We are approaching bloom in the southern part of the state, and temperatures are expected to drop close to 32 °F on Friday and Saturday night. I’m revisiting an article I wrote in 2016 with some new tips on the use of Promalin as a frost rescue treatment.

Studies in New York and North Carolina have reported that applications of the plant growth regulator Promalin (gibberellin plus cytokinin) following a severe frost event can significantly increase fruit set and yield in apple trees by mitigating frost damage to flowers. Promalin stimulates the retention of fruit with unviable seeds that otherwise would have dropped. Essentially, it signals to fruitlets that the seeds inside are still viable and should continue growing. This often results in the development of parthenocarpic (seedless) fruit, which, however, will be similar in size to seeded fruit. Yield improvement with the use of Promalin will only be a fraction of a full crop.

The recommended application rate is 1-2 pints of Promalin per 100-150 gallons of water. Apply Promalin as soon as possible after the frost event. Research shows that even if applied 5 days after the frost, it can still increase fruit set. The product can be applied from the pink stage to 10mm fruit size; if applied later, once fruit size is over 10 mm, it will provide no benefit. Ideally, the product should be applied once temperatures rise to 65°F. However, if temperatures do not warm up, the product can still be applied with cooler temperatures, as long as it is within the 5-day window of application.

Applications of Promalin work best after a significant frost event (below 28°F), when there is extensive flower damage. If the frost is not severe (29-31°F), the application of Promalin may not be necessary because there will likely be more flowers alive to achieve a full crop. Growers should assess the level of damage to their crop and determine the stage of development of flowers and fruitlets before applying the product. The studies conducted in NY and NC States were performed on Gingergold, Jonagold, and Gala on M.9, and Taylor Spur Rome on M.26, respectively.

There are still many questions to be answered regarding this “rescue treatment,” and it certainly does not guarantee a crop. Each grower will have to consider on his or her own if it is worth trying an application of Promalin to rescue the crop.

This article was posted in Apples and tagged Amaya Atucha, Apples, frost rescue, Promalin.