4 CIRCULAR 685, U. S. DEPT. OF AGRICULTURE a 
approximately 30,000 to 35,000 acres of apples were sprayed; in 1941, © 
approximately 50,000 to 55,000 acres of apples were sprayed; and in — 
1942, 75,000 to 80,000 acres of apples and pears received harvest-spray — 
applications.? . 
With the rapid expansion in the use of harvest sprays for retarding 
fruit drop, there have been variations in results under various condi- — 
tions; it seems desirable to bring together in this publication both 
experimental and observational information that might aid in the 
more effective use of these sprays. 
EFFECTIVENESS OF VARIOUS CHEMICALS ON APPLES# 
Of a number of compounds tested on apples at Beltsville, Md., in” 
1939, «-naphthaleneacetic acid, a-naphthaleneacetamide, and the metal- — 
he salts of a-naphthaleneacetic acid were outstanding in effectiveness | 
(7). Somewhat less effective were the methyl and ethyl esters of 
a-naphthaleneacetic acid. The indole compounds (indolebutyric, 
indoleacetic, and indolepropionic acids) were found to retard fruit — 
drop somewhat, but they were regarded as definitely inferior to the 
a-naphthaleneacetic acid group. Additional chemicals were tested in 
1940 (2), and tetralin—6—acetamide was found to be effective when — 
used at relatively high concentrations. Other compounds closely 
related to w-naphthaleneacetic acid, namely, 8-naphthoxyacetic acid, 
a-naphthalene acetonitrile, «naphthalene methyl thiocyanate, and _ 
a-naphthalene methyl isothiocyanate, proved totally ineffective when — 
tested on the Williams variety of apple. 
In the first experiments conducted at Beltsville, where direct com- — 