Soybean rust has a range of hosts but in the U.S. we know of only soybeans and kudzu. Hosts other than soybeans are important because they serve as a green bridge for rust and keep it alive until the next soybean crop is planted.

In east central Mexico in the state of Veracruz soybeans are grown and there is some kudzu, but it is uncommon. It is considered possible for soybean rust to move up out of Mexico over the winter into Texas in the early spring and create an infection that could eventually feed into the Midwest. However that was considered unlikely since soybeans are not grown over the winter months in this part of Mexico.

This summer there have been extensive soybean rust sightings in Texas. Tom Isakeit, extension plant pathologist with Texas A&M University at College Station believes that the infection in Texas started when spores blew in from Mexico. Isakeit believes that Jicama or yam bean was the original source of spores. Once the spores were produced they just followed the rust pathway to Texas where they first landed on soybeans in the Rio Grande Valley. Then spores and infections flowed north.

Yam bean is a legume grown for its large tuberous roots and seeds in Mexico. Last January Mexican offices confirmed the presence of soybean rust on yam bean leaves in the state of Veracruz.

According to a report posted in July on the North American Plant Protection Organization's Phytosanitary Alert System, "by the middle of January 2007, the Plant Health General Directorate received samples of yam bean (Pachyrhizus erosus) crop foliage from the communities of Agua Dulce and Pajapan, in Papantla township, in the state of Veracruz. These samples showed signs and symptoms of Asian soybean rust (Phakopsora pachyrhizi)."

Additional laboratory and morphology analysis was done. This analysis confirmed the presence of Phakopsora pachyrhizi, the causal agent of soybean rust.

Yam bean was not listed among the more than forty known hosts for soybean rust but now it can be added to the list. Knowing that yam bean is an alternate host for rust means that it can serve as a green bridge for the pathogen.

In Mexico farmers that grow yam bean will have to monitor for the disease on yam bean and spray on fungicides to minimize the spread of the disease on soybeans the following season and to prevent the release of spores that could travel to Texas, jump starting and infection there.