Soybean rust has been positively identified in nine states in the US. This surprised all the experts because most assumed that this pathogen would not arrive for another year or two. Unfortunately, you will receive a great deal of misinformation over the next few months about soybean rust. Here are the facts…

This disease is caused by one of two fungal pathogens:

1. Phakopsora pachyrhizi or Asian Rust
2. Phakopsora meibomiae or New World species

For this article we will be discussing the pachyrhizi species (Asian rust), because it is the more aggressive one. It causes major economic losses in soybeans and other leguminous crops across the globe if not controlled in a timely manner.

The disease was first identified in Japan in 1902. Ever since that time it has traveled across the globe mainly as spores riding on air currents. In 2000 it was found in South America. In Brazil, the airborne spores traveled 1000 miles in just one year, infecting crops and causing losses to growers in its path. It infected about 100,000 acres in 2002 and spread to some 10 million acres in 2003. In 2004 it is speculated that the spores traveled from South America on Hurricane Ivan and landed in the US. The fact is that the disease is here; and even if you do not believe you will have rust this year, you need to understand its potential rapid spread, its destructive nature, and the various crops that are susceptible. You need a definite plan for optimum methods of control because losses could vary from 10 to 80%, depending on time of infection and environmental conditions.

In order for the disease to flourish, it requires that the pathogen be present on a suitable host under the proper environmental conditions. Midwestern winter temperatures are hostile to spore survival. However, these spores will survive in the southern states on plants like kudzu and numerous others that stay green during the winter months and possess some degree of susceptibility to this pathogen. For rust to get established in the Midwest, the spores must be blown in from the south. Disease development is favored by long periods of wetness if temperatures are between 50 and 80 degrees F. Under such ideal conditions, the spores will germinate on the underside of the lower leaves and penetrate the leaf cuticle and the epidermal layers. They do not depend on the open stomata to gain entry into the leaves.

Once in the plant the disease moves upwards, causing defoliation and preventing adequate pod fill. Plants under stress of any sort, including other diseases and insects, will be affected more severely, even in the early stages of this disease development. The problem is that it is difficult to identify during its early stages of development on the host plant.

There are monitoring processes in place to trap spores in rain water, etc., as they attempt to make entry into our soybean growing areas. Once identified in any region, the most difficult decisions will be when to start treatment and what products to use. As far as products go, before you pick which one(s) you’ll use, keep in mind that some are preventative, some are curative, some are both. Also, there are 3 main fungicide families: triazols, strobilurins, and chlorothalonils. There is much more to explain about this disease, but for now, here are my suggestions for you:

1. If you raise any legume crop, learn all you can about soybean rust. Attend the Ag PhD Weed, Insect, and Disease Workshop in your area in February, as we will spend a great deal of time teaching you everything you need to know about soybean rust.
2. Buy some fungicide now! If you wait, you may not get the product(s) you want, and worst-case scenario, you may not get anything.
3. Scout your fields this summer and watch for our continued updates on Ag PhD TV, Ag PhD radio, and with our free Daily Crop Scouting Reports (sign up at agphd.com – did I mention these daily in-season reports are free!?!)
4. Don’t panic. If you manage things well, rust will not hurt you severely. In fact, you may profit from it, because even though you will have increased costs, you may see $8 or $10 soybeans again. On the other hand, rust may not become a major problem in 2005 either. Only time will tell. Good luck!