Spraying Promalin at full bloom increased fruit set and yield. Fruit size and shape were normal at harvest, but most of the fruit were without seeds (parthenocarpic).

Spraying Promalin at full bloom increased fruit set and yield. Fruit size and shape were normal at harvest, but most of the fruit were without seeds (parthenocarpic).

PHOTOS COURTESY of NORTH CAROLINA STATE UNIVERSITY

The Southeast wasn’t spared from the spring freezes that plagued much of the Midwest and eastern apple-­producing states during 2012. Rome apples are widely planted in the Southeast because they bloom two to three weeks later than other cultivars and will normally escape early freeze events to crop consistently in most years.

But 2012 was exceptional. A mild winter and warm spring left Rome flowering three weeks earlier than normal, and two freeze events were forecast when this cultivar was in full bloom. Without any frost protection options in the orchard, we were faced with the potential for significant crop losses. This scenario was used to test the practice, used in Europe, of applying Promalin within 24 hours following a freeze as a frost-rescue ­treatment.

The freeze events

A commercial orchard of Taylor Spur Rome in a low-lying area of Henderson County, North Carolina, was used for the study. Temperature loggers were positioned in solar radiation shields at a height of 1 meter in the canopy. The loggers were programmed to record air temperatures at 10-minute intervals.

Things didn’t look promising when the loggers were downloaded after the first freeze event on April 12. The temperatures were below freezing for about 6 hours, and reached a minimum of 23.9°F. According to the textbooks (see accompanying table), exposure to temperatures this low result in death of approximately 90 percent of the blooms.

Trees were sprayed with 1 pint of Promalin in 100 ­gallons of water per acre just before lunch that morning, approximately 5 hours after the minimum temperature was recorded. The petals were already browning when the Promalin was sprayed.

Then, adding insult to injury, there was a second freeze the following morning. Although it didn’t get quite as cold on April 13, temperatures were again below freezing for 6 hours, and reached a low of 28.4°F.

We couldn’t find any live ovules on April 13, even in the plots that were sprayed with Promalin on April 12. Even though we seriously doubted the benefits of making a second application, we went ahead anyway and resprayed trees that had been sprayed the previous day.

The Promalin effect

Given how low the temperatures had been in the two freeze events, we didn’t hold out much hope for any crop at all. Our expectations were met in the unsprayed control plots, where fewer than 3 percent of flower ­clusters set a fruit, and the crop load at harvest was only around 94 bushels per acre, or less than 10 percent of a full crop. The apples that did set in these trees probably resulted from the few, late blooms that opened after the freeze events.

As the season progressed, more and more fruit started showing up on the treated trees. Fruit set was 18 percent on trees sprayed with Promalin, and the trees ended up producing just shy of 300 bushels per acre, or around 25 percent of a full crop.

The Promalin treatment significantly increased fruit set and yield after two freeze events, but even then, the crop was only a quarter of what it might have been on these trees in a normal year. At the prices received for processing fruit in 2012, the increased yield was ­equivalent to an additional $2,424 in fruit value per acre.

How could Promalin increase fruit set when we couldn’t find any live ovules after the first freeze? Aren’t seeds needed to ensure good fruit set and to keep the fruit growing normally?

While fruit on the treated trees grew to a normal size and shape at harvest, they had one distinguishing ­feature—they were largely without seeds, or parthenocarpic. It seems that Promalin increased fruit set and yield by stimulating the retention and growth of fruit that would otherwise have dropped due to the absence of viable seeds.

Promalin did not elongate the fruit, but Rome does not have pronounced calyx lobes, so it was not expected. This is not to say it wouldn’t elongate fruit of other ­cultivars. Fruit with few or no seeds may not be suitable for long-term storage, since they may be more likely to develop physiological disorders such as bitter pit. We did not carry out poststorage ­evaluations in our study.

The bottom line

The positive effect of Promalin that we observed in this study was surprising, given how low the temperatures dropped. Applying Promalin five hours after freeze events on two consecutive mornings during full bloom increased fruit set and yield by stimulating parthenocarpic fruit development.

Similar fruit set and yield responses to Promalin frost-rescue treatments were observed in separate studies in New York in 2012.

Fruit set and yield were partially recovered by Promalin, but still only around 25 percent of a full crop. Then again, the minimum temperature during each freeze event was extremely low. To have a positive response at temperatures this low was encouraging, and begs the question what sort of response might we have seen if the freezes were less severe, i.e., in the 28° to 32°F range?

There are several other questions regarding the use of Promalin as a frost-rescue treatment that need to be answered.

First, is the treatment only effective if freeze events occur during bloom, or are they also effective during the pink or postbloom stages?

Second, how long after a freeze can Promalin be applied and still have efficacy? Can application be delayed until 24 or 48 hours after a freeze and still have an effect?

Unfortunately, attempts to answer these questions will have to wait until the next time we get a spring freeze, which may not happen for many years—hopefully!

If you need apples to sell and are not concerned about keeping the fruit in storage for too long, then, yes, our results in 2012 suggest that Promalin might be used as a (partial) frost-rescue ­treatment.