Soybean rust is a serious foliar disease causing millions of dollars of yield loss in soybean production areas around the world. In November 2004 it was first detected in the United States and since then has been identified in several southeastern states. The disease is not expected to have overwintered at all the sites identified last fall. The first U.S. case of soybean rust to have overwintered was reported in Florida the week of March 1. This, if it is the only location of overwintering, will supply the needed initial inoculum for the disease to spread and develop over the U.S. soybean crop.
Soybean rust, also referred to as Asian soybean rust, is an aggressive pathogen that has spread in the past 10 years from Asia to Zimbabwe, South Africa, Paraguay, Brazil, and now the United States. Yield losses can be severe and have ranged from 10% to 80% of a field. In the last few years, Brazilian soybean producers have been significantly affected by soybean rust. In 2002-2003 they spent $600 million for fungicide applications and in 2003-04 it’s estimated they spent more than $1 billion. USDA estimates U.S. losses could be in the range of $640 million to $1.3 billion in the first year and $240 million to $2 billion in subsequent years, depending on the severity and extent of spread. I want to make note of a key point: This disease is manageable. U.S. producers will adapt to the needed management and our soybean crop will continue to be strong.
Symptoms Soybean rust symptoms start in the lower canopy and are most commonly observed on the leaves. Lesions also can develop on petioles, pods and stems. Symptom development occurs rapidly once pod set occurs and can result in significant levels of defoliation under favorable environmental conditions. Lesions first appear as small yellow and irregularly shaped spots. As the disease progresses, lesions enlarge to 1/16- to 1/12-inch in diameter and are tan to dark reddish brown. Within each lesion are a few to several volcano-shaped spore-producing structures. These features can only be seen under magnification (20X recommended). As rust severity increases, plants prematurely lose their leaves and commonly mature early. Lesions from soybean rust can appear similar to other foliar diseases of soybean and can be confused with brown spot and bacterial pustule. See Identifying Soybean Rust (Cooperative Extension EC05-1892) for more information. This I.D. card is available from local Cooperative Extension offices or online at http://www.aphis.usda.gov/lpa/issues/sbr/SBR_IDcard_11-04.pdf
Spores are produced in great numbers and are readily dispersed by wind. The cycle repeats multiple times throughout the season, moving with the wind into new areas. The soybean rust fungus will not overwinter in Nebraska and will have to be reintroduced each year, similar to other crop rusts. After contacting a host, spores germinate to produce a germ tube that grows across the leaf surface.
Soybean rust development is favored by temperatures of 59-84°F and a relative humidity above 90% for more than 12 hours. In order for spores to germinate and infect the plant, six hours of continuous leaf wetness is required. Infection increases with longer leaf wetness periods up to 12 hours. In South America, significant rust development is associated with rain.
Soybean rust is airborne and spreads via spores. A spore can infect a plant and cause a new lesion which will produce spores in 7-10 days. Under favorable temperature and humidity conditions, spore production in a field can double every two days and over 400 billion spores per acre are produced. There are more than 87 million acres of soybeans in the United States which can serve as a building inoculum source for the disease as it progresses northward each year.
Dave Varner is an Extension Educator with the University of Nebraska Cooperative Extension office in Dodge County. Dave may be contacted at 727-2775, 1206 West 23rd Street in Fremont, or at dvarner1@unl.edu
