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Corn+Soybean Digest

Fight With Fungicides

Asian soybean rust made its debut last November in Louisiana and is now already showing its ugly pustules this year in the U.S.

But growers are far from being panicky over this potentially devastating disease that, left untreated, could torment the U.S. $18 billion soybean industry.

A recent multi-university bulletin using USDA/ERS data states that annual yield losses from rust for North American soybean production are predicted to be at least 10% in the upper Midwest, Northeast and Canada; and 50% or greater in the Mississippi Delta and southeastern states. However, without effective management tactics, losses in hard-hit areas could exceed 80%.

“Farmers must realize that this disease can be controlled,” says Greg Anderson, United Soybean Board (USB) chairman and soybean farmer from Newman Grove, NE. “While it should be respected, rust can be treated like any other soybean disease. As long as farmers stay up to date and properly scout fields, yield losses can be minimized.”

With the information explosion from agricultural media and industry, farmers appear to be on top of the disease. In fact, according to a USB survey in April, 77% of farmers are aware of what can be done to manage rust.

Growers should realize, however, that rust will be variable from year to year. “Be diligent and keep looking for the disease in your fields. You must scout,” says Burleson Smith, USDA's special assistant for pest management policy.

Across the board, soybean specialists agree that timely scouting and early detection can make or break your ability to control the fungus.

Around the world where rust has occurred, fungicide use has become as commonplace as glyphosate use here in the U.S. For example, in 2003-2004 Brazil spent close to $1 billion on fungicides to control soybean rust.

“To start, you need to know if you have rust in your fields when you're trying to decide on treatment,” says Marty Draper, Extension plant pathologist at South Dakota State University. “You also need to be watching rust spread maps so you know where the disease is throughout the country.”

Scout for rust early, from beginning flowers (R1) through full seed (R6), advises Geir Friisoe, manager of plant protection for the Minnesota Department of Agriculture. “Rust could appear earlier, but R1 is where treatment becomes economical,” he adds.

If rust is confirmed within your county, or within 300 miles of your farm, a fungicide with preventative properties should be considered. Preventatives should be used before the fungus is present. Treatment just prior to rust development is typically the most effective strategy for fighting rust, Draper says.

If you confirm rust in your own fields, a fungicide with curative properties will be required. Curatives should be used after the fungus has arrived.

Soybean rust fungicides fall into three families: chlorothalonil, strobilurins and triazoles. Both chlorothalonil and the strobilurins are preventative fungicides. Triazoles, on the other hand, have both preventative and curative activity.

At press time, five products were registered for rust and 10 products had been approved for Section 18 use. Section 18 allows off-label use — see fungicide table on page 17.

“When you know the disease is getting close, use preventatives like strobilurins (i.e. Quadris or Headline) or chlorothalonil (i.e. Echo or Bravo). Or, you can also use a combination product or a triazole,” says Draper. “As long as the disease hasn't been identified in your field, anything is available for use.”

Once you find rust, it's a different story. You're now in a curative mode and should only use a triazole or a combination product with a full component of active triazole, he adds.

After you find the disease, you cannot use the strobilurins; and remember chlorothalonil will only protect from additional infections, Draper says.

“If you come in with a triazole, with curative and preventative activity, you'll also get protection against new infections as well has having activity against existing infections,” Draper says.

If you choose to use chlorothalonil in the preventative stage, there's a 14-day maximum reapplication period. That means you should spray within that 14-day period. With the other fungicides, there's a 14-day minimum, meaning you should spray again 14-21 days later.

Chlorothalonil can be applied any number of times, however. Draper points out that it's the only product chemistry for rust that's never shown resistance development in any pathogen population.

“Overall, we think rust has a relatively low risk for resistance. However, it's a fungus that mutates readily,” Draper explains. “That's why we have to rotate product chemistries.”

Generally, experts say tankmixing isn't a problem either, as long as you mix fungicides with fungicides.

“We don't recommend tankmixing fungicides with herbicides,” Draper says. “The practices you want to follow to get good weed control are different from the practices you'd follow to get good disease control. For example, carrier volume and droplet size are not going to be the same.

