May 8, 2017
Michigan apple growers see some frost damage

Growers around “The Ridge” near Sparta, Michigan are currently assessing the frost damage from the cold overnight temperatures on May 7-8, with more cold expected for the morning of May 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 and it’s really too early at this time to tell what our crop might be. 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 will be challenged to assess whether they should apply Promalin (Valent) or Perlan (Fine Americas) to frosted blocks. Promalin/Perlan are a mixture of naturally occurring plant growth regulators (PGR), most specifically a 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 timing is longer than the 24 hours, perhaps as much as 4 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 can be applied from pink to petal fall. Once the fruit has been fertilized and begins to size (4-20mm), Promalin/Perlan will provide little to no benefit. 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.

We recommend that Promalin/Perlan be used 1 pt/acre. As with most PGRs, temperatures should be warm (65+F) before application. However, if temperatures never warm, then the Promalin/Perlan needs to be applied within the 4 days even under cold temperatures. We do not want the fruitlet to whither on the cool days and earlier sprays will start the fruit setting process.

In addition to setting fruit in frosty conditions, Promalin/Perlan will do the following: 1. Increase cell division, 2. Increase fruit weight, 3. Increase apple typiness, 4. Increase fruitset, and 5. Start the thinning process.

You could add the Promalin/Perlan with your fungicide cover sprays, but be careful with other PGR’s such as Apogee (leave at least 2 to 4 days between PGR applications).

Frost Risk – There is a risk for some frost again Tuesday morning. You can find the new MSU Enviroweather Sparta Tower on the Enviroweather website.

Look for the “Weather observations and summaries” section where several weather tools are available.

The “New Meteogram” shows the inversion strength and is updated every 5 minutes at the tower site only – it does not predict inversions, but shows in real time what it happening at the tower site in Sparta. The “Overnight temperatures/ hours below freezing” link takes you to a page with three tables – the hourly average overnight low temperatures, the predicted overnight temperatures for the next day and the predicted dew point for the coming evening. All are useful to help you decide if it’s necessary to run your frost fans.

You can hover over the graph to see the data for each five-minute point. Or, if you select “show data” in the bottom right corner of the graph, you can see the data in table form.

— Amy Irish-Brown, MSU Extension Educator – Production Tree Fruit

75 Applewood Drive, Suite A
P.O. Box 128
Sparta, MI 49345
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