Doesn’t the cold just frost your apples?
Frost or freezing temperatures can cause fruit drop in apples. A stop drop application like NAA may be needed, so be aware of temperatures across the orchard and pay special attention to frost pockets.
With threats of frost or freezes around Michigan predicted for later this week, there have been questions about the effects of frost on apples yet to be harvested. Light frosts (temperatures of 28°F to 32°F) are usually not an issue, but several nights of just mild frosts with temperatures around 28°F can cause some fruit drop. A hard frost or freeze when temperatures are down around 24°F in the overnight – even for just one night – can cause significant drop.
These cold weather events turn on ethylene production and create or hasten the development of abscission layers. Fruit drop typically occurs about one week after a cold weather event – either a one night cold snap or several nights of just below freezing.
Fruit drop can be minimized by applying a stop drop material like NAA within two days of the cold event. Soon after a frost event, the abscission process will be too far along to stop and an NAA treatment will not prevent fruit from dropping. Significant drop will occur about a week after the cold event.
To determine if stop drop applications are needed, growers should be aware of the temperature differences across orchards and pay special attention to frost pockets where it could be much colder than at the tops of hills. It might be feasible to only spray the frost pockets with NAA to prevent drop.
Also, after a cold event, harvest should wait until apples have had enough time to thaw. If growers harvest when fruit is still frozen, even slightly frozen, pickers can cause significant finger bruising when tiny ice crystals in the cells puncture cell walls.
Related article: The nuances of stop drop applications in apples, MSU Extension