
Hawke’s Bay Fruitgrowers Newsletter
Jack Wilson
9th October 2024
Post Blossom Thinners
What keeps chemical thinning interesting is the fact it’s just as much an art than a science. A great chemical thinning result is often caused by a plan based on history, a good analysis on the seasonal conditions you have in front of you and plenty of walking blocks. This season we are going into an early compressed bloom with great spring conditions allowing the bees to go hard and therefore the probability of fruit set being high.
With a number of primary thinners been used in those early weather windows we are starting to see their effects, some better than others.
Getting out and walking blocks is critical at this time of year to ensure that any tweaks you make to your plan are based on good observation. Because of the current trend of planting large blocks of single varieties, I am seeing a fruit set gradient across blocks with headland trees and those in rows near adjacent varieties showing heavier set than trees further into the block. Blocks need to be carefully checked for fruit set gradients and the thinning programme adjusted accordingly.
The post blossom thinning window is quite wide so there is no need to panic about getting thinning sprays on if the weather doesn’t allow. Weather always trumps fruitlet stage for secondary thinners and these days we are lucky because we have chemistry for most weather conditions.
Benzyladenine (BA) thinners are not very effective on fruitlets less than 7 or 8mm diameter, irrespective of weather conditions. 10 to 12mm diameter king fruitlet size is the preferred size range and I have also seen good results out of 18 to 20mm diameter fruitlets , due to the tree being under significant stress at that time. The addition of NAA to BA will give you more effective thinning, particularly if you are going in at the smaller fruit size end of the desired range, however do not use BA with NAA on Fuji. BA and BA+NAA is temperature sensitive requiring >18oC for the next 2-3 days post application.
Metamitron is another post blossom thinner with a photosynthetic inhibitor mode of action. It has a wide window of application (6-20mm), with the best effect at 10-12mm. Metamitron has a greater thinning result under carbohydrate deficit favouring conditions, including warm nights, cloudy overcast days, high interfruit competition, shaded trees and general tree stress (waterlogging, frost etc.).
Keep walking blocks and observing, some say it’s the best chemical thinning strategy out there.