Setting apples with plant growth regulators

Recent cold injury requires consideration of Promalin or Perlan on frost-damaged apple sites.

Growers around the Ridge and in other areas of Michigan are currently assessing the frost damage from the cold overnight temperatures on May 7-8, 2017, and again on May 8-9. As of now, there appears to be some damage in many apple blocks, but the damage depends on the stage of development in each block, the variety and, of course, the elevation and frost protection methods used or available.

While there are cold-damaged fruits, there are still many with no visible damage. The flowers in the tops of trees are much less hurt than the lower parts of trees. There seems to be damage in low areas as expected with frosty conditions, but also some reports of higher levels of damage on the tops of hills.

In this situation, growers need to assess the potential of applying Promalin (Valent) or Perlan (Fine Americas) to frosted blocks. Promalin and Perlan are the same essential product from different manufactures. Both products are a mixture of naturally occurring plant growth regulators (PGR), most specifically gibberellic acid 4 and 7 (GA4+7), which cause cell enlargement. This PGR can impact apples in many ways depending on when it is applied.

In the case of apple trees that have been frosted, these gibberellins can stimulate parthenocarpic fruit development. Parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules. Fruit that develop through this method are seedless.

Much of the early research recommends applying Promalin/Perlan within 24 hours of the frost event. However, more recent studies have found that the application window is longer than the 24 hours, perhaps as much as four days. According to Michigan State University Extension educator Phil Schwallier’s observations, he has seen this PGR work when applied within a few days after the frost event and still increase fruitset.

This product is best applied from pink to petal fall, which most apple flowers in the Grand Rapids, Michigan, area are in at this time. For apples in earlier or even later stages of development, how fruit will respond to GA4+7 is an unknown, but perhaps worth an application if flowers are not completely killed. The application can be made up to four days from the May 8 frost event, so growers will have a bit of time to determine the level of damage and their stage of development for each of their blocks.

As with other PGR applications, temperatures on the day of application are important, but the days following are even more critical. In the case of Promalin/Perlan to set fruit for this season, getting the application made by Wednesday, May 10, is highly recommended to take advantage of warming temperatures. You can start applying right away, but try to get it done by May 10.

Promalin/Perlan are recommended to be used at 1 pint per acre. In addition to setting fruit in frosty conditions, Promalin/Perlan will do the following:

  • Increase cell division.
  • Increase fruit weight.
  • Increase apple typiness.
  • Increase fruitset.
  • Start the thinning process.

You could add the Promalin/Perlan with your fungicide cover sprays, but be careful with other PGRs such as Apogee—leave at least two to four days between PGR applications.

Special thanks for MSU Extension’s Nikki Rothwell and Emily Pochubay for the use of their 2016 article on this topic from the Northern Michigan FruitNet Report – May 26, 2015.

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