What is an acre?? This is a question that has been floating around for some time and has driven many of us slightly crazy at times. When we are talking about fertilizer applications or herbicide applications, the calculation of an acre is a little more straight forward than when we are talking about drop boom applications of insecticides or fungicides on crops such as tomatoes. Some want to talk about a "vertical acre" or a three dimensional acre. Other variables that keep popping up in the conversations include the row width or spray swath width, length of nozzle extensions off the boom, and the number of nozzles open.

In light of current concerns about resistance management with silverleaf whitefly (SWF) and in light of concerns about perceived poor control with some chemicals, it’s probably good to review how we calculate an acre to determine spray application rates. In other words, should we be spraying based on so much per 100 gal or so much per acre? In talking with several people recently, including growers, industry and university researchers, it seems that we may have some inconsistencies going on that may be causing problems. In some cases, we have found that materials are being applied in terms of so much per 100 gal and when we go back and calculate it out, we are actually applying 1/2 or even less of the labeled per acre rate. The research and labeling of products (both by the manufacturer and by most University researchers) is done based on how the EPA defines a treated acre which is a swath width (for most of us here that's 6 feet since most are on 6 foot centers) times the row length required to equal 43,560 square feet. If you are on 5 foot centers, your row acre is different than if you are on 6 foot centers. The EPA doesn’t care how many nozzles are open, or whether you have extensions off the drop boom, or whether the actual concentration in that spray solution changes as that crop grows. All they care about is from a residue standpoint what goes out on that net acre which is 43,560 square feet.

The big question is whether this discrepancy in application rates may be contributing to poor efficacy or even increased resistance development due to the lower rates being applied. In addition, increased frequency of application due to poor efficacy may actually be costing growers money. It's probably not quite as critical for many of our standard fungicides, since most of these go out preventively and often, but for hard to control insects and/or those organisms that seem to develop resistance (i.e. whitefly, late blight) and even for some of the newer fungicides where we have to worry about resistance development, these discrepancies may be a problem.

I am hearing reports that some growers who have switched to the per acre method of calculating over the last season or two are seeing increased efficacy with some pest control materials. Considering current problems with SWF control and virus, it’s more important than ever for growers to pay close attention to calibration and make sure that applications are based on the labeled rate per acre and not on concentration.

(P. R. Gilreath, ext. agt. IV, Manatee Co. - Vegetarian 04-04)