When considering seed treatments, we usually think first about their necessity and then what products will work for a particular field. We don’t often think about stewardship of these products. I am guilty of it as well. When discussing seed treatments, I usually talk about their benefits and product selection with a mind toward disease control. Stewardship is a second thought.
However, when we look at available seed treatment products compared to the spectrum of seedling diseases, we quickly realize the importance of stewardship. There are few effective modes of actions available. To conserve seed treatments as a crop protection tool, they must be used in conjunction with sound cultural practices.
In this part of the world, when we think about resistance, herbicides are our first concern. But resistance management should be considered when dealing with any pest, including seedling diseases. When you break down our driver diseases and effective sites of action, the need for resistance management becomes much more apparent. There are essentially six different sites of action, or classes of fungicides, used in seed treatments today, compared to 19 classes of herbicides.
But, just as not all herbicides are effective on all weeds, not all seed treatment fungicides are effective on all seed-borne or seedling pathogens. When we look at our driver diseases used to make seed treatment decisions, they fall into two classes: water molds (phytopthora and pythium) and true fungi (rhizoctonia and fusarium). When we look at these groups separately, there are only three modes of action and four sites of action to control true fungi and a mere two sites of action effective against water molds. With so few choices coupled with logistical difficulties of handling multiple seed treatments, rotation of fungicides becomes complicated to implement as a resistance management strategy.
So what are our options? First, seed treatments cannot be our only defense against disease. Just because a fungicide is applied to the seed does not mean we can ignore other factors that favor disease development. Remember the components of the disease triangle from biology class. To have a disease, we need three things: a host, a pathogen and a favorable environment. Seed treatments attempt to address one piece—the pathogen—but additional management can take care of both the environment and the host.
We can manage the environment by waiting for soil conditions to be correct. While not always possible, this is the best line of defense against most soybean seedling diseases. The diseases of concern all have a preferred temperature range. Avoiding temperatures in the ranges outlined in the accompanying table can decrease the likelihood of infection.
Perhaps more important than disease is proper soil moisture. Planting into fields with adequate—but not excessive—moisture is ideal for numerous reasons. Not only do many fungal diseases thrive in saturated conditions, but you also want to avoid compaction, which compound saturated conditions, hamper seedling growth and contribute to disease.
We can also manage the susceptible host component of the disease triangle. Selecting varieties with resistance to soil-borne diseases such as phytopthora or sudden death syndrome is the obvious place to start, but soil conditions can also have a huge impact on the amount of time a seedling remains vulnerable to infection. Anything that limits the amount of time a seed spends underground and promotes rapid growth of a young plant lessens the time a plant remains vulnerable to infection. We see this time and again in years where SDS is common. The fields affected most severely often have seedlings that endured stressed or prolonged emergence periods. Think about how much more vulnerable an infant is to the flu than a healthy adult. Proper temperature, moisture, depth and seed placement all impact the growth and development of the soybean plant just as much as the disease.
Managing diseases takes a multi-faceted approach. Over-reliance on any one method of control—cultural or chemical—can lead to disappointing short-term results. In the long term, that reliance can lead to the development of resistant pathogens. It is important keep integrated pest management strategies at the front of our decision-making process for immediate success and continuing efficacy of our control options.