Citrus Pesticide Application Tool

Chris Oswalt
Multi-County Commercial Citrus Agent
Hillsborough & Polk Counties

In any pesticide application, the possibility of drift and rainfall are significant concerns for those who are making decisions about when to go into the field to apply these chemicals. While the decision to spray or not spray must ultimately be made by the applicator in compliance with all label instructions, the FAWN Citrus Pesticide Application Tool is a decision aid for agricultural producers that routinely make pesticide applications using ground spray equipment. The tool displays in a table the current conditions of windspeed, wind direction, rainfall, relative humidity and atmospheric stability - referenced as “inversion” (whether the temperature increases or decreases with height), at a selected FAWN station. The tool also displays a 2-day National Weather Service (NWS) forecast for windspeed and percentage probability of rainfall for that location, both in the table, and on a graph.

The graphical display shows windspeed on the left y-axis and percentage probability of rainfall on the right y-axis. Time of day is used as the scale for the x-axis. Acceptable conditions for ground spray application are categorized using predetermined critical values for windspeed and these values are color coded: green for good/favorable (0-4 mph), yellow for fair/caution (4-8 mph) and red for poor/unfavorable (greater than 8 mph). The black solid line represents the NWS forecast for windspeed over the next 2 days and conditions for spraying are indicated by the amplitude of the line and the colored coded area in which it falls. Percentage probability of rainfall is displayed on the graph as vertical blue bars and conditions are categorized using the following predetermine values: good/favorable (less than 25%), fair/caution (25 to 50%), and poor/unfavorable (greater than 50%). The NWS interprets percentage probability of rainfall as the percent chance that rain will occur at any given point in the forecast area during the forecast period.

The table provides additional information on environmental conditions, including wind direction and relative humidity, for the 2-day period. Depending on droplet size relative humidity can be a limiting factor in spray applications. For example, relative humidity greater than 70% is considered favorable, 50 - 70% favorable (depending on droplet size), and less than 50% either potentially unfavorable (for small droplets - less than 100 microns), or cautionary (for larger droplet sizes). The table also includes the time of day and a color coded/single word description of wind speed conditions for ground spray applications. To enhance record-keeping, users can select “Print Data Sheet” at the top right of the graphical display to retrieve a printable PDF document that includes the forecast graph, current conditions at the selected FAWN site, a place to record on-site weather conditions, and space for notes. Accurate on-side records can provide valuable information when and where drift compliance is a potential concern. This form can easily be filed with other pesticide application records, and should be maintained for future reference.

The method of ground application (low volume versus high volume) and spray droplet size, along with other environmental conditions at the time and location of the spray application will determine if the information provided by this tool is applicable for each individual situation. Ultimately, the decision to spray or not spray must still be made by the applicator at the time and location of the planned spray application and in compliance with all pesticide label instructions. Additional information on pesticide application technology can be found at the following: