Soybean Rust

Soybean rust has now been positively diagnosed in seven Illinois counties. It was first confirmed on October 13, 2006 on a sample collected from Pope County. The sample was taken from a soybean research plot at the U of I Dixon Springs Agricultural Research Center. U of I Plant Clinic director Nancy Pataky and Dr. Glen Hartman USDA-ARS Soybean Plant Pathologist observed the sample and sent the sample to the National Mycologist USDA-ARS in Beltsville Maryland for positive confirmation and species verification as indicated by the National protocol for handling of first soybean rust samples in a state. Since that time soybean rust has been confirmed in six additional counties, Massac, Hardin, White, Alexander, Johnson and Pulaski. So as not to test anyone's geography memory too much, for the most part the positive counties are our southern most counties in Illinois. To see the distribution of rust in Illinois, and other states, look on the national soybean rust website (Pest Information Platform for Extension and Education) http://www.sbrusa.net/ for the counties colored red.

The finding of soybean rust in southern Illinois was not un-expected. Kentucky has recently reported soybean rust and now numerous other states have reported new rust detections including Missouri and Indiana. The finding of soybean rust in Illinois at this time will have NO impact on our 2006 soybean crop. Soybean harvest is well under way across the state. All areas of Illinois that have experienced a hard frost also will not be affected in any way. So why are we still sampling non-frosted areas? Because information on the extent of this outbreak will greatly facilitate research on soybean rust and aide in the refinement of predictive models for soybean rust. No management actions should be undertaken by growers or commercial applicators at this time

Green Stem of Soybean

Now, on to some other troubles showing up in the soybeans as harvest progresses. A number of producers have called regarding 'green stems' on some plants in an otherwise mature field. In particular, R. Munsterman called and left me a voicemail, when I returned the call, I kept getting the phone in an ambulance, this panicked me a bit but they assured me Mr. Munsterman wasn't calling from the vehicle so I thought I'd answer his question here. What is causing the green stems in the field? Well, two things could be responsible.

First, it is possible that the beans in question were infected with a viral disease called bean pod mottle virus. Bean pod mottle virus (BPMV) causes a mottling and distortion of leaves in the upper canopy of the plant during periods of rapid growth and cooler temperatures. Another symptom that can be exhibited by BPMV infected plants is 'green stems' after the plant matures. Plants may also exhibit death of new terminal leaf growth. Seeds of BPMV infected plants may have a very light purplish discoloration of their seed coat. BPMV natural and experimental host range is limited to three families of legumes. It's natural host range of concern being soybean and green bean. Virus testing can be obtained through AgDia.com.

Several kinds of beetles can move infective sap around to spread the virus disease, the most prevalent being bean leaf beetle. Other beetles can transmit the virus including grape colaspis, striped blister beetle and several types of rootworm beetles. It can also be mechanically transmitted, graft transmitted or seed transmitted. So particularly in fields that experience heavy bean leaf beetle feeding this past spring BPMV is a real possibility for those green stems. The problem is though that BPMV infected plants do not always have green stems.

So the second possibility for those green stems is something called "green stem syndrome". As you know if you've heard me talk about syndromes before, we name things syndromes when we don't know all the causal factors. In looking for possible explanations, researchers at the University's of Illinois and Wisconsin recently published results of a multi-state, multi-location project investigating whether or not "green stem disorder" was related to or a result of BPMV infection. They found that BPMV incidence in surveyed and tested fields was often higher than the amount of 'green stem' present. Additionally, they found 'green stem disorder' developed in the absence of BPMV infection. Also, the study found that the same varieties in different locations showed varying levels of BPMV infection but similar levels of 'green stem' incidence. The researchers concluded that 'green stem disorder' is independent of BPMV infection.

So, from a production viewpoint neither of these possibilities lends itself to management. We do not currently have BPMV resistant varieties, trying to reduce vector population hasn't been shown to be consistent and we still don't know all the factors that are responsible for 'green stem syndrome'. Maybe that call really was from an ambulance. Continue to stay safe as harvest continues.