Date of this Version
Chapter in 2016 Crop Production Clinic Proceedings, University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources, pp. 149-153.
Wet weather that persisted during much of the 2015 growing season proved to be very favorable for disease development in several crops across Nebraska. In particular, diseases caused by fungi were favored by repeated rainfall events and high relative humidity, fog, etc., that supported the production of more spores and continuous plant infection. On some susceptible varieties and hybrids, diseases developed early and quickly eventually reaching moderate to severe levels. Foliar fungicides were applied one or more times to some crops in an effort to mitigate losses due to disease. However, low crop prices and narrow profit margins made foliar fungicide application decisions more difficult as producers weighed the application costs versus the potential protection fungicides can provide from some diseases. One of the most important factors to consider when making a foliar fungicide application is the timing of the application and how long can you expect protection from the product. In general, most strobilurin fungicides are expected to provide about 21-28 days of protection from infection. Triazoles are systemic and have curative activity but only for infections that have just occurred. These fungicides can’t restore diseased, necrotic leaf tissue. And, unfortunately, gaining a return on the foliar fungicide application is determined by several additional factors that impact the anticipated disease severity making the decision more difficult, such as: disease history (for those pathogens that overwinter here), susceptibility of the hybrid or variety, weather forecast, availability of irrigation, crop stage, crop rotation vs. continuous cropping, and tillage regime.
The protection provided by fungicides applied too early could leave crops vulnerable to increased disease severity once they’ve worn off. Although, applications made too late in disease development will likely not provide the desired level of control. This may also be impacted by the seeming delay in development of disease symptoms from earlier infections (latent period). Some pathogens may require up to three weeks for symptom development after infections that often occurred during an earlier period when weather conditions were more favorable. Fungicides applied during this time period won’t likely be able to stop most lesions from developing. And, the development of disease lesions after product application may give producers the impression that the products didn’t work as expected. Listed are specific examples of disease control with foliar fungicides in several crops.