May 18, 2006

Rain in US state Florida causing soybean rust to reappear

The return of rain to drought-stricken regions of Florida is leading to a resurgence of Asian soybean rust, as well.

"Soybean rust has just been reconfirmed in the Quincey, Florida area after having disappeared from that site, due to extremely dry weather," reported University of Kentucky extension plant pathologist Dr Don Hershman, via the USDA's public rust website Tuesday (May 16).

Although fresh infections of rust have not been reported in the US since Mar 3, Florida state rust experts Wednesday warned, "We once again have positive finds of soybean rust on kudzu in north Florida. We have had several days of rain after a prolonged dry period and we expect the disease to continue to increase if the rains continue."

The airborne fungus flourishes in overcast conditions, when atmospheric humidity is high and temperatures are warm, posing a particular threat to soybeans once plants begin to flower--a stage of development which some of Florida's early-planted sentinel plots of soybeans (specifically designed to track rust movement) are just now beginning to reach.

The North American Plant Disease Forecast Centre said, although this week's rainfall, "may give rise to localised disease development," in southern Georgia and northern Florida, prevailing wind currents should move any rust spores aloft offshore, thereby posing a low risk of infection to susceptible plants in new areas of the South-east this week.

Active infections of soybean rust are currently thought to be limited to kudzu weed in 20 counties of Florida, Georgia and Alabama.

Asian soybean rust is caused by Phakopsora pachyrhizi, a fungus that cuts soybean yields via premature defoliation of infected plants and can cause heavy yield losses unless countered with fungicide.

The disease first appeared in the continental US in Nov 2004, and was found in 9 southern states last season, but has thus far caused little appreciable damage to US soybean production.