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Common Soybean Leaf Diseases and Soybean Rust

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Common Soybean Leaf Diseases and Soybean Rust Common Soybean Leaf Diseases and Soybean Rust oybean rust was found in Iowa State University Extension Each event is equally important S several states in the southern plant disease scientists expect because an outbreak will not occur United States in November of that outbreaks of soybean rust in if one of those events is missing. 2004. The rust pathogen can only Iowa will happen only after several The early symptoms of soybean survive on living plant material. connected, timely events. The rust are similar to other diseases Although the soybean rust fungus three primary events that must that are more common to Iowa may not overwinter in the central happen each crop season are the soybean fields, so information on or northern soybean production production of spores early in the the similarities and differences regions of the United States, it season in the South, the spread between soybean rust and these does survive the winter months on of large numbers of those spores diseases have been included in this hosts, such as kudzu, in the south- from there to Iowa, and then the publication. ern United States. occurrence of weather in Iowa that is conducive for disease establish- ment and development. Soybean leaf top showing rust Soybean leaf bottom showing rust Soybean field showing rust PM 1989 Revised May 2007 SYMPTOMS Weather conditions in Iowa will Early symptoms of rust infection not always favor widespread or begin on the lower leaves deep severe rust development, even in the canopy as small spots that when spores are present. However, increase in size and change from dense canopies will provide an gray to tan or reddish brown usu- ideal microclimate that encourages ally on the lower leaf surfaces. disease development. Mature spots consist of small pustules (pimple-like structures) SCOUTING FOR SOYBEAN RUST surrounded by slightly discolored Follow the national movement dying areas with masses of spores of rust closely. If rust on the lower leaf surface (Figure development in the southern 1). Lesions are scattered within © X. B. Yang U.S. is slow, scouting efforts yellow areas that appear see- Figure 1. Advanced disease produces spots can be delayed. If needed, through (translucent) if the affected usually clustered alongside the veins. local scouting should be done leaves are held up to the sun between the R1 growth stage (Figure 2). Once pod set begins on and R5 growth stages (mid to soybean, infection can spread rap- late August). idly to the middle and upper leaves of the plant. 1. Walk through an entire field in a standard scouting pattern. After infection, pustules (Figure For example, walk across 3) can be seen after about 10-14 the field in a W-shaped days. The pustules produce mass- © J. T. Yorinori pattern. es of spores (Figure 4), and spore Figure 2. Yellow areas appear translucent. 2. Periodically stop and examine production may continue for soybean plants. Look low and weeks. Spores are easily spread deep into the canopy of the by the wind. Soybean plants are plants. susceptible to soybean rust at 3. Closely examine the plants any stage of development, but for mottled yellow leaves symptoms are most common with pustules (pimple-like during and after flowering. The structures) on the underside. disease usually starts within the If you suspect rust but no low to mid canopy and moves up pustules are present, a the plant (Figure 5). © X. B. Yang sample of leaves can be Figure 3. Pustules (pimple-like structures) found place in a self-closing plastic DEVELOPMENT on the underside of the leaf. bag. The leaves can be The development of soybean rust inspected daily for up to a is favored by prolonged periods week for the development of of leaf wetness (6–12 hours) pustules. and moderate temperatures of 4. Areas in the field with distinct 60 to 85 degrees F. Extended yellowing or browning of the periods of cool, wet weather or leaves or areas with dense high humidity (75-80 percent) canopy should be targeted in addition to those areas during the growing season would covered by the standard favor soybean rust epidemics. scouting pattern. Figure 4. The pustules produce masses of orange to brick-red spores. Brown Spot Bacterial Blight Bacterial Pustule Septoria glycines Pseudomonas savastanoi pv. glycinea Xanthomonas campestris pv. glycinea Causes small, irregular-shaped, dark Causes small, angular, yellow-to-brown Causes small, yellow-green spots with brown spots scattered on the upper and spots surrounded by yellow halos. angular reddish-brown centers. The spots lower surface of leaves. Adjacent spots The angular spots enlarge and join may join together to form large, irregular frequently join together to form dead together producing large, irregular dead