Soybean Production in Mississippi
Soybean Rust - Causal Organism
Soybean rust (SBR) can be caused by two fungal pathogens: Phakopsora pachyrizi, the most notable causal agent (and the one currently found in North America), and internationally to a lesser extent Phakopsora meibomiae. To date, P. meibomiae has never been found in the contiguous United States.
Phakopsora pachyrizi is widely known as the Australasian or Asian strain of the fungus (now just referred to as soybean rust due to its geographical distribution). This particular pathogen is the more aggressive of the two fungi associated with SBR. Since first being identified, the fungus has been reported from Argentina, Australia, Bolivia, Brazil, China, Indonesia, Japan, Korea, Malaysia, Nigeria, Paraguay, Philippines, Rwanda, South Africa, Uganda, Zimbabwe, and is now found in the United States.
Phakopsora meibomiae is a less aggressive rust causing fungal species. It can occur on soybeans and other legumes, but has not been reported to have a significant impact on yield. This particular species has been reported from Argentina, Brazil, Costa Rica, Cuba, Guatemala, Mexico, Paraguay, and Puerto Rico.
SBR has many different hosts. Research has documented more than 95 plant species that are affected by this fungus. One common host found throughout Mississippi and the southeast in general is kudzu. Estimates on kudzu acres in Mississippi range from 250,000 to over 530,000 acres. In addition, kudzu has both resistant and susceptible genotypes throughout the state making scouting kudzu for SBR a challenge unless the disease has previously been detected at that location. Other hosts include: certain clovers, coral bean, crown vetch, cowpeas, Florida beggarweed, green beans, lespedeza, and lima beans. It should be noted that while there are numerous additional host plants most of them lose their leaves in response to infection from the fungus and thus would not serve as a major source of inoculum. Moreover, most of the additional host information has been derived from greenhouse studies in situations where the plants are inoculated with the fungus and not due to the plants having been identified as susceptible in the field. Even when the disease was detected, following inoculation with the fungus, it has been at low levels and has not been commonplace (for a more extensive list of potential hosts from the USDA see the attached table).
For SBR to occur, three specific characteristics need to coincide with one another. A susceptible host, the pathogen and a favorable environment that is conducive for disease development (generally considered to be 61 to 82°F with approximately 100 percent RH). Higher temperatures (above 82°F) are generally considered to be unfavorable for the development or spread of the fungus. SBR can affect the stems, cotyledons, petioles, leaves, and pods. However, to date the disease has only been detected on kudzu and soybean leaves in Mississippi.
The disease 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 environmental conditions it will germinate and produce an appresorium that will penetrate the leaf surface directly. Uredinia begin to appear and in approximately 9 days will sporulate. It generally requires about 7 days from the time infection occurs until lesions first appear. Infection generally begins in the lower third of the canopy and as the disease becomes worse the infection will move up into the middle and upper canopy of the plant. Lesions developing on the leaf tissue first appear as chlorotic regions and progress into tan to reddish brown areas surrounded by necrotic areas. However, scouting for early symptoms of the disease is a difficult task since the chlorotic areas can be similar to numerous other diseases that occur on soybeans in Mississippi. Pustules form on the underside of the leaf surface. In rare cases teliospores will develop but this typically occurs at the end of the season. Teliospores are part of the sexual reproduction process of the fungus. This accounts for genetic variability in the pathogen population. To date, teliospores have not been detected within Mississippi; however, several states have detected them including, Florida and Texas.
Other Rust Information