Get Reacquainted With Soybean Rust, Aphids This Winter

By Jane Fyksen Crops Editor
Thursday, November 6, 2008 2:03 PM CST

Soybean rust quietly tightens its headlock on the U.S. soybean industry. According to the USDA’s Asian Soybean Rust website, on Oct. 31, the dreaded disease was reported for the first time ever in Maryland. That’s the sixteenth state to report soybean rust this year.

This includes 56 counties in Alabama, 45 counties in Arkansas, 43 counties in Georgia, 24 counties in Florida, four counties in Illinois, 33 parishes in Louisiana, three counties in Kentucky, one county in Maryland, 79 counties in Mississippi, one county in Missouri; five counties in North Carolina, one county in Oklahoma, 16 counties in South Carolina, five counties in Tennessee, five counties in Texas, and seven counties in Virginia. Rust was also reported in 10 municipalities (counties) in Mexico. Soybean rust is expected to spread northward until frost.

Asian soybean rust was first found in the U.S. in November 2004, and has since caused significant damage in the Southeast.

Illinois announced its first confirmed finding of soybean rust for 2008 on Oct. 23, as did Oklahoma and Missouri. The Illinois discovery in McLean County, right above the middle of the state, is the northernmost sighting of soybean rust so far this year. The infected field was late-planted and double-cropped and was still green. The incidence was low, about 3 percent. It was also too late in the season to have an impact on yield. It was anticipated that freezing temperatures would do in the central Illinois rust spores.

Wisconsin growers are urged to remain vigilant in looking for soybean rust next year. They can brush up on the disease this winter online at USDA’s rust website at http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi.

According to Pioneer, after several growing seasons of Asian soybean rust in the U.S., the disease is showing signs of potentially expanding northward. “Growers need to continue to be aware of ASR,” says Paul Stephens, Pioneer research fellow.

Soybean rust is a wind-borne fungal disease that’s aggressive. Look on the underside of leaves, starting from the middle to lower portions of the plants. Infected leaves have red to tan lesions with small spores. This pathogen survives winter in the South, and continues to push further north into the Midwest each growing season. As was the case this year, if the Midwest sees rust, it’s late in the season.

Pioneer has an intensive molecular marker research program to incorporate into soybeans native genes for resistance to rust. Pioneer plans to commercialize soybean varieties carrying multiple sources of Asian Soybean Rust resistance by 2013 in the U.S. However, Asian soybean rust is a diverse pathogen that can change in some environments to overcome a resistance gene, increasing the challenge for developing durable resistance. According to Pioneer, experience with soybean rust in South America and Asia have demonstrated that single genes for Asian soybean rust resistance can be overcome. Pioneer is evaluating stacking rust resistance genes to offer broader resistance.

USDA also has a soybean aphid tracking website at

http://sba.ipmpipe.org/cgi-bin/sbr/public.cgi?host=All%20Legumes/Kudzu&pest=soybean_aphid.

This year, soybean aphids went west, reaching the western most cultivation of soybeans in the U.S. The Wyoming find on sentinel plants was confirmed, leaving only several states in the South where soybean aphids haven’t yet been found and/or verified.

Over 8,500 soybean aphids were caught in the Midwest suction trap network as of the end of September. Soybean aphids caught in the traps during September and October are migrating from soybeans to buckthorn where the overwintering eggs are deposited. This number exceeds any seen in previous years during the same time period and will most likely result in heavy egg deposition similar to that seen in the fall of 2006 when, in some areas, 100 percent of randomly sampled twigs had eggs on them. Predators can have a major impact on the populations of the soybean aphid on buckthorn and may limit egg deposition. Male aphids have been caught in the traps so fertilized eggs can be expected until a heavy freeze stops the development of aphids on buckthorn.

The North Central Soybean Research Program has published a 12-page, full-color report on aphids that includes biological control, genetic resistance (aphid resistant varieties are coming soon), aphid early-warning systems, revisiting the “250 threshold” and treatment recommendations. A free copy can be ordered by calling 800-383-1423, or you can see it on line at www.plantpath.wisc.edu/soyhealth/pdf/sba_update08.pdf.

The University of Wisconsin Nutrient and Pest Management program has published a Visual Guide to the Number of Soybean Aphids per Leaflet to help growers with scouting. Copies can be ordered from the NPM Program at 608-265-2660 or it can be viewed online at www.plantpath.wisc.edu/soyhealth/pdf/sba_update08.pdf. Each soybean leaflet pictured has a specific number of aphids displayed to help you count.

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