Rust Home Page Causal Organism Symptomology Control Sentinel Plots Current News FAQ's Other Links Publications Contact     Soybeans Home Page     MSUcares Home Page		Causal Organism Soybean Rust is caused by two pathogens: Phakopsora pachyrizi and Phakopsora meibomiae. Phakopsora pachyrizi is of the Australasian strain or Asian strain. This particular pathogen is the more aggressive of the two fungi associated with soybean rust. It is found in Brazil, Argentina, Paraguay, Australia, China, Korea, Malaysia, Indonesia, Japan, Philippines, Nigeria, Rwanda, Uganda and is now found in the United States. Phakopsora meibomiae is a less aggressive strain of rust. It can occur on soybeans and other legumes, but does not have a significant impact on yield. This particular strain has been found in Brazil, Argentina, Paraguay, Costa Rica, Cuba, Guatemala, Mexico and the Caribbean islands. Soybean rust has many different hosts. Research has documented more than 95 plant species that are affected by this pathogen. A common host that is found throughout the state of Mississippi and the south east in general is Kudzu. Other hosts include: certain clovers, crown vetch, lespedeza, lima beans, green beans and cowpeas (Figure 6). For disease to be present, three things must be present. You must have a susceptible host, the pathogen and the right environment that is conducive for disease development. Soybean rust can affect the stems, cotyledons, petioles, leaves and pods. This will typically lead to premature defoliation which in turn reduces the amount of photosynthates the plant can utilize. The limiting amount of photosynthates reduce yield by affecting seed weight and size. Once a rust urediniospore lands on a susceptible host under favorable conditions (Figure 7) it will germinate and produce an appersorium that will pentrate the leaf surface directly. Uredinia begin to appear and in approximately 9 days they begin to sporulate. It generally takes about 7 days from the time infection occurs till lesions begin to appear. Infection begins in the lower third of the canopy and as the disease gets worse the infection will move up into the middle and upper canopy of the plant. Lesions developing on the leaf tissue begin as chlorotic regions and progress into tan to reddish brown areas that are surrounded by necrotic areas. Pustules form on the underside of the leaf surface. In rare cases teliospores will develop. Teliospores are part of sexual reproduction with the fungus. This accounts for genetic variable in the pathogen.
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