areas that rupture and tear away dur- blotches. Late in the season, affected dead areas. The centers of old dead ing windy, rainy weather. Pustules may be leaves may turn yellow or orange-brown areas tear away so infected leaves have seen on the underside of the leaf surface. and drop prematurely. a ragged appearance. This disease is seen on the leaves at the This disease is very common, and is This disease is seen on the leaves at the top of the plant. Favorable conditions are usually one of the first to appear on top of the plant. It is very common and high temperatures and higher-than-aver- young plants, starting at V2 stage. It usually one of the first to appear on young age rainfall. begins at the bottom of the plant and plants, starting at V2 stage. It is common moves up the plant if conditions are after heavy rains and if temperatures favorable (warm and wet). remain cool. How does this differ from soybean rust? How does this differ from soybean rust? How does this differ from soybean rust? Spots more angular; no pustules on the Angular spots; ragged leaves; no Pustules not associated with each lesion; underside of the leaf. pustules on underside of the leaf. pustules do not have spores; pustule openings are cracks instead of round pores. Frogeye Leaf Spot Cercospora Leaf Blight Downy Mildew Cercospora sojina Cercospora kikuchii Peronospora manshurica Causes small angular spots with gray Starts as a mottled purple-to-orange Causes light-green to yellow, irregular centers and distinct purple to reddish discoloration that becomes orange or shaped spots on the upper surface of the brown margins. In older spots, dark bronze. The leaves become leathery topmost leaves. On the underside of the fungal structures form in the center in texture. leaves, the fungus may be seen growing of the spot and the spots look like out of the center of the spots. frog eyes. Usually occurs on topmost three to four trifoliate leaves and on the upper surface Usually occurs on topmost three to four This disease is found in the mid and of the leaf in warm, wet weather. It usually trifoliate leaves and on the upper surface upper canopy during warm and humid occurs in mid to late season. of the leaf in warm, wet weather. Usually weather. It usually occurs in mid to late occurs in mid to late season. season. © X. B. Yang How does this differ from soybean rust? How does this differ from soybean rust? How does this differ from soybean rust? Spots are larger; spots have dark, Only upper leaf surface discolored; few Spots are larger; no pustules on under- defined edges; no pustules on underside spots and no pustules on underside of the side of leaf; fuzzy fungal growth may be of leaf. leaf. seen. Fungicides currently Updated by Daren Mueller, Soybean Producer are the only viable program specialist, Department option for manage- of Pathology. First Detector ment of soybean rust. To manage the By Alison Robertson, assistant Triage Team Member disease effectively professor, and Greg Tylka, and economically, professor, Department of Plant ISU Plant Disease Clinic spray the chemicals Pathology. at the recommend- Figure 6. Iowa Fast Track System ed time. File: Pest Management 2-5 For more information on fungicide FAST TRACK SYSTEM— . and justice for all use for management of soybean The U.S. Department of Agriculture If I Suspect My Soybeans rust, please see ISU bulletin PM (USDA) prohibits discrimination in all its Have Soybean Rust? 2028 Managing Soybean Rust programs and activities on the basis of race, color, national origin, gender, religion, Please use the Iowa Fast Track with Fungicides 2006 or visit age, disability, political beliefs, sexual ori- System (Figure 6) to get confirma- www.soybeanrust.info. entation, and marital or family status. (Not tion. This system was put in place all prohibited bases apply to all programs.) Many materials can be made available in to alert the ISU Plant Disease Iowa Soybean Rust Team alternative formats for ADA clients. To file Clinic that a suspected sample is a complaint of discrimination, write USDA, To help Iowa producers prepare for on its way for examination, and to Office of Civil Rights, Room 326-W, Whitten soybean rust, Iowa State Univer- Building, 14th and Independence Avenue, provide quick, no-cost confirmation SW, Washington, DC 20250-9410 or call sity (ISU), the Iowa Soybean As- 202-720-5964. to Iowa soybean producers. sociation, the Iowa Department of Agriculture and Land Stewardship Issued in furtherance of Cooperative Therefore, immediately contact and the United States Department Extension work, Acts of May 8 and June an Iowa Soybean Rust First Detec- 30, 1914, in cooperation with the U.S. of Agriculture—Animal and Plant Department of Agriculture. Jack M. Payne, tor if you suspect soybean rust Health Inspection Service formed director, Cooperative Extension Service, in your soybeans.
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