Do foliar fungicides fit into our soybean production practices in Iowa?
Palle Pedersen, Soybean Extension Agronomist
Originally published in the Iowa Soybean Review, January 2007

Probably the hottest topic in the fall besides the election and bio-renewable energy was the confirmation of Asian soybean rust (ASR) in central Indiana. It was found in research plots of late-maturing soybeans at Purdue's Agronomy Center for Research and Education near West Lafayette on October 22.

Since ASR was found on November 11, 2004, there has been much speculation on how it will overwinter in the U.S., spore movements from the southern U.S. to the major soybean growing region in the northern U.S., and the complex interactions with moisture, temperature, and radiation vs. incidence and severity in the fields. Some would say it is like “happy hour” for any plant pathologist and epidemiologist. However, for the rest of us it seems ….bewildering.

Some thought it was too complicated to scout and they would just spray a preventative fungicide, because of the complexity of ASR. Iowa has little history of foliar pathogens we can manage that cause significant yield loss, so it would not be a wise economic decision. In addition, spraying a foliar fungicide in absence of disease seems to be against all the rules of Integrated Pest Management we have worked so hard to introduce over the last couple of decades. However, there is a reason why people contemplate using a strobilurin as a preventative fungicide in the absence of disease.

Research has been published documenting that strobilurins can induce non-fungicidal physiological changes in wheat and tobacco, including a darker green appearance of the leaves, delayed leaf senescence, increased stress tolerance, plant biomass and yield. However, very little information exists if they have a physiological effect on soybean. Therefore, we started a large project in 2005 to look at the use of foliar fungicides in Iowa. We have been working at two locations and with four varieties and have had treatments out at R1 (flowering), R3 (beginning pod set) and R5 (beginning seed fill).

After two years of research we have not been able to find any consistent yield benefit for any variety or at any location. This is consistent with what most farmers have been facing, which fits well with the literature that Iowa does not have a history of foliar pathogens we can manage that cause a significant yield loss. I think most farmers that have done an On-Farm trial have seen an average of one to three bu/acre increase over the last two years, which would not pay for the time nor the product. In addition, there have also been cases where a yield loss has been observed. It has been speculated that this yield loss is a function of increased insect activity after the fungicide application. It is important to recognize that although we do not find any yield advantage for using a strobilurin in absence of disease, it is an extremely efficient fungicide group for managing many diseases. However, our data from the last two years does not give us any significant economic advantage for using it in absence of disease for improved plant health. However, I will continue working on it in 2007, since there are still many things we do not know about this fungicide group.

What about ASR next year? Even though it has not been found in Iowa during the last two years, we do know that a small change in our weather pattern can change the disease triangle for many pathogens. Even though we have two years of experience with ASR in the U.S., it is still too early to predict the outcome of this disease for 2007. It is highly recommended that everyone in Iowa still takes this disease very seriously, scouts their fields and listens to the information gathered from the sentinel plot system.

Check out the poster that was presented at the 2006 ASA meetings on Effect of strobilurin fungicide on soybean yield components.