What is soybean rust? Soybean rust is an extremely serious fungal disease of soybean that was first reported in the continental United States in November of 2004. The disease has never been reported in Wisconsin. Soybean rust had previously been reported in Asia, Australia, Africa and South America, where yield losses due to the disease ranged from 10 to 80%. In addition to soybean (Glycine max), soybean rust affects approximately 90 other plant species in the legume family. In Wisconsin, other potential hosts include snap and kidney bean (Phaseolus vulgaris), American bird’s-foot trefoil (Lotus unifoliolatus), crimson clover (Trifolium incarnatum), Korean clover (Kummerowia stipulacea), white clover (Trifolium repens), purple crownvetch (Coronilla varia), Chinese lespedeza (Lespedeza cuneata), lupine (Lupinus spp.), pea (Pisum sativum), rattlebox (Crotalaria spp.), yellow sweetclover (Melilotus officinalis), ticktrefoil (Desmodium spp.), and winter vetch (Vicia villosa).
What does soybean rust look like? Initial symptoms of soybean rust include formation of small, gray spots on soybean leaves, particularly on the undersides of leaves. Spots are most likely to occur first on lower leaves where conditions are more favorable for spores to germinate and infect. Infections can also occur on petioles, stems and pods. Spots increase in size over time and change color from gray, to tan or reddish-brown. Tan lesions mature to form small pimple-like structures (called pustules) on the lower leaf surface. Pustules contain powdery, tan spores that give the leaves the appearance that they have dandruff. Reddish-brown lesions are composed of primarily necrotic (i.e., dead) tissue and typically have only a limited number of pustules. As plant canopies close and pods begin to set, the soybean rust fungus can rapidly spread from lower to upper foliage of plants. Other diseases of soybean including brown spot [see UW Plant Disease Facts, Brown Spot (Septoria Leaf Spot)], bacterial pustule and particularly downy mildew could potentially be confused with soybean rust.
Where does soybean rust come from? Soybean rust is caused by the fungi Phakopsora pachyrhizi and Phakopsora meibomiae. P. pachyrhizi is the more aggressive of the two species, and the fungus that was introduced into the continental United States in 2004. P. pachyrhizi is thought to have been brought to the U.S. through hurricane activity in the late summer of 2004. Soybean rust fungi must overwinter on living plant tissue. Therefore, if soybean rust fungi ever reach Wisconsin, they are not likely to survive Wisconsin winters. In the South, however, plants such as kudzu (Pueraria montana var. lobata) can serve as overwintering hosts. Soybean rust spores produced on these plants could be moved north each year by prevailing winds, as is known to occur with other rust fungi (e.g., the corn rust pathogen). Soybean rust fungi may eventually reach Wisconsin via this route. This movement of spores via prevailing winds could occur each year, thus making soybean rust a recurring problem.
How do I save a soybean plants infected with soybean rust? If you suspect that your soybeans are suffering from soybean rust, proper diagnosis is crucial to document the presence of the disease in Wisconsin. Contact the UW-Madison Plant Disease Diagnostics Clinic (https://pddc.wisc.edu/) about submitting a sample for diagnosis. Keep in mind however that once soybean plants are infected and the soybean rust fungus has begun to produce spores, control of the disease is difficult and significant yield losses are likely. Fungicides with “curative” properties are registered for use against soybean rust in Wisconsin. However, curative fungicides have a very limited ability to eliminate existing disease and by the time soybean rust is observed, these products will likely not provide adequate control. Therefore, every attempt should be made to prevent infections (see below), rather than to attempt to control soybean rust after infections have occurred.
How do I prevent problems with soybean rust? Plant soybeans as early as possible, so that if soybean rust does occur, plants are as mature as possible when infection occurs, and yield loss can be minimized. Researchers throughout the soybean-producing regions in the United States monitor for soybean rust each growing season (see https://soybean.ipmpipe.org/soybeanrust/). Watch for reports of the disease to the south of Wisconsin and consider preventative fungicide treatments as the rust fungus approaches the state. Products containing chlorothalonil, strobilurins and triazoles (the latter two types of active ingredients often combined into a single product) are labeled for preventative control of soybean rust. Combining strobilurins with triazoles helps reduce the risk of selecting for variants of the soybean rust pathogen that will no longer be controlled by these active ingredients. If you decide to use fungicides for control, be sure to select a formulation that is labeled for use on soybeans, and be sure to read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.
For more information on soybean rust or help in diagnosing this disease: Contact Damon Smith, Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI 53706-1598, [phone: (608) 286-9706, fax (608) 263-3322, email: email@example.com]; or contact Brian Hudelson, Plant Disease Diagnostics Clinic, University of Wisconsin-Madison/Extension, 1630 Linden Drive, Madison, WI 53706-1598 [phone: (608) 262-2863, fax: (608) 263-3322, email: firstname.lastname@example.org]; or contact your county Extension agent.
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