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We propose that return flowering of `Fuji' apple can be improved if sufficient flower clusters are removed during or shortly after bloom. In this study conducted at Corvallis, Ore., we evaluated two synthetic auxins, MCPB-ethyl and the Na salt of NAA, each at 0, 4, 8 and 16 ppm, as blossom cluster thinners. Each auxin treatment was applied alone or with 100 ppm ethephon as a tank mix. Six-year-old `Fuji'/M.26 trees were sprayed at full bloom of the king flowers (≈85% of whole-tree full bloom). A follow-up treatment of Sevin XLR (800 ppm carbaryl) was made at 11-mm fruit diameter to determine if carbaryl's known effectiveness as a fruitlet thinner was influenced by the bloom-time auxin or auxin + ethephon treatments. MCPB-ethyl proved ineffective as a bloom-time thinner, whereas the NAA effect on cluster removal was linear with concentration, 16 ppm NAA completely defruiting 33% of initial flower clusters. On control trees fewer than 12% of flowering clusters failed to set fruit. Ethephon alone defruited 25% of the clusters and NAA+ethephon defruited 51% of clusters. It is notable that the NAA and ethephon + NAA treatments did not reduce fruit set on the remaining clusters, resulting in considerable need for hand-thinning. Carbaryl effectively reduced total crop load by increasing the number of defruited clusters and reducing the incidence of doubles and triples. There was evidence to suggest that its effectiveness was compromised by the bloom-time NAA and/or ethephon sprays.

296 Chemical Thinning of `Fuji' Apple with Ethephon, NAA, MCPB-ethyl, and Carbaryl