August 6, 2008


Tropical storm Edouard may bring soy rust into Midwest


Tropical storm Edouard is expected to cause modest damage to soy crops but the greatest threat carried by the storm may be Asian soy rust spores that are now on their way to the US interior.


Soy rust has thus far been confined to 27 counties in a half dozen Gulf Coast states during 2008, which is less than half the number of US counties where the devastating crop disease had been found by this point in the 2007 growing season.


"Because of Edouard, there will be the potential to transport the disease as far north as northern Kansas and southern Nebraska on Tuesday," said the USDA in an infection forecast published on the agency's public rust Web site on late Monday.


"The risk for deposition and survival will be high, as clouds and rain from Edouardo move across the state of Texas."


The storm made landfall at daybreak Tuesday between Galveston, Texas and the Louisiana border.


The fast-acting fungus - which is spread via windborne spores that act as seeds - flourishes in warm, damp, overcast conditions typical of hurricanes and tropical storms. In fact, soy rust was first brought to the continental US in 2004 by a hurricane which carried spores from infected fields in South America to Louisiana.


The latest discovery of soy rust in the US was reported July 28, within soy fields across Cameron County, Texas; an area that is widely regarded as the leading spore-source threat for soy fields in intensive soy-production areas of the Midwest.


Although the disease has thus far been a negligible problem for the US soy industry, the USDA's Economic Research Service estimates that soy rust could cause economic losses of between US$200 million and US$2 billion, depending on the severity and extent of any given outbreak. Conservative estimates of yield-loss range from 50 percent in the Mississippi Delta and Southeast, to 10 percent in other soy production areas.


Asian soy rust is caused by the fungus Phakospora pachyrhizi. The disease causes lesions on plant leaves, eventually leading to premature defoliation and outright death of the infected plant. Spores of the soy rust fungus are killed by UV radiation (sunlight) and by desiccation (drying out). The disease can be controlled through the timely application of chemical fungicides, which cost about US$15-US$25 per acre.

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