Dr. Paul's Research Perspectives
New hope for organic management of Asian rust in soybeans
ASR confined so far to Deep South where its kudzu host gives winter lodging.

By Paul Hepperly, PhD

July 13, 2006: The ancient Romans prayed to Rosbigus, the god of rust who could protect their crops from (to them) mysterious diseases in harvested wheat and in stored bread during winter months. Ceremonies were given to allay this deity so the cereal crops—needed for the daily sustenance of Rome—were productive and safe to eat.

Today, the huge multi-billion dollar United States soybean crop is threatened by Asiatic Soybean Rust (ASR), a fungal disease caused by Phakopsora pachirhizi. This pathogen can eliminate up to 90 percent of soybean yield under severe epidemic environments. While varieties vary in their tolerance, no commercial strain has resistance to this rust.

The ASR fungus originally wrought havoc in Asia and Africa, hitting more recently Brazil and the United States. Hurricane Ivan is attributed with bringing ASR spores to Florida from South America or Africa in the fall of 2004. By the end of 2004, soybean rust was identified in about eight counties, mostly in the panhandle of northwestern Florida. By the fall of 2005, the same rust was identified in about 10 states and in 138 counties.

In 2004 Hurricane Ivan came out of the South Caribbean sweeping rust spores north to Florida panhandle and Gulf State area.

As important as soybeans are to the US ag economy in terms of quantity—hence, the threat of a catastrophic disease exposure—they are prominent in organic production due to their distinctive quality. Because of consumer concerns about Roundup Ready genetically modified soybeans in food products, there has been increasing demand for organic soybeans over the last decade. Because there has been no confirmed defense against ASR that was permissible under the rules of the USDA’s National Organic Program, the viability of organic beans seemed to be in danger, especially in the southern states.

Better news in 2006

But wait. If you are an organic soybean farmer it’s way too early to give up on the crop, given differences developing this season. By early July, the number of counties with soybean rust was only up to 24, all of them in the Deep South relatively close to the Gulf of Mexico. Soybean rust, like soybeans, will overwinter only in southern Florida and Texas. The overwintering host has been shown to be kudzu (Pueraria spp.), the notorious runaway forage vine that cascades over trees in the Southern summer but only survives unfrozen on live leaves at the most southerly points of the continental US.

By July 2006, the epidemic of Asiatic Soybean had receded due to its inability to over winter where there is frost. Soybeans in early vegetative stages usually do not show the fungus. (Map from July 12, 2006. For an updated map, visit www.sbrusa.net.)

More good news: AgraQuest, a venture capital group developing biocontrol organisms for disease and pest management, has registered the first organically approved biocontrol specifically formulated for rust. In 2005, results both within and outside the United States shows Ballad biocontrol fungicide significantly improves both yield and protects against many non-rust diseases to boot.

Bacillus pumilus (BP) is a patented biocontrol bacteria in the same genus as Bacillus thuringiensis (BT). Many organic farmers are well aware of BT for its ability to control caterpillar pests. These bacteria produce a resistant endospore, which allows a biological agent to survive commercial storage and distribution without decreasing insect or disease control in BT or BP, respectively.

In addition to direct action to kill rust, B. pumilus works by activating the soybean plant defense system as well as both blocking leaf-to-rust-spore contact and through the action of potent amino sugar antifungal antibiotics. This last action is expressed by inhibiting fungal cross-wall formation, new cell development, and destroying membranes, which leads to pathogen cell death. In addition, the soybean rust action has been demonstrated on septoria brown spot, frogeye leafspot (Cercospora sojina), soybean powdery mildew (Microsphaera diffusa) and purple leaf blight and seed stain (Cercospora kikuchii).

Controls pay for organic beans

But does the application pay? Average yield increases from two to four applications is 17 percent. This would cost about $20 to $40 approximately per acre for a seven-bushel bump on 45 bu/ac beans. Whereas for conventional beans this might have a value of about $40 an acre—being near a wash—in the case of $10 organic beans, this would be a value of at least $70 dollars per acre on the same yield potential. Wholesale prices reported from seven terminal markets as July 11 ranged from $9.50 to $14.50 per bushel for certified organic soybeans, according to The New Farm Organic Price Index.

Ballad fungicide is used at two quarts per acre and applied at 14-day intervals starting at or slightly before or after start of flowering at the R1 stage. For growth stage details, visit:
www.ext.nodak.edu/extpubs/plantsci/rowcrops/a1174/
a1174w.htm.

Soybeans in the center were not sprayed with ASR fungicide (all the soybeans are the same variety). Biological fungicides such as Ballad® have shown ASR control similar to synthetic fungicides as seen in this demonstration test. Photo was taken at Mississippi State University field test site.

Trials in South Africa concluding in March 2005 showed Ballad applications of two quarts per acre gave 95 percent control of soybean rust—not statistically more significant than the synthetic chemical control azoxystrobin. There were also two trials in 2005 in Argentina. In the first trial, yield was increased five bushels per acre with a 10-percent decrease in soybean rust. In the second, one application at the R5 stage (just before full green-bean stage) reduced rust by 96 percent and bumped yield by 17 bushels—two bushels per acre more than the mixture of synthetic fungicides tested.

We are collaborating with Iowa State University, University of Florida and Michigan State University to examine ASR controls for organic farmers. Our Florida collaborators tested Ballad with three applications at R1 that showed rust decreased about two-thirds.

Dr. Boyd Padgett of Louisiana State University showed Ballad bumped the yield by seven bushels per acre—almost 50 percent over non-treated by controlling Cercospora Leafspot in the absence of rust in 2005.

We plan to look at Ballad closely in Pennsylvania this year and, although we do not expect soybean rust, we will see how it (in addition to lime sulfur and soybean oil treatments) does on other diseases and get our feet wet on the product. Soybean rust was reported as far north as North Carolina and Missouri last year, so you may want to be particularly careful this year scouting your fields, particularly below the Mason-Dixon line.

I don’t know if praying to the rust god will work for you, but being prepared with information on ASR and its management will definitely help put you ahead in the rust game.

The multi-partner ASR research referenced in this article is funded by the Cooperative State Research Education and Extension Service (CSREES) of the USDA through its competitive grant program. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the USDA or the Iowa State University.