Good Fruit Grower

February 2013

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GOOD TO KNOW A research report from Steven McArtney, Promalin as a frost rescue for apples In tests, Promalin applied five hours after freeze events during apple bloom increased fruit set and yield. T PHOTOS COURTESY Of NORTH CAROLINA STATE UNIVERSITY he Southeast wasn't spared from the State University 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. North Carolina The freeze events A commercial orchard of Taylor Spur Rome in a lowlying 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. 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). 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. (Continued on page 38) GOOD FRUIT GROWER FEBRUARY 1, 2013 37

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