Each year, as spring unfolds in the fertile fields of southeast and central Pennsylvania, a young farmer's fancy turns to soybeans. Between May and June, 400,000 acres of Glycine max will be planted in the Keystone State, a dramatic increase from the 50-60,000 acres planted 15 years ago. The vogue of soy as an ingredient in everything from breakfast cereal to shampoo explains the planting trend, and processing plants throughout the state help meet the demand for the humble legume once grown mainly for livestock feed and soybean oil.
This booming business should put smiles on the faces of Pennsylvania's soybean growers, but their brows could be a bit more furrowed this year as they walk among the soybean rows. Trouble may be traveling rapidly toward them in currents high above their heads. Its form: an airborne fungal pathogen. Its name: Phakopsora pachyrhizi, also known as Asian soybean rust.
Martin Draper, SDSU, 2004
Evidence of damage from sever soybean rust infection.
Rust hunters
The existence of Asian rust was first recorded in Japan at the dawn of the last century. Since then, it has been on a steady march through the continents, moving from Asia to Australia and Africa before taking up residence in South America in 2000.
North America has been on "rust alert" ever since, with scientists scrutinizing data to predict how—and when—Asian rust would land on our shores. This kind of mystery appealed to the sleuthing skills of aerobiologist Scott Isard and plant pathologist Erick DeWolf. When these Penn State professors teamed up in 2001, Asian rust had not yet encroached into North America. But pachyrhizi spores—like birds and allergens—depend on aerial transport for long-distance travel and, when Hurricanes Frances and Ivan hit in succession last August and September, Isard and DeWolf guessed correctly that Asian rust would hop a ride into the U.S. on the strong upper-level winds. Right on schedule, the spores touched down in Louisiana last November. To date, Asian rust infection has been reported in nine states and there's no mistaking its inexorable spread northward.
Protecting fields and yields
In contrast to the more benign spores of Phakopsora meibomiae, the aggressive pachyrhizi strain is "a serious threat to soybean production in the U.S.," and its management is not expected to be easy. Researchers, including Isard and DeWolf, are creating predictive models to help them second-guess the path of pachyrhizi's infectious spores (also called "inoculum"). Since 2002, Isard and DeWolf have worked in collaboration with the U.S. Department of Agriculture (USDA), employing weather data, plant and disease pathology, and survey crews on the ground to create a complex, composite portrait of soybean rust activity.
There's a lot at stake in understanding the fungus's trajectories as it extends into North America. In the United States, 355,000 farms grow soybeans over a total of 72 million acres. Soybeans represent 23 percent of all crops harvested in the country. Without early detection and carefully-timed applications of fungicides, Asian rust—with its telltale reddish-brown pustules that erupt with infectious spores—is capable of mass field contamination that could decimate yields. Many factors—including weather and fungal reproduction levels—influence the disease's destructive capacity. The USDA can only say that 2005 yields may be reduced "by 10-80 percent," with total losses approaching $1.3 billion dollars.
Wintering in Florida?
Pachyrhizi spores are remarkably adaptable and "can infect at least 100 known plant species," in addition to soybeans, says DeWolf. (Green beans, dry edible beans, sweet clover, and kudzu are all potential hosts.) But the one thing the pathogen can't adapt to is cold temperatures, favoring the same warm, moist conditions that soybeans prefer. Like all rust pathogens, pachyrhizi survives on green living tissue and few North American host plants are green all year. (Kudzu is an important exception.) So far, Florida is the only state where live spores have been detected—on the hardy kudzu vine, as predicted—during the winter months.
But Pennsylvania's cold winters won't necessarily protect its soybean fields from rust, thinks Isard. "Even if soybean rust doesn't overwinter," he points out, "it probably will be reintroduced from the Caribbean region every year. The rust will then migrate north each year into the soybean-producing regions." Spores can also blow into Pennsylvania each spring from infected Southern fields.
Timing is everything
Soybean plants are most vulnerable to rust between the sprouting and flowering stages. Once soybean rust arrives in Pennsylvania—and few doubt it will—DeWolf believes that the timing of spore migrations as well as weather conditions and plant growth stage will determine the level of crop damage.
"Each year, we'll have to watch where the rust is," notes DeWolf. "In some years, it may arrive late in the season and cause only limited damage. Then no fungicides will be required. However, if rust arrives early, and wind currents are right, multiple fungicide applications may be needed."
The fungicides DeWolf mentions include some with "section 18" status, meaning they weren't developed for use on soybeans and require special permission from the Environmental Protection Agency for use against Asian rust. Penn State and the Pennsylvania Department of Agriculture have successfully applied to use section 18 fungicides and are now collaborating on educating growers about the most effective timing of applications. ("Some estimate that total use of fungicides in this country for all purposes could double if next year's soy rust infestation is severe," DeWolf cautions.)
Digital know-how
Let's return to our Pennsylvania farmers, worriedly surveying their soybean fields. They clearly need more than the "wet finger in the air" test to see which way the winds are blowing. Team members from the North American Plant Disease Forecast Center (NAPDFC)—including Scott Isard—have created a different kind of digital resource, in the form of a Web site with state-by-state maps of disease observations, management recommendations, and scouting information. Funded by the USDA's Animal and Plant Health Inspection Service, the site should be a helpful tool for farmers in all states, including Pennsylvania, as they attempt to prevent and limit Asian rust damage.
"We want to provide daily risk maps online for farmers," says Isard. "How good they'll be ultimately will depend on how good our information is on pathogen distribution in the Caribbean basin and the southern U.S." No doubt Isard and colleagues will be monitoring these regions closely to better alert growers when the peripatetic pachyrhizi is on the move once more.
Scott Isard, Ph.D., is professor of aerobiology in the Department of Plant Pathology in the College of Agricultural Sciences. He can be reached at sai10@psu.edu. Erick DeWolf, Ph.D., is professor of plant pathology in the same department. He can be reached at edd10@psu.edu.