doesn’t the saying go, those who fail to plan, plan to fail. I believe that the area of northern Minnesota and most of North Dakota is sitting in about the best geography possible to bypass the possibility of a major threat, but that doesn’t mean it couldn’t happen. Chances are that something else will be a bigger issue for our soybean acres this next growing season – but let’s not be unprepared.  

This image demonstrates the effectiveness of well-timed fungicide applications to control Asian rust. This is a picture of a rust-infected field in Brazil. The brown/dead area down the middle of this rust-infected field was not sprayed with fungicides—a dramatic difference compared to the treated soybean plants.  Photo: United Soybean

Hanson, Devils Lake, N.D., is a regional agronomist with Agriliance.

Several online sources for Asian soybean rust info

• www.dtnsoybeanrustcenter.com
• www.aphis.usda.gov/lpa/issues/sbr/sbr.html
• www.stopsoybeanrust.com
• www.soybeans.umn.edu/crop/diseases
• www.planthealth.info