Apple Crop Load Management Update: 5/4/2023

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For those that have a crop of apples, opportunities to apply chemical thinners have been limited. Inclement weather (high winds, cool temperatures, and rain) and uncertainty of initial fruit set due to multiple frost/freeze events have seriously complicated an already difficult series of decisions.

A quick recap of the current situation:

  • Some orchards have not received a chemical thinner to date (at least in western NC).
  • While fruit growth rates were slowed by cool temperatures, many cultivars have fruit size greater than 15 mm. Some blocks have disturbingly large fruit. Late blooming cultivars (Rome) are in or approaching the 8-14 mm window
  • The 1st thinner application that some orchards will receive will be a rescue thinner. I suspect that this is the case for significant acreage in the region.
  • Those that were able to apply chemical thinners in mid-April should be able to see thinner responses at this time. In many cases, these responses have been mild…. and re-thinning is necessary.

Today and tomorrow (May 4 & 5, 2023) present marginal temperatures for chemical thinner activity and we are currently in a carbon surplus. Then, rain, some heat (finally), and an unsettled weather pattern are predicted for Saturday – Tuesday. Once we get into the weekend and early next week, a mild/moderate carbon deficit is forecast in most locations, which is desirable for thinner responses. The trade-off and critical decision for all affected parties:

While the forecast for next week suggests a better opportunity for chemical thinning, will fruit size be too large? This decision is not one-size fits all.

Given the potential for pop-up thunderstorms early next week here are a few reminders regarding rainfall after a chemical thinner application and rescue thinning:

  • Concerned about rainfall after a thinner was applied? Most PGR labels suggest that 6 to 8 hours of drying time is required for full activity. However, for most chemical thinners (6-BA, NAA, and carbaryl): once the chemical thinner dries, you can expect at least 80% of the response. This may come into play for those who thin early next week.
  • Ethephon is a different animal when it comes to drying time. Work by Dick Unrath and JD Obermiller demonstrated that 8 hours of drying time is needed. It would be inadvisable to re-apply this material in the event of a rain event following application.
  • Good spray coverage is important with all chemical thinners, but is an absolute necessity with ethephon.
  • Once apples are greater than 22 mm, they become unresponsive to chemical thinner. In warm weather, apples grow ~ 1mm per day. Cool weather slows growth rate, but does not stop it. Once apples are 18 mm, you have about 4 days to apply thinner.

Below are the Apple Re-Thinning Recommendations for the Southeast for your reference.

Apple rescue thinning reccomendations.