We have tested dozens of chemicals that have potential for thinning peaches, plums and nectarines. Most are caustic materials that "burn" off flowers when applied at the time of bloom. Although thinning was generally achieved in our trials, we felt each chemical had some drawback such as inconsistency, excessive phytotoxicity or difficulties in registering the product. The material we had the most success with was Armothin that is still sold by Wilbur-Ellis under the name of N-TER, although it is no longer registered for thinning. With some experimentation and by taking into account tree "health" and stage of bloom (see 2001 CTFA Plum Chemical Thinning Report), we concluded growers could develop a workable chemical thinning program to supplement hand thinning.
CTFA sponsored projects on chemical thinning are listed below with links to the full report.
1991 - Test of UN32, zinc sulfate and 6 surfactants (1991 CTFA Chemical Thinning Report)
1996 - Comparison of Armothin, Wilthin and Endothal (1996 CTFA Chemical Thinning Report)
1997 - Comparison of Armothin, Wilthin and Urea (1997 CTFA Chemical Thinning Report)
1997 - Summary of trials with Armothin, Wilthin and Ralex (1997 KTFR Chemical Thinning Summary)
2000 - Armothin trials on plums (2000 CTFA Plum Chemical Thinning Report)
2001 - Further Armothin trials on plums (2001 CTFA Plum Chemical Thinning Report)
2005 - Trials with soybean oil, cottonseed oil, Ethrel and Tergitol (2005 CTFA Thinning Report)
2006 - Same as 2005 plus ammonium thiosulfate (ATS) (2006 CTFA Chemical Thinning Report)