November 16, 2004
From: Reuters, 16 Nov 2004 [edited]

USDA tests show soybean rust spreading

USDA tests have confirmed the spread of soybean rust, a yield-slashing fungus, in 3 more fields in Louisiana, a university scientist familiar with United States Department of Agriculture (USDA) activities told Reuters on
Tuesday [16 Nov 2004]. "We have 3 of the results. They were positive (for soybean rust)," said Clayton Hollier, plant pathologist with Louisiana State University.

The 1st case of soybean rust on the US mainland was confirmed last week [2nd week of Nov] at 2 LSU research plots near Baton Rouge. The disease does not harm humans, but can reduce soybean crop yields by as much as 80 per cent.

The newly discovered infected fields are located in 3 parishes or counties: St Mary, St John the Baptist, and Iberia. It was still unclear how many acres were affected. Hollier said the USDA would soon announce the test
results of a suspect soybean field in Mississippi. USDA officials were not immediately available for comment.

Soybean rust is a windborne disease characterized by reddish brown lesions on soybean plants, resulting in plants with shredded leaves. The fungus has spread from Asia to Africa and into Latin America, devastating crops in
many countries. Officials in Mississippi and Arkansas were also checking soybean fields for signs of possible infection. Soybean rust infection can be confirmed by USDA scientists by testing the DNA of spores or leaf samples from suspicious plants, or by examining spores by microscopy.

[Soybean rust, caused by the fungus _Phakopsora pachyrhizi_, [Pr], has now been reported in 2 states (Louisiana and Mississippi). The question can be asked: are we dealing with "discovery" or "spread" of soybean rust [SR]? A
more appropriate description of the current status might be that SR has now been discovered at 4 sites in these 2 states. Is it therefore already present in other areas in the south eastern US, and, if so, at how many?

Disease management will depend upon the use of chemical fungicides, because there are no resistant commercial soybean cultivars in the US. The development of resistant cultivars is under way, but it will be several
years before commercial cultivars are available. Classical plant breeding approaches, as well as DNA sequencing strategies, are being employed. The development effort is considerable. At least 20 000 soybean lines have been screened for resistance to SR. Some have been screened for a 2nd time, but less than 300 appear to have some tolerance to SR, and none have broad spectrum resistance to all Pr isolates in the USDA collection.

Effective disease management will be based on the use of chemical fungicides for control of SR, as has been demonstrated in Zimbabwe, South Africa, and Brazil. The most likely fungicide candidates appear to be the
strobilurins (azoxystrobin), ethylenebisdithiocarbamate (EDBC) fungicides, and triazoles (triadimefon, difenoconazole, tebuconazole). Several fungicidal products labeled for SR are available, but effective dosage
rates and application methods must be determined. The US is initiating cooperative efforts to conduct field trials in Africa and South America.

Links: <http://www.plantprotection.org/news/NewsDec02.htm>
<http://spdn.ifas.ufl.edu/soybean_rust.htm> - Mod.DH]