July 8, 2005
Hurricane could pack soybean rust along with wind, rain
WEST LAFAYETTE, Ind. - Hurricane Dennis could bring some much-needed rain to moisture-deprived Indiana crops as he travels toward the United States. He also might carry some unwanted excess baggage on his trip: Asian soybean rust, said a Purdue University plant pathologist.
"The predicted path of Hurricane Dennis will move across known areas of soybean rust and then progress on up into Indiana," said pathologist Greg Shaner. "There is the potential for that storm to bring rust spores into this area."
As of 5 a.m. EDT Friday (July 8) the hurricane was centered near Cuba's southern coast and is on a track to make landfall in the U.S. early Monday near the Alabama-Florida state line. Dennis is packing winds of up to 110 mph and could strengthen as he nears the U.S. Remnants of Dennis are predicted to reach Indiana Tuesday or Wednesday.
Dennis' path could take him over areas in Alabama, Georgia and Florida with confirmed soybean rust - a yield-reducing fungal disease that attacks a soybean plant's leaves and seed-producing functions. The disease is spread when infectious rust spores are picked up by wind or other means and transported to uninfected fields.
Should the hurricane bear rust on its mighty winds, it is unlikely that large spore volumes would reach Indiana, Shaner said.
"We still have the situation that even though there is some active soybean rust in the South, to the best of our knowledge, there still isn't a lot of rust there," Shaner said. "So if spores are picked up by that storm, there's going to be a lot of dilution before those air masses reach us. We wouldn't anticipate a heavy load of spores arriving in Indiana, but there could be some."
Rust symptoms would not appear immediately, Shaner said.
"It would take a minimum of five days for infections to be visible," he said. "At that stage, all we would expect to see would be some very small brown flecks on the leaves, which, frankly, are not very diagnostic. Those spots could be any of several diseases. It would take about nine days for the characteristic rust pustule to appear."
Pustules are raised areas that form on soybean leaves after the plant is infected. The pustules break, releasing thousands of spores into the air. A moderately infected soybean plant can produce about 6 million rust spores per day. In an average acre of 160,000 to 180,000 soybean plants, that equals around 1 trillion spores.
Soybean producers are urged to scout their fields for signs of rust. If rust is found in a field, fungicide applications might be necessary. Fungicide is the only control option available.
Farmers could transport spores into uninfected fields as they scout but such inadvertent disease transmission would be insignificant, Shaner said.
"If a farmer were to go into a field and find rust or a few pustules, it is possible that some of those spores would adhere to his clothing," Shaner said. "Then, if that farmer were to go into another field, those spores would be transported and could be rubbed off on soybean leaves.
"However, if you find rust in a field and you go to fields anywhere in the general area, it's quite likely that the same weather that brought the spores into the first field brought them into the second field, as well. So spores that are transported on a scout probably are very few in number compared to the number being transported by wind."
For more information about Asian soybean rust and fungicides, visit the Purdue Plant & Pest Diagnostic Laboratory soybean rust Web page.
Writer: Steve Leer, (765) 494-8415, firstname.lastname@example.org
Source: Greg Shaner, (765) 494-4651, email@example.com
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