The arrival of Phakopsora pachyrhizi, the fungus that causes soybean rust, has been confirmed in 9 southern states and has caused great concern for everyone in the soybean industry.  Although no yield loss occurred this year due to its late arrival, the potential now exists for damage this coming season.

Opinion varies on the time required for soybean rust to reach its potential range throughout the U.S., but it could take one season or several years. Each year may differ as the pathogen will not over season in temperate areas unless it is kept inside on soybean plants in the greenhouse.

All soybean-growing areas of the U.S. are at risk with the most favorable weather conditions (based on 30-year averages) in the Mississippi delta region and the Midwest.  In general, southern growing areas are at greater risk than northern areas because of the longer growing season, the warm humid climate that is favorable for rust development, and the presence of other hosts such as kudzu.

Risk analyses indicate that P. pachyrhizi could cause yield losses of greater than 10% in any U.S. soybean-growing region, while in southeastern coastal states, losses up to 50% or more are possible.  Of course this is all dependent on weather conditions during the growing season and the actual point where the pathogen overwinters.

Not likely, as the main infective spore of soybean rust, the urediniospores cannot survive temperate conditions since there is no living host.  Areas that are likely to support year-round occurrence of P. pachyrhizi are the southern parts of the very southernmost United States and farther south of the U.S. where freezing temperatures are rare.  The presence of kudzu and several other hosts in those southernmost areas would create conditions for the rust fungus to become established.

No, the fungus is not seed-borne.  There is a very slight chance that spores could be associated with debris in seed.  Research is ongoing to determine the influence of storage time and temperature on the survivability of rust spores.  For meal, there is virtually no chance of contamination, since both the meal and the foreign materials it contained were heat-treated, eliminating the risk that rust spores could survive.  The American Soybean Association (ASA) has worked closely with the U.S. Animal and Plant Health Inspection Service (APHIS) to reduce the risk of accidental introduction of rust.

Yes, there are effective fungicides in use in other countries.  If soybean rust survives in the U.S., fungicides will be the major tool to protect yield.

In addition to efficacy, there are other important questions being addressed in current USDA-ARS fungicide trials in Africa and South America:  application methods, the effect of fungicides on the crop, timing, minimum number of applications, and effective rates for each compound, as well as the economics of using fungicides on soybean.  The rust fungus is a highly variable pathogen, and resistance management will be an issue.

It depends when the rust pathogen arrives and on the extent of the outbreak.  Section 18 Emergency Quarantine Exemption requests have already been drafted in order to expand the number of fungicides available if needed.  Once approved, the Section 18 Emergency Quarantine Exemption will be activated.

When will varieties be available that are genetically resistant to soybean rust?

Best estimates say at least 4 to 6 years.  The good news is that such groundwork has been done.  Partial resistance or tolerance is likely to be the most effective strategy, where fewer lesions and a reduced number of spores develop.  Planting cultivars with resistance to rust pathogens has been very successful in managing rust in wheat and corn and is expected to be a successful approach with soybean rust.

State and national soybean associations funded by soybean checkoff dollars have been the leaders in preparing for soybean rust.  They have funded research on rust movement and occurrence and research on soybean resistance and the screening of commercial soybean varieties.  They have been at the forefront of preventing accidental introduction of rust into the U.S., promoting federal funding for research and encouraging the EPA and chemical companies to work on the availability of fungicides.  Soybean checkoff dollars have funded much of the information on rust available to soybean growers.

Continue to stay informed, as you are now.  If you suspect soybean rust is present in your fields this season, contact your local county Extension office, a certified crop advisor, Extension plant pathologist, or your state university plant diagnostic lab.  Any of these resource people will be able to determine if it could be rust or another soybean foliar disease.

The most common symptom of Phakopsora pachyrhizi is small tan to dark brown or reddish brown lesions.  The disease begins with small, water-soaked lesions, which gradually increase in size, turning from gray to tan or brown.  They assume a polygonal shape restricted by leaf veins and usually coalesce to form larger lesions.  Lesions can appear on petioles, pods, and stems but are most abundant on leaves, particularly on the underside of the leaf.  As the plant matures and sets pods, the symptoms spread rapidly to the middle and upper parts of the plant.

Especially at the early stages, it is easy to confuse the symptoms of soybean rust with symptoms of three other soybean leaf diseases:  brown spot caused by the fungus Septoria gycines, bacterial pustule caused by Xanthomonas axonopodis pv. glycines, and bacterial blight (also called angular leaf spot) caused by Pseudomonas savastanoi pv. glycinea.

Asian soybean rust is an aggressive disease that destroys photosynthetic tissue, causing premature defoliation, early maturation, and lower yields.  The most severe epidemics occur when soybean leaves are infected early in the growing season; however, leaves are susceptible at all stages of plant growth.

Some common leguminous crop plants and weeds are also hosts, including yellow sweet clover (Melilotus officinalis), vetch (Vicia dasycarpa), medic (Medicago arborea), lupine (Lupinus hirsutus), green and kidney bean (Phaseolus vulgaris), lima and butter bean (Phaseolus lunatus).

Corn and other grain crops are not hosts to Asian soybean rust.

Soybean rust dispersal is highly dependent on environmental conditions.  Once the pathogen is present, abundant spore production occurs during wet leaf periods of at least 8 hours (including extended dew periods) and moderate temperatures of 60 to 80^oF.  The spread of the disease within a field can occur quickly.  Long-distance dispersal is dependent on wind patterns and weather conditions and is the subject of current research.

The timing of scouting will depend on your geographic location within the United States.  Soybean producers in southern growing areas will need to begin scouting before their counterparts in more northern growing areas because of where Asian soybean rust is expected to overwinter.

In southern growing areas begin scouting in mid-June and continue through mid-August.  Check the undersides of the lower leaves for a yellow mosaic discoloration and reddish-brown to tan pustules when leaflets are well developed.

Current research from Brazil suggests that the first application should be applied at the initiation of flowering (R1) before rust infection becomes established.  The second application, if needed should be applied 20 days later.

Based on current research from Brazil, soybean producers will likely need to apply two to three applications of a fungicide to obtain optimal results.  In Brazil during the 2003-2004 season, it was estimated that 95% of the acreage was treated with 1½ to 2 sprays.  Short-season varieties only required one spray, while long-season varieties needed 2-3 applications.

Opinion varies on the time it will require for Asian soybean rust to reach its potential range after the initial introduction into the United States.  It could take one season or several years.

Once established in the southernmost areas of the U.S., the spread of rust from south to north will be different each year.  The movement will depend on the amount of inoculum that overwinters in the south each year and on spring weather conditions.

No, the spread of soybean rust has been a natural event.  There is no evidence of intentional introduction by humans.

No.  As more land was planted into soybean around the world, the spread of soybean rust eventually came with it.