Sumithion E.C. [0.0-dimethyl-o-(3-methyl-4-nitro phenyl) thiophosphate 50%] and Sevin W.P. (I-naphthyl N-methyl carbamate) were evaluated for their fruit thinning effect and their suitability as thinning chemicals for apple trees. The results are as follows:
1. By the application of Sumithion, a strong thinning effect was found on the cultivars, Jonathan and Ralls, however the latter hardly responded to Sevin. Especially, side fruits were strongly thinned, leaving mostly one center fruit. Ralls showed a satisfactory commercial thinning effect by Sumithion application 10 days after full bloom, and Jonathan on 20 days after. Delicious was over-thinned even 30 days after full bloom.
2. Both Ralls and Jonathan were effectively thinned by Sevin application from the 10th to 20th day after full bloom, but in the case of Delicious, the danger of over-thinning was eliminated by delaying the application until after the 20th day from full bloom.
3. Both Sumithion and Sevin produce decreasing degree of thinning from the dilution of 300X down to 3000X; however, the decrease is not significant.
4. Although thinning was observed during 4 weeks after full bloom, it was most sensitive around the 10th day after.
5. Comparing 1, 2 and 3 times of application, at an interval of 3 to 5 days, starting from the 7th day after full bloom, the effect of the number of applications was not significant; however, tends to increase the thinning effect.
6. The thinning effect of sprays was more effective, when applied directly to fruitlets, than when applied to leaves only (i.e. by covering the fruits with polyethylene film).
7. When only one fruit remains in a cluster, spraying does not remove this fruit, regardless of its position on the cluster.
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