Comparison of application methods on deposition and redistribution of chlorothalonil in a potato canopy and potential for control of late blight

The effect of application method by aircraft (fixed wing), ground, and chemigation on deposition of chlorothalonil in a potato canopy was compared over 2 years. initial chlorothalonil deposition was greatest with ground application, less by aircraft, and least with chemigation. After application by aircraft or ground, the upper canopy had more chlorothalonil than the middle, and the lower canopy had the least. With chemigation, the upper canopy also had more chlorothalonil, but amounts in the lower and middle canopy were similar. Redistribution occurred with time as material from the upper canopy moved downward. With a 9-day application frequency using ground or aircraft methods, residue levels throughout the treated potato canopy were maintained, even if distribution of the fungicide within the canopy was poor from the initial application. With chemigation, low initial chlorothalonil levels were further decreased due to redistribution, increasing the likelihood that levels could fall below that which may be needed to adequately control late blight, particularly beyond 7 days after application.