There’s a new plant fungus among us in the United States and it’s not afraid to cross the Mason-Dixon line. Asian soybean rust, an aggressive fungus of legumes, was first detected in Louisiana in November, and in eight other states since then. Severe infestations can devastate soybean fields. But Craig Grau and Brian Hudelson, plant pathologists at the University of Wisconsin-Madison, are poised and ready for it, should it enter Wisconsin during the 2005 soybean growing season.
ASR spores are carried on wind currents. They probably hitchhiked from South America to North America in September during the active hurricane season, most likely as a result of Hurricane Ivan.
The fungus was not detected in the Western Hemisphere until 1994, when it was positively identified in the Hawaiian Islands, says Grau. It spread to South America in 2001 and to the U.S. mainland this year, where it was first detected Nov. 10 in Louisiana. Samples of the infected plants were sent for confirmation to the USDA National Plant Germplasm and Biotechnology Laboratory in Beltsville, Maryland. In recent weeks, the national lab has confirmed ASR in soybean samples from Mississippi, Florida, Arkansas, Alabama, Georgia, South Carolina, Tennesee and the boot heel of Missouri.
It”s anybody”s guess where ASR will show up in 2005. Since it needs a host plant in which to overwinter, winter weather and spring rains play key roles in determining the extent of the spread and subsequent damage.
Kudzu is one of the primary hosts of the fungus, along with other perennials, including some species of clover, vetch, peas and beans. “The question is: how far will kudzu be beat back by frost?” says Grau. “It should die back to the Gulf states.”
Grau contends that for growers in the Midwest, time is on their side. “We (in the upper Midwest) have the advantage of being farther north, watching what happens in the South. Our growing season is later by several months, and the spread will be gradual.”
Furthermore, sentinel test plots of soybeans will be set out by many land-grant universities. These plots will serve as early warning systems to identify the fungus as it moves across the country.
Still, Grau knows that some will argue with his stance, but he is not ready to hit the panic button just yet. “Even with strong prevailing winds, which can occur in the spring, you need a critical mass of spores. It takes time to generate that mass of spores.”
As for the consequences in the South, Grau is not as optimistic. “This fungus could end soybean production in the southern states. They already have low yields and use other inputs.” Grau notes that 80 percent of the U.S. soybean crop comes from the Midwest; the other 20 percent of the yield comes from the South.
Brian Hudelson, senior outreach specialist and director of the Plant Disease Diagnostics Clinic in the plant pathology department at UW-Madison, is preparing for an active 2005 season. The clinic he directs, along with the State Department of Agriculture lab, are mandated to run tests on plant samples that indicate symptoms of the new soybean fungus.
“I”m gearing up for a high volume, maybe up to 1,000 samples,” he says. “I”ve never seen it myself, but I have identified other types of rust. It will be a learning experience for all of us.”
Hudelson believes that vigilance is the key to keeping the fungus at bay. That”s why he is working to get the word out about some of the tell-tale symptoms of Asian soybean rust. The initial symptoms, he says, are small gray spots on the leaves, which occur first on lower leaves, then on upper leaves. But these spots can also appear on the plant”s petioles, stems and pods. Over time, the spots change, enlarging and becoming reddish-brown or tan. The tan lesions mature to form small pustules, pimple-like blemishes that contain the spores. The powdery tan spores give the leaf a dandruff-like coating.
The earlier the infection occurs in the plant, Hudelson says, the more damage it causes. Once in the leaf, the fungus can defoliate the plant, which fails to produce pods and suffers an early death.
Although there are several look-alike diseases, such as downy mildew, brown spot and bacterial pustule, Hudelson says if in doubt, send it out. He encourage growers to send plant samples to the PDDC for examination. His clinic will provide soybean rust diagnostics free of charge. In return, growers will receive a written report and current information on management of the fungus. Plant submission forms and other information on soybean rust can be found on the PDDC website.
If Asian soybean rust shows up in the state, it should be manageable, says Grau. “We have fungicides to deal with this fungus. But spraying fungicides is different than herbicides. There is a steep learning curve for use and application. That”s why my personal agenda is education.”
To that end, Grau and others are conducting teleconferences and meetings, educating what he calls the “first tier”: county extension agents, seed industry representatives and crop consultants. That tier will then get the word out to the growers.
If knowledge is power, then soybean growers in Wisconsin will be powerfully equipped to deal with the Asian soybean rust fungus when it makes its debut in 2005.