P. pachyrhizi is the species that as gotten all of the attention. It has quickly moved from its place of origin in southeast China in to Africa then to South America. In South America, it first arrived in Paraguay during 2001 – and from there it has been confirmed in all of the soybean regions in Brazil with reports from northern Argentina. Yield losses in South America ranged all over the board – Speculation on potential yield losses in the U.S. ranged from as high 50% or more. Rust will change the way we produce soybeans – when in the US ranged from as high 50% or more. Rust will change the way we produce soybeans – when it gets here.
Four routes have been proposed for soybean rust to reach the U.S.
- Via the Central American land bridge. This rust has a broad host range – so conceivably it could infect and colonize hosts all through central America to reach the U.S. This would take awhile and is dependent on hosts all the way through Central America.
- On the winds of a hurricane. Conceivably a storm could pick up lots of rust spores from production fields in northern Brazil and carry these across the Caribbean and drop spores on winter nurseries in Southern Florida or on the Kudzu that grows through out the South. Once the rust is established here, then with spring and summer storms it would track via the rust pathway we are familiar with for both corn and wheat. There are no reports of this happening (February 2, 2004). Of all of the possible outcomes – this may have the highest probability. This is also the easiest for us to implement management strategies – because we already have recommendations and procedures in place for wheat and corn – they would only need to be tweaked for soybeans.
- Spores that survive in debris in shipments of seed or meal. Residue or debris is that is left near production fields were the spores could infect soybean plants. This is questionable. We know from corn and wheat rust fungi that the spores do not survive and serve as inoculum sources in or on seed. The debris is buried with the seed or spores are no longer alive from the time of harvest to planting season. We can take additional precautions here, bury debris from containers away from production fields, and monitor fields that are close to ports or facilities were seed cleaning may take place.
- Bioterrorism - this is the big unknown. However, on a practical level, I also think this is unlikely. It is very difficult to get successful infections with many fungi by just spreading them onto plants, there are very specific environmental parameters that have to be met in order for such an attack to be successful.
For 2004, we should be vigilant in scouting and watching our fields for unusual leaf spots or large numbers of leaf spots. I don’t think it’s wise to stockpile fungicides. Efficacy, cost, and timing are all data that you will want before making a wise decision to managing this pathogen and BEFORE you make that financial investment. But, knowing how much Roundup is sprayed in this state over such a sort period of time, I am highly confident that Ohio producers and custom applicators are up to the task to make fungicide applications should they become necessary.