Apple Carbohydrate Thinning Model-Updates-(May 08 – 12)

At such advanced stage of apple fruit development, 6-BA and NAA are no longer effective as chemical thinners and it is the time to start the “rescue thinning” applications. The fruit size for most varieties in Winchester is close to 20 mm and it would likely be above 23 mm in Central Virginia. Ethephon is the most effective thinner at this stage. Ethephon thins more effectively when the temperature is in the 70s to low 80s. Although the weather forecast predicts cool temperatures this week, increasing the rate of chemical thinners by 30% might mitigate this negative impact of low temperatures. Mixing carbaryl (1 pint/100 gal) and oil (1-2 qt/100 gal) with ethephon (0.5-1.5 pints/100 gal) enhances its effectiveness and it might show desirable effects under the weather conditions of this week. The addition of NAA (5 ppm) to ethephon/carbaryl mix might be an option if more aggressive thinning is still needed. However, this application should be avoided with Delicious and Fuji as it will lead to the development of pigmies. Adjusting the pH to 3-5 is recommended with ethephon applications. Non-ionic surfactants (e.g. Regulaid) can also increase the response to ethephon applications.
As shown in the graphs below, the apple carbohydrate-thinning model predicts carbohydrate surplus for the whole week and likely during the weekend in Winchester and Central Virginia. Under such conditions, the recommendations are to increase the rate of chemical thinners by 30%.

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