As regular visitors of ILSoyAdvisor are aware, fungicides are an important component of a high-yielding soybean system. Progressive growers have become increasingly aware of the value of fungicide application not only for disease control, but also for improved stress tolerance and plant health. Perhaps surprisingly, many of today’s most popular varieties with the most inherent yield potential also show the biggest responses to fungicide applications.
In fact, varietal response to fungicides appears to be increasing. There are several potential reasons for this phenomenon. One is that modern varieties are often more robust in their growth habits, with more branching and denser canopies than their ancestors. This type of lush growth lays the foundation for strong yield potential, but also provides an environment in which diseases can thrive. Another factor in the positive response to fungicide application is the more widespread use of conservation tillage practices. Reduced tillage is often great for soil health but leaves much residue on the soil surface, which provides inoculum for infection from diseases. A final factor is improvements in fungicides themselves.
There are three main classes of fungicides that are commonly applied to soybeans, each with specific characteristics. Quinone outside inhibitors include strobilurons and prevent diseases from taking hold. They control disease by inhibiting germination of fungal spores, and as such cannot cure an active infection. They also benefit plant health and stress tolerance by inhibiting ethylene production. For more information on why inhibition of ethylene is important, check out this excellent ILSoyAdvisor blog written by fellow CCA Soy Envoy, Jared Greuel.
Demethylation inhibitors include triazoles which attack fungi in many ways. They prevent infection from occurring and also cure active infections. It is important to note that when we talk about curative effects of fungicide, that they do not reverse damage that has already occurred, but triazoles can stop disease from progressing any further.
The final class of fungicides used on soybeans are the succinate dehydrogenase inhibitors (SDHIs). These chemicals are generally preventative, with some limited curative effects. Another characteristic of SDHIs is that they are somewhat limited in the range of diseases against which they are effective. One well-known example is fluopyram, which is the active ingredient in Ilevo. Fluopyram is incredibly effective against Sudden Death Syndrome but has limited activity against other soil-borne fungi.
For several reasons, it is important to use multiple modes of action when applying fungicides. One primary reason is highlighted in the preceding paragraphs, which explain the differing mechanisms of each type of fungicide. Taking advantage of the unique characteristics of each class provides a multi-faceted approach to disease control and stress relief. As in herbicide application, the use of multiple active ingredients also helps ensure the longevity of the product by making it much more difficult for resistance to develop in any given fungal population. There are many quality products in the marketplace today that incorporate multiple modes of action.
At this point in the growing season, most of the fields slated for a fungicide application have already received it. There may be a few later planted fields that have not yet reached the R2 growth stage, and growers may have the perception that there is less value in applying fungicide to these fields. However, recent research, including that which I reported on in a previous blog showed large yield responses even in late-planted soybeans. If you have already applied your fungicide this year, you are likely aware of its value. If you haven’t, it’s not too late to test it out in your own operation.