The way scientists currently combat rusts is they go down to northern Mexico each winter to survey the new strains each year so they can try to create pesticides and resistant wheat strains accordingly. The reason they go to Mexico is that the wheat grows there first, and then in Texas and the other border states when it gets a little warmer, and then finally you get the wheat crop in midwest and Canada. The spores are produced first in Mexico and then blow northwards and infect the new wheat in each climate region as it begins to grow. However, Ug99 is able to completely destroy 99% of the world's wheat population. If someone were to bring a single teaspoon of this stuff back from the middle east and put it in just the right field in northern Mexico then the potential exists to destroy a very large percentage of the north american wheat crop. Really scary stuff. It's been in the news this week because the Gates foundation just gave Cornell a 26 million dollar grant to work on the problem:
http://www.news.cornell.edu/stories/April08/wheat.rust.lm.ht...