There are two species of soybean rust Phakopsora pachyrhizi and Phakopsora meibomiae but P. pachyrhizi is the species that is of concern in the continental US. This pathogen is native to eastern Australia and eastern Asia.
Phakopsora pachyrhizi is an obligate parasite, which means that it must have live, green tissue to survive. It is mainly a foliar disease; eventually infection may also be seen on leaf petioles and pods. This pathogen is spread by airborne spores that are capable of remaining airborne throughout large sections of the soybean growing areas. Fungal spores blow into the fields and land on soybean leaves. Under favorable conditions leaf spots and pustules (uredinia) form on infected leaves. Spots are visible 4 days after infection and pustules can be seen within 10 days. Each pustule can produce spores for about 3 weeks, these spores are easily carried and dispersed by wind causing more infections. Disease incidence and severity increase when canopy closes and crop begins flowering. The cycle continues over and over until the entire crop is defoliated or when environmental conditions are no longer favorable for disease development. Premature defoliation can occur 4 to 6 weeks after initial infection.
Apparently Asian Soybean Rust moved over the years from Asia and Australia to Africa (1997) then to South America (2001) and then to North America (2004). It was first discovered in continental U.S. in November 2004. Appearance of the pathogen simultaneously among several states suggested that the spores were introduced into the U.S. by hurricane Ivan. Since its arrival it has dispersed within the country and as of 2008 soybean rust was found in 392 counties within 16 states. This included 56 counties in Alabama, 66 counties in Arkansas, 82 counties in Georgia, 24 counties in Florida, five counties in Illinois, 32 parishes in Louisiana, 4 counties in Kentucky, 1 county in Maryland, 79 counties in Mississippi, 1 county in Missouri, 5 counties in North Carolina, 1 county in Oklahoma, 16 counties in South Carolina, 5 counties in Tennessee, 5 counties in Texas, and 10 counties in Virginia. In Mexico, rust was also reported in 14 municipalities (counties) within 4 states. Rust remains a threat especially under mild winters but it can be controlled when detected early.
Constant crop monitoring plays a major role because within 10 days one leaf can be severely infested. When scouting, focus on early planted fields with early maturing varieties, low lying or protected fields that have prolonged dew periods and fields with early canopy closure. Make sure to check the lower canopy if scouting before flowering. After flowering look for rust half way up the plant. A long stick for parting the canopy, a marker to mark suspected areas on the leaf and 10 to 20X hand lens help detect pustules on the underside of the leaves. It is recommended to place the hand lens close to your eye and bring the leaf towards the hand lens. Look at the leaf facing the sun, so that you have a light background. When sampling a field, select 20 locations and sample 5 plants per location, for a total of 100 plants sampled. Contact your local extension agent if you suspect rust in a new area or if management recommendations are needed. Submit a sample to your local plant diagnostic clinic (http://www.sbrusa.net/NPDN_Pathology.html). It is very important to remember that rust adhere very easily to clothing and boots so if possible put on a disposable spray suit and change and wash before moving to the next location.