Many years ago, I was told that to successfully use fungicides on vegetables, one must use high spray pressures and hollow cone nozzles. However, I had trouble finding any research on this topic, just rumors. So, I did my own research.
Dennis Nowaskie, Superintendent at the Southwest Purdue Agricultural Center (SWPAC) built a single row sprayer that could be used to vary nozzle types between flat fans and hollow cones and spray pressures from 30 to 150 PSI. We used the sprayer to conduct experiments on Alternaria leaf blight of cantaloupe during three years of field tests. The fungicide we used to try to manage this disease was the contact product chlorothalonil (trade names include Agronil®, Bravo®, Echo® and Terranil®). Phillip Harmon, now a professor at the University of Florida, was my co-author on this paper.
Try as we might, we could not find any statistical differences in disease severity or yield between any of the nozzle type or spray pressure treatments. We also used water sensitive paper to measure coverage at each treatment; we found no statistical differences in the coverage of water sensitive paper as a result of spray pressure or nozzle type. Regardless, of the nozzle type or spray pressure there was no statistical difference in coverage. This research was on cantaloupe and Alternaria leaf blight. However, I have reason to believe that the research applies to other situations as well. Read on for research on other crops and diseases.
University of Florida researchers lead by Tom Kucharek found that regardless of whether flat fan or hollow cone nozzles were used, no difference in disease severity was observed in the following diseases: early or late leaf spot of peanut, bacterial spot of pepper and blast or purple blotch of onions. Kucharek also found that spray pressures ranging from 50 to 250 psi made no differences in disease control in early or late leaf spot of peanut.
In general, higher spray pressures (and to some extent, hollow cone nozzles) are often linked with smaller droplet size. Small droplet sizes are often associated with better coverage. If this is so, why wasn’t I able to find better disease control or coverage using hollow cones and high spray pressures? One reason may have to do with where the spray ends up. If one watches a boom sprayer from a distance, it is often possible to see a mist that is generated by the spray that floats across the field. Clearly, this mist represents water plus active ingredients that may very well not end up in contact with the crop. It may be that while small droplet size helps to increase coverage, too high a population of small droplets increase the amount of non-target impacts.
I was not able to obtain funding for the question of how much water per acre to use for fungicide applications. My observations are that one should use between 20 and 50 gallons per acre. My personal opinion is the more the better, within this approximate range.
Based on this research and the work of others, it might be that we have overestimated the importance of high pressures and small droplets in protecting vegetables from diseases. It seemed reasonable that more drops would provide better coverage and less disease. However, the data doesn’t indicate this at all. If buying a house is location, location, and location, then using fungicides on cantaloupe, and perhaps other vegetables, is all about timing.
The timing of fungicide applications is usually more important than nozzles and pressures. Some of my thoughts on fungicide timing are given in this issue of the Hotline on fungicide application hints.