Ostioles releasing urediniospores

Ostioles releasing urediniospores

In 2002 the United States Department of Agriculture began preparing for the potential entry of soybean rust into the conterminous U.S. (APHIS National Strategic Action Plan). The rust is caused by two fungal species: Phakopsora meibomiae and P. pachyrhizi, with the latter being considerably more aggressive than the former. P. meibomiae has only been found in the Western Hemisphere and it is not known to cause severe yield losses in soybean (Miles et al. 2003). For many years, P. pachyrhizi has impacted soybean production in Australia and Asia. However in the past decade, this more virulent species has spread to soybean fields in Hawaii (1994), Africa (1996), Paraguay, Brazil, and Argentina (2001), Bolivia (2003), and Uruguay and Columbia (2004) (Sinclair and Hartman 1996, Miles et al. 2003, APHIS) (link to maps on web site).

In November 2004, P. pachyrhizi was detected for the first time in the southern United States. Fortunately, cold winter weather limited the area where soybean rust was able to survive to a few counties in central Florida and subsequent spread during the 2005 growing season was very gradual and confined to the southeastern U.S.

Yield Reduction in soybeanSoybean rust has the potential to be the most destructive foliar disease of soybean in the U.S. due to the rapid aerial spread of P. pachyrhizi and the high risk of severe yield losses caused by this pathogen (Sinclair and Hartman 1996). Soybean rust has caused significant yield reduction in many Asian countries with losses as high as 40% in Japan (Bromfield 1984) and 80% in Taiwan (Yang et al. 1992). During the past few years, yield losses in commercial crops ranged from 60-80% in Zimbabwe and from 10-80% in South Africa (Caldwell and Laing 2001). During 2003, the pathogen was detected in most of the soybean-growing regions in Brazil with a conservative yield loss estimate of 5% of the annual production (Miles et al. 2003).

The U.S. plants approximately 30 million hectares of soybean annually that yields ca. 75 million metric tons with an on-farm value of $12 billion (USDA 2001). If soybean rust caused a 5% loss to the U.S. soybean crop, such as Brazil recently experienced, this would have a major economic impact on U.S. agriculture. Kuchler et al. (1984) estimated total losses to consumers and other sectors of the U.S. economy from soybean rust could exceed $7.2 billion/year, even with a conservative estimate of damage. A risk analysis by Yang (1996) indicates that P. pachyrhizi could cause yield losses greater than 10% in any U.S. soybean-growing region; while in southeastern states where climatic conditions would favor the spread and development of disease, losses up to 50% are possible. During the first year of the soybean rust establishment in the US, the USDA Economic Research Service estimates that the expected value of the net economic loss caused by the pathogen will range from $640 million to $1.3 billion, depending on the severity and extent of the outbreaks (http://www.ers.usda.gov/publications/OCS/Apr04/OCS04D02/).

Urediniospores with germ tubeU.S. soybean producers are preparing to control P. pachyrhizi if it becomes established in North America. Soybean rust resistant or tolerant cultivars have not been developed for the U.S. (Miles et al. 2003). However, research programs focused on breeding for resistance to soybean rust are underway in the U.S. and other countries, such as Brazil. Fungicide applications were able to successfully manage the soybean rust pathogen in southeastern US production areas during 2005 and 2006.

Should soybean rust become established in the U.S., fungicides will be the primary method for reducing crop losses until such time when soybean cultivars resistant to the rust become available to producers. Foliar fungicides require frequent applications, 7 to 20 days apart, and thus can add significantly to production costs (Miles et al. 2003).


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