“You're also going to compromise either weed control or disease control if you use one application method over another,” he adds. “The key with tankmixing is timing. If you want the product to do the job, it should be applied as recommended or you'll compromise control. That holds true for tankmixing insecticides, too. If you're going to target aphids as well as rust, make sure they're both there.”

Depending on where you live in the U.S., the number of fungicide applications you'll make will vary. The potential for rust in the V stage is minimal, says Draper. “If you look at Brazil last year, most of its first reports from sentinel plots occurred from R3 through R5. Only about 3% occurred in the V stage.

“After spraying, come back in two weeks and do additional scouting,” Minnesota's Friisoe says. “Before spraying again you need to see if new infections are occurring.”

As a guideline, Friisoe says you can apply fungicides through the R5 stage. You won't have to apply after R6.

If you live in the South, where maturity groups are longer and plants are in the reproductive stage longer, Draper says it's likely you'll need multiple applications. “V stages don't change nearly as much as the length of time in the R stage,” he adds.

When applying, watch recommended rates. “Some products say a 10 gal./acre minimum, some say 15,” Draper points out. “Maybe you can get by with 10 gal. of water up to about R3 or R4, but as you build canopy you'll want to wet the foliage more and will need more water than you did earlier in the season.” (See “Spray Smart,” on page 30.)

Application cost, including product, will run about $10-20/acre, depending on the products you choose. Draper reports that some of the fungicide products alone now cost as little as $6/acre, not including application fees.

Still, a big question remains about whether there will be enough product available and if farmers will begin hoarding fungicides.

Draper's not overly worried because he says companies are allocating product across the country. “They're metering it out so they don't have to worry about redistributing later,” he says. “I think we'll have enough fungicide because I don't think we'll need to treat all 70 million acres.

“From my perspective in the North, I'd like to see the South get all it needs,” Draper says. “If it does a good job of controlling the disease, the chance of significant inoculum coming north is dramatically reduced.”

According to the plant pathologist, “Companies are churning out products as fast as they can. But it's important that we have multiple product chemistries available because no one product is ever going to meet the demand.”

Draper also says one chemical company told him it's running its plant 24/7, 48 weeks of the year. It's only closed four weeks for maintenance.

With any product chemistry, resistance is an issue. Although chlorothalonil is not considered a high risk for resistance, that's not the case for strobilurins, Draper says. They're high risk and can only be applied once, unless they're applied in combination with other chemistries. Triazoles are considered low to moderate risk for resistance.

As always when using fungicides, be sure to read labels and ask questions of your dealer or sales representative.

For example, each product has a specific preharvest interval to follow that ranges from 14 to 42 days. “Chlorothalonil, with a 42-day preharvest interval, means you'll want to use it on the front-end of the season and it's probably not something you're going to use beyond R4,” says Draper.

“I don't think the preharvest issue is much of an issue, especially if you're looking at a 28-day preharvest interval and you can only apply up to R6,” he says.

Above all, experts urge you to scout and treat following label recommendations.

“We have to treat this disease with respect because it can be too sneaky, too fast and too damaging,” says Draper.

New Fungicide Guide Available

A new multi-university bulletin, SR-2005, “Using Foliar Fungicides To Manage Soybean Rust,” is now available from your state Extension office or by logging on to

The Scoop On Section 18s

Section 18s are special exemptions to the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) that governs pesticide use and allows special uses for products.

Usually, Section 18s are emergency exemptions that allow use of a product for one year. In the case of Asian soybean rust, they're called a Quarantine Section 18, which allows use on exotic pests for three years.

“Although complicated, the three-year approval is going to allow the industry a longer period of time to respond and get full labels,” says Marty Draper, Extension plant pathologist at South Dakota State University. He has led the charge in South Dakota and nationwide on getting Section 18s.

According to Draper, the Section 18s were activated nationwide on Nov. 10, 2004, when rust was identified in Louisiana. That means if your state has applied for Section 18s, they're automatically activated so you'll be able to use them.

Check with your local state departments of agriculture for any special restrictions relevant in your state.

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