A recent New England Tree Fruit Extension Seminar focused on Tree Row Volume (TRV): What it is, why it matters, and how to use it. (You can watch the recording here when it becomes available.) After a brief introduction about TRV by the host, a guest speaker -- Jason Deveau, of sprayers101.com fame -- cut to the chase about what (IMHO too) is really important, that is spray COVERAGE. Because, modern pesticide labels make no mention of TRV or rate per 100 gallons, a rate that was based on dilute spray coverage using TRV. Forget TRV, just worry about getting good coverage with an adequate amount of water. Then apply the amount of pesticide per acre as noted on the label in that amount of water that is getting you good coverage. Bingo.
I am not going to talk about how to get good airblast sprayer coverage here. For that you need to get your copy of sprayers101.com/airblast101/. But let's look at a scenario. Let's say you have a tall-spindle apple orchard. Using TRV, it calculates out to about using 100 to 125 gallons of water per acre that would give you really good coverage. Fine if you are spraying oil. But when not, most growers I know spray tall-spindle using 50 to 60 gallons per acre and get quite adequate coverage. Adequate is fine because you can only fill up the sprayer so many times, right? There are other things to do!
OK now, let's look at a very modern pesticide label, that of Valent's Excalia. For apple scab, it says use 3 to 4 fluid ounces of Excalia per acre in a minimum of 100 gallons of water per acre. (Interesting on the minimum 100 gallons of water per acre, that is unfortunate, I would ignore it and as long as you are getting good coverage -- the label says "Equip sprayer with nozzles that provide accurate and uniform application" -- you should be fine.) So, if your tall-spindle orchard has a little smaller trees or pest pressure is on the lower side, use 3 ounces of Excalia in that 50 to 60 gallons of water. Alternately, if pest pressure is high and trees are just a little bigger, use 4 ounces. Simple. You got good coverage and are applying the label-correct amount of pesticide per acre. Modify this train of thought accordingly to your tree size/acreage/pesticide label. Point is to get good COVERAGE with the label-recommended amount per acre of pesticide.
One little monkey wrench thrown into this thought process might be when using Plant Growth Regulators (PGR's). Because, their label rate typically DOES NOT use rate per acre, but instead amount per 100 gallons (dilute TRV). (Or even worse, Parts Per Million (PPM). Ugh.) Don't stress though. Let's look at NAA in the brand name PoMaxa. The label says to use 1 to fluid 4 ounces of PoMaxa in 100 gallons (TRV) for apple thinning. So my tall-spindle orchard, to keep it simple, requires 100 gallons per acre TRV. So if I wanted to use a 4 ounce rate (equivalent to 10 PPM) I would need 4 ounces of Pomaxa in 100 gallons of water to cover that acre. But I am using 50 gallons to cover an acre, so I still need to add that 4 ounces in the 50 gallons of water, right! (Which is now technically equivalent to a 20 PPM solution, isnt' it? Why I don't like PPM.) Interestingly again, in the text of the label, it says "A typical rate for a moderate-to-thin apple cultivar in an orchard which requires 100 gallons of water per acre to achieve drip is 2 fl oz of PoMaxa per acre. For an acre requiring 200 gallons to achieve drip, use at least 4 fl oz per acre." Makes perfect sense, doesn't it? So why not just say that on the label and leave it at that! My advice would be for apple thining to say "use 2 to 8 oz. per acre depending on tree size and amount of thinning desired!" That would cover most dwarf and semi-dwarf orchards. And that is what I tell growers! For more on this topic: Spray Mixing Instructions (for PGR's) Considering Tree Row Volume
So, even though my start-out premise was TRV is dead, I came about and with PGR's not so fast! Sorry about that. I still contend, however, focus on getting good COVERAGE. Then for fungicides and pesticides use the label rate per acre, adjust the amount of product based on tree size and pest pressure. But always within the label amount (minimum and maximum) per acre. For PGR's, put on your thinking cap, analyze the situation a bit, and convert the label recommendation, whether it's amount per 100 gallons TRV or PPM, to rate per acre. Small trees, less PGR per acre, large trees, more PGR per acre. Less efficacy -- need for less thinning for example -- less PGR per acre, more efficacy, more PGR per acre. Makes kind of common sense doesn't it?