Here is an update from UGA Extension Pecan Specialist Dr. Lenny Wells:
We have had a heavy bloom of female flowers and a good early nut set on ‘Desirable’ this year, but anyone that has been around pecans for long knows this cultivar has a nut drop in June each year. When this occurs the trees may drop as much as 30-60% of the nuts on the tree. The June drop continues to about 40-45 days after pollination, and is due to the lack of successful fertilization of the egg and sperm. This is an inherent characteristic of ‘Desirable’ in particular and this self-thinning is also the reason this cultivar can have consistent production from one year to the next
‘Stuart’ will also have a June drop but it is usually not as severe or as noticeable as the ‘Desirable’ drop. There is nothing you can do about this drop and there is nothing wrong with the trees when it occurs. If seeing this drop bothers you I recommend not going into the orchard to look until its over. Desirables always drop down from 4 and 5 nuts per cluster to 0, 1 and 2 nuts per cluster. The trees will still have good production. I began seeing this last week and it will likely continue for a week or so.