Soybean Rust: Its History, Biology, Epidemiology and Importance to Crop Production

James Kurle
Department of Plant Pathology,
University of Minnesota,
St. Paul, MN USA

Soybean Rust (SR) caused by the fungus Phakopsora pachyrhizi (Pp) is a destructive leaf disease of soybean that can cause severe yield losses. Discovered in Japan in 1902, SR spread to Africa by 1997, South America by 2001, and the U.S. by 2004.

Pp is an obligate parasite that relies on continuous production of aerially disseminated spores on a live host for survival and spread. Pp can survive on a variety of species including soybeans, dry and snap beans, peas, alfalfa and kudzu. In the United States Pp probably overwinters on annual and perennial alternative hosts in Florida, along the Gulf coast, and in southern Texas. Favorable environmental conditions, viable spores, and a susceptible host are necessary for SR to occur. Soybean infection is favored by temperatures from 21.8 to 24.3o C. and six hours or more of leaf wetness. Soybean is susceptible whenever green leaf tissue is present.

In the U.S, because of synoptic atmospheric conditions that occur during the growing season, it is probable that aerial dispersal of spores from infected hosts in southern states such as Texas, Louisiana, and Oklahoma, or Mexico, to susceptible crops in the north will determine if SR can occur in the North Central states. Sentinel plots for detection of SR were established at 25 locations in MN during the 2006 and 2007 growing seasons. Samplers for collection of spores deposited by both wet and dry deposition were located in all plots. Leaf samples and particulate filters were collected weekly and leaf samples analyzed visually for SR symptoms. Particulate matter was recovered from the filters by sonication, DNA extracted, and Pp spores detected with a nested PCR assay. Results indicating that Pp spores were present were confirmed by DNA sequence analysis.

Spores of Pp were detected in MN on four filter samples collected between July 31 and August 29, 2006 and seven filter samples collected between June 22 and August 23, 2007, periods with precipitation. However, no symptoms of SR were observed in MN in either in 2006 or 2007. The absence of SR symptoms may be a consequence of unfavorable environmental conditions, deposition of non-viable spores, or failure to detect SR symptoms at low disease levels, since the disease has been reported from near the vicinity in Iowa.