Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
This year I have been encouraging growers to leave at least one untreated strip in fields in cases where they were on the fence about making a fungicide application and pulled the trigger. Although a single strip will not account for all the variability in disease and yield at the field level, it can give you an idea of the effect of the application on disease and yield in a given year. This information is particularly useful when profit margins are tight and every trip across the field needs to count.
When leaving an untreated strip, try to run it through the center of the field and avoid field edges, tree lines, or extremely low/high areas. Ideally the strip will be wide enough to allow you to do a yield check to see not only if disease was controlled, but if the disease caused enough yield reduction to result in the application paying off. Disease presence does not guarantee yield loss, but you will not know if this was true in your case unless there is an untreated strip in the field. Comparisons at the field level (e.g. Field A vs Field B) are not valid for this type of purpose due to all of the confounding factors that could ultimately impact yield, in addition to disease. For example, variety, planting date, soil type, nutrient levels, disease presence, severity, and onset, presence of other diseases, insects, weeds, and other agricultural factors, all could play a role in making the yield of Field A different from Field B. By leaving an untreated strip, you at least can say something about the effect of a particular application within a field. If you are interested in conducting on farm research, which is a little more involved than simply leaving an untreated strip but easy to do given a little effort, I encourage you to read Bob Nielsen’s “Practical Guide to On Farm Research”, which is located at the following location: https://www.agry.purdue.edu/ext/corn/news/timeless/onfarmresearch.pdf .