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Updated: August 8, 2017
At the spring orchard meeting yesterday in Bedford County there were some comments from the growers about using 6-BA products earlier when temperatures were cooler. It will be interesting to see how those products performed. Certainly the past two days they should have done a good job. Other comments were made about the use of oil seeming more effective than some of the commercial surfactants. The daily carbohydrate balance for the next few days shows a lot of fluctuation looking at the graphs; swinging from positive to slightly negative.
Many orchards may have fruit that is the range of 14+ mm in diameter and may best be thinned by the use of ethephon. 6-BA and NAA tend to be less effective when diameters reach 17-18 mm. Ethephon can thin fruit up to 25 mm in diameter but is influenced by temperature with higher temperatures taking more fruit off. It is also affected by spray water pH and an acidifying surfactant may be needed if pH is above 7. If there is still too many fruit or you have not been able to see any results from an earlier application(s) you may need to add carbaryl at 0.5 - 1.0 lb. of material. Remember that Golden Delicious and Rome are very sensitive to ethephon.
For more information on the carbohydrate thinning model, visit Carbohydrate Thinning Model. For more details on factors that affect plant growth regulator performance and chemical thinner rates, visit Growth Regulators in Apple and Pear Production.
Table 1. Decision rules for using the output of the Carbohydrate model to adjust chemical thinning rate
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