For both Metamitron and the benzyladenine thinners sufficient water is necessary to give adequate spray covers, particularly in the upper tree. In our Metamitron development work, we standardised on 1 litre on spray to 10 cubic metres of tree row volume. This means that a full canopy intensive orchard in Hawke’s Bay requires somewhere between 1,200 and 2,000L/ha of spray. Recent experience with the BA thinners is also pointing towards higher water rates for full canopies.
It has come to our notice that many spray rigs are calibrated to apply 1,000L/ha which is insufficient wetting for good post blossom thinner response. This probably explains the disappointing thinning responses we often see.
Two or three thinning sprays at the most are applied, not the 20 to 24 general cover sprays so taking a bit more time to apply critical thinning sprays is not that onerous.
Sprayers should be equipped with flow meters so by altering speed of travel and pressures, it should be relatively simple to lift water rates into the optimum range for these thinning sprays. About 70% of the spray volume should be directed to the upper third of the canopy and about 70% of the total canopy sprayed for satisfactory results. Drip and drift should give sufficient wetting of the lower 30% of the canopy for good response.
Tree size variation has been a problem with thinning sprays leading to over spraying areas of smaller trees. Where there is variable tree size, speeding up where trees are small will reduce risk of over spray. A half size tree will only need about 1,000L/ha. Canopy density increases as the season advances so thinning sprays later in the window may require more water to allow for higher foliage area.