Soybean Pest Management

George Cummins, Crops Field Specialist, Northeast Area

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 raised awareness of the impact of this and other soybean diseases on soybean yields. Various seed treatments and fungicides were 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 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 from each County Extension Education Director, 41 sampling sites and 33 cooperators were identified in the 11 counties I serve. Cooperators selected 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 , a private company soybean breeder; chemical/ seed dealers; and the crops instructors at North Iowa Area Community College and Hawkeye Community College. Included in the sampling were: a continuous bean rotation; specialty varieties that included ISU and Asgrow low-linolenic varieties and Vinton 81s; and replicated comparisons W/WO fungicide seed treatments and W/WO fungicide treatments at R-R2.  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 local cooperators on a recurring basis. Others have asked to be included as cooperators in the future. The Iowa Soybean Association was satisfied with the study results and has agreed to support the survey for another year. 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 and prevalence of little known diseases bacterial blight was the most prevalent disease in the 11 counties. Bacterial pustule was also confirmed in several North Iowa fields. White mold, which was common in 2004, was rarely found in 2005. 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. One of the three seed treatment comparisons produced an economic response. Two of four fungicide treatments at R1-R2 produced a positive yield response none of the four produced an economic response.  Soybean cyst nematodes were identified in two of the soybean fields. Soybean aphid, bean leaf beetle, grasshopper, thistle caterpillar, northern corn rootworm beetle and beneficial insect numbers were noted during each sampling period.  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. A major contributing factor identified by the survey was the susceptibility to and prevalence of brown stem rot. The low-linolenic varieties (Vistive) from Asgrow yielded as well as their commercial isolines and had similar disease incidence and prevalence. Survey results will provide localized information for use in future Extension programming and to assist local producers with their soybean production decisions.
November 21, 2005
142 -- Integrated Pest Management/ Integrated Crop Management

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