George Cummins, Extension Field Agronomist, Northeast
In recent years corn yields have increased steadily while soybean yields have plateaued for many farmers. Many producers are interested in products and practices which will make soybean production more competitive with corn production. The identification of asian soybean rust (ASR) in the continental US in November, 2004, raised awareness of the impact of this and other soybean diseases on soybean yields. Various seed treatments and fungicides have been heavily promoted as a way to increase soybean yields and profits.
Soybean pathologists at Iowa State received a grant from the Iowa Soybean Association to conduct a 3-year statewide soybean disease survey. Each crop specialist was asked to coordinate the sampling in the counties they serve. This survey is the first comprehensive survey of soybean diseases ever conducted in the state.
With assistance of County Extension Education Directors and Corn/ Soybean Initiative (CSI) Partners, collection sites and cooperators were identified in each of the 11 counties I serve. Cooperators in 2007 included: John Hoffman, the President of the American Soybean Association and Tom Thompson, Past President of the Iowa Crop Improvement Association. Other cooperators included members/ leaders of county extension councils, the Iowa Soybean Association/ Soybean Promotion Board, and Farm Bureau; directors of the research associations at Nashua and Kanawha ; and the crops instructors at North Iowa Area Community College and Hawkeye Community College. Over the 3 years of the project, unique sampling sites included: a continuous bean rotation; specialty varieties such as ISU and Asgrow low-linolenic varieties and Vinton 81s; and as part of replicated, on-farm demonstration projects comparing varieties, fungicide seed treatments or foliar fungicide treatments at R-R2. In 2007, this disease survey provided disease assessment for an on-farm research project funded by ISA which compared 4 soybean varieties with differing SCN susceptibility/ resistance at a site infected with SCN. In 2007 dug samples were collected from each county as part of a statewide fusarium study.
Four CEEDs (Pat Derdzinski, Bill Arndorfer, Jim Hill and Neil Wubben) and I sampled each of the selected sites 4 times during the growing season (V2-V3, R1-R2, R4-R5, and R6-R7). Samples were evaluated at the Plant Disease Lab and the data posted to determine the prevalence, incidence and geographic distribution of foliar diseases. Harvest data was collected from the replicated comparisons and summarized.
The survey provided ISU Extension field staff the opportunity to work closely with CSI Partners and local cooperators on a recurring basis. Others offered to be included in this project or as future cooperators for on-farm ISU studies. The Iowa Soybean Association provided continuing support for the 3 years of the survey and shared the results widely.
The project provided cost recovery funds to help defray expenses incurred by field staff conducting the survey.
The survey raised general awareness of the incidence, prevalence and potential agronomic and economic impact of soybean diseases in North Iowa. The 3-year survey documented the importance of environment on disease establishment and spread. Because of weather variation, disease incidence and prevalence differed from year to year. Bacterial blight and brown spot were the most prevalent diseases in the 11 counties during the survey. Rhizoctonia root rot was common in early 2007 samplings. Bacterial pustule, frogeye and sudden death syndrome at were confirmed at low levels in several North Iowa fields. White mold, which was common in 2004, was rarely found in during the 3 years of the survey. The study will try to correlate prevailing weather patterns (moisture and temperature) with specific disease occurrence. This information will help to predict disease pressures in the future.
On-farm demonstrations with seed treatments and foliar fungicides mirror the results of similar trials at Nashua and Kanawha. Fungicide seed treatments are an inexpensive insurance while fungicide/ insecticide seed treatments have not given an economic response unless there were heavy overwintering bean leaf beetle pressures. Foliar fungicides have generally produced a modest positive yield response. With higher market prices, a positive economic response to foliar fungicides will be more common.
During sampling, other soybean pests were also noted - soybean cyst nematodes soybean aphid, bean leaf beetles, grasshoppers, thistle caterpillars, northern corn rootworm beetles as well as beneficial insect numbers. The cooperators were notified of pest numbers when they approached threshold levels and were informed of recommended management practices.
A new low-linolenic soybean variety developed at Iowa State yielded at least 20 bu/acre less than other commercial varieties in the area in 2005. A major contributing factor identified by the survey was the susceptibility to and prevalence of brown stem rot. This field was also a collection site in 2007. The cooperator selected a soybean variety with good BSR tolerance and expected yields were obtained. The low-linolenic varieties (Vistive) from Asgrow yielded as well as their commercial isolines and had similar disease incidence and prevalence in all years of the survey.
Survey results will provide localized information for use in future Extension programming and to assist local producers with their soybean production decisions.
100 Corn and Soybean Production
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February 14, 2008
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