NPIPM:Asian soybean rust
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Asian soybean rust has been known in the eastern hemisphere for a long time. In 1994, the pathogen was reported in Hawaii. Throughout the first half of the 2000s, the pathogen was subsequently found in Paraguay, Bolivia, Brazil and Argentina. In November 2004, Asian soybean rust was first reported in the continental US. The pathogen was first found in a soybean field near Baton Rouge, LA.
Causal Organism
Asian soybean rust is caused by the fungus Phakopsora pachyrizi. The pathogen can infect a wide host range, most of which are leguminous plants. Another pathogen causing rust on soybean is Phakopsora meibomiae. Phakopsora meibomiae originates in Latin America and is known to be less aggressive than P. pachyrizi. There has been no report of P. meibomiae in the continental US. But it has been present in a localized area of Puerto Rico for many years
Symptoms and Signs
Symptoms start as small grayish-brown irregularly-shaped pustules (raised volcano-like structures) . The lesions that develop as the disease progresses may contain several pustules that may turn to a tan or reddish-brown color, depending on the host genetics. Symptoms are usually found first on the lower half of the canopy, spreading later intothe upper canopy. Lesions can also occasionally be found on pods and stems. The pustules produce powdery spores known as urediniospores. As the pustules mature, they break open and the urediniospores are released to the air. The presence of pustules and urediniospores are unique signs of soybean rust and should be used to differentiate this disease from other diseases such as bacterial blight or brown spot. Bacterial pustule is a particularly convincing look-alike disease. As rust severity increases, it may cause early defoliation and prematurate death.
Disease Impacts
Asian soybean rust suppresses yield by reducing pod number, seeds per pod and seed weight produced by infected plants. Yield losses of ten to eighty percenthas been reported due to Asian soybean rust: however, there are no documented losses above ten percent in the continental US.
Life Cycle and Epidemiology
The pathogen causing Asian soybean rust has been reported to survive mild winter conditions. P. pachyrizihas been shown to overwinter in kudzu in protected areas along the Gulf coast and in Mexico. Spores produced by P. pachyrizi are airborne and can be spread to nearby soybeans or longer distances by winds and storm systems. Soybean plants can be infected at all growth stages. Spores deposited on plant surface require the presence of free moisture for 6-7 hours and temperatures ranging between 65 and 80˚F (18-26.5˚C) to start infection, what are generally thought to be cooler conditions. About 9-10 days after initial infection, resulting spore producing structures can start producing spores for secondary infections. Leaf moisture or frequent rains are needed to support widespread infections.
Management Approaches
Scouting and Threshold
Sentinel plots have been set up throughout the US to monitor the movement of Asian soybean rust. Check the USDA ipmPIPE soybean rust map to follow the progression of the disease in a given year. Scouting for soybean rust is critical during the R1 (first flower) to R5 (beginning seed formation) growth stages. To scout for soybean rust, walk the field in a 'W' or 'Z' pattern and check the plants as you move through the field. As symptoms appear first on lower and mid-canopy, check the leaves in these zones for soybean rust symptoms and signs. It is important to identify soybean rust correctly since it may affect the management decision. A guide to differentiate soybean rust from other similar-looking diseases can be found atUSDA website. If soybean rust is suspected in a field, collect samples and send them to your state diagnostic clinic for a confirmation. Select leaves, stems or pods that show the symptoms of Asian soybean rust. Arrange the leaves between layers of cardboard and dry paper towels to keep them flat (i.e. a sandwich consisting of cardboard, paper towel, leaves, paper towel, cardboard). Place this arrangement within a self-sealing plastic bag to retain moisture. DO NOT add water to the bag, Care should be taken that the outer surface of the plastic bag is not contaminated by Asian soybean rust spores. Include a detailed information of collection (e.g. location, date, host plant) when sending the samples.
Host Plant Resistance
Currently no commercial soybean cultivar resistant to Asian soybean rust is available. Gene conditioning resistance against this pathogen has been reported and breeding programs to incorporate this gene to commercial cultivar are underway.
Chemical Control
In absence of resistant soybean cultivars, application of foliar fungicide is critical in managing Asian soybean rust. Generally fungicide should not be applied until the risk of infection is high and the crop is at the susceptible stages of R1-R6. A forecasting system provided by theUSDA ipmPIPE can be used to guide spray decisions.
