As predicted we are seeing a banner year for water stage fruit split. We are currently seeing nut drop from water split even on varieties on which we don’t normally see it (such as Desirable). It has been abnormally wet this growing season and especially through the late water stage and early shell hardening phase of the last couple weeks. If you have noticed a lot of nuts falling from the trees recently , it is likely this is the problem you are seeing.
As a reminder, water-stage fruit split occurs when there is a sudden influx of water into the tree from heavy rain or even just high humidity during the late water stage just before the shells harden. The sudden influx of water causes an internal splitting of the nut from the water pressure that builds inside. The nuts fall from the tree about 7 days after the splitting event occurs. Sometimes when the influx of water is very rapid and the splitting event is especially violent, thin shelled varieties may show a longitudinal split in the shuck. Most of the time, however, the split is not visible from the outside and you will see a bruise or internal staining develop on the shuck.
There is nothing that can eliminate water split. It is a naturally occurring physiological event that is related to the how the tree uses water. With the amount of rain we have had at this period, it is inevitable. Boron and Nickel sprays can minimize it as can maintaining consistent soil moisture, but nothing eliminates the problem. The heavier the crop load, the more nuts appear to fall from the tree but trees have been known to drop nuts from 30-40% of the terminals. The lightening of the crop load on heavy bearing trees can help improve quality in the current year but the event occurs too late to be of any help with return bloom.
I have had the first report of shuck split of the 2021 season on the Eclipse variety and it occurred about 4 days later than for 2020. According to the grower, initial split of Pawnee usually occurs about a week later than Eclipse at this particular orchard.