Try different volumes of water per acre with your sprayer in your orchard and observe which volume results in runoff.
Estimates of dilute volume of water per acre are shown in the chart on page 19. The chart is based on tree size and spacing. If tree size within an orchard block is variable, then choose the measurements for the average size of the larger trees. For any combination of tree size and spacing, the chart shows what the dilute spray volume should be. The dilute spray volume is based on the rule that it takes 0.7 to 1.0 gallon to treat 1,000 cubic feet of tree canopy to the point of runoff. A canopy volume of 1,000 cubic feet is a section that is 10 feet wide by 10 feet tall by 10 feet deep. Most growers with well-pruned trees can use the minimum dilute volume, which is 0.7 gallon per 1,000 cubic feet of tree canopy. The maximum dilute volume is 1.0 gallon per 1,000 cubic feet of tree canopy. The maximum dilute volume should be used in an unpruned orchard with a thick canopy.
For example, if your trees are about 12 feet wide and 10 feet tall in rows spaced 20 feet apart, the chart shows that you have a maximum of about 261,000 cubic feet of tree per acre. If these trees are relatively well-pruned, then your dilute volume will be about 183 gallons of water per acre, as shown in the column for Minimum. If these trees are not pruned, then your dilute volume will be about 261 gallons of water per acre, as shown in the column for Maximum.
If your tree size and spacing are not in the chart on page 22, then use the formulas given here.
Tree-row volume is canopy width, times tree height, times row length per acre. Row length per acre is 43,560 square feet per acre divided by the distance between rows, in feet. This number can be considered the maximum volume of trees this size.
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| Figure 3. Orchard measurements |
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Canopy thickness can be factored into tree-row volume calculations. You will need to choose a density factor between 0.7 and 1.0, depending on the relative canopy density or tree shape, rather than using either the minimum or maximum volume as described previously. However, many orchardists have found that such an adjustment does not make any difference, and in most commercial orchards 0.7 is a good choice. Only in a completely unpruned orchard would the full 1.0 density factor be needed.
Pesticide type can be factored into tree-row volume calculations. Multiply the dilute volume by certain percentages because some thinners require lower volume and some stop-drop materials require higher volume. For example, use 4050% for Promalin and Pro-Vide and 7090% for thinners used on varieties other than spur Red Delicious. Some stop-drop materials require higher volume; use 100120% for Ethrel, for instance.
Some applicators use a lower limit of 200 gallons per acre as the minimum dilute volume even when tree-row volume calculations show that a smaller volume should be adequate. Not all orchardists agree with this, but it could be considered in cases where a smaller number of gallons is used and control is not satisfactory.
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