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Bagenal et al. (2) recognized that the drop of immature fruit from a healthy tree was increased by chemical sprays applied to control pests. They observed that "lime-sulphur" (calcium polysulfide) induced excessive drop of young apples. The first conscious attempts at chemical elimination of flowers were made by Auchter and Roberts (1). Several of the common spray materials of that era were used in their tests and included calcium and sodium polysulfide, copper sulfate, oil emulsion, and zinc sulfate. In many instances, prebloom applications of these materials not only killed the flowers but also injured the spur tissues and foliage. The tar distillates, when applied at the cluster bud stage, were the most effective in preventing fruit set. The main object of early work on chemical thinning was to find a practical method of entirely preventing fruit set on certain apple cultivars. In 1940, promising results were reported in reducing fruit set with a commercial preparation of dinitrocyclohexylphenol (32).

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A colorimetric method for the determination of Alar residues in apples with a sensitivity of about 1.5 p.p.m. has been developed. The method involves alkaline extraction and hydrolysis of Alar to dimethylhydrazine and steam distillation of the latter. Dimethylhydrazine is determined colorimetrically based on its reduction of phosphomolybdic acid to the heteropoly blue. Recovery of

Radioactive (2-chloroethyl)phosphonic acid (ethephon) was applied to leaf and fruit surfaces 6 to 10 days before normal harvest date. Samples were collected periodically following application and analyzed with appropriate extraction and counting procedures. The level of radioactive ethephon increased in the fruit for about 48 to 72 hr, then decreased to a low level after 6 days. No intermediate metabolites were detected in the fruits. It was found that the majority of the ethephon in the fruits moved there from the application on adjacent leaves; relatively small amounts moved directly into the fruit from surface application. Radioactive ethylene was detected within 12 hr after application of the 14C-ethephon on the leaf surfaces.

Visible injury to apple foliage from exposure to ozone includes brown stippling on lower leaf surfaces and a light yellow mosaic on upper surfaces. Histologically, chlorophyll is destroyed and palisade and spongy mesophyll cells collapse following disintegration of their protoplasm. Foliar spray applications of the antioxidant, diphenylamine (DPA) or the antitranspirant, Wilt Pruf offer protection against this visible and histological injury. Superior protection results from application of both agents in combination.

2-chloroethylphosphonic acid (Ethrel) was applied alone and in combination with NAA or 2,4,5-TP on apple cultivars which had previously been treated with succinic acid 2,2-dimethyl hydrazide (Alar). On ‘McIntosh,’ Ethrel applied 1 to 2 weeks before harvest overcame the abscission delay induced by the earlier Alar application, hastened red color development and slightly diminished the increased firmness imparted by Alar. The addition of NAA to the Ethrel treatment counteracted the abscission stimulation but did not alter the other effects of Ethrel associated with its release of C2H4. Thus, Alar-treated fruits reached maturity earlier under the influence of Ethrel, and with the addition of NAA or 2,4,5-TP a satisfactory retention force was maintained until harvest. ‘McIntosh’ receiving these combinations were of comparable or better quality in comparison with control fruits or fruits treated with Alar alone after storage in CA and in 32°F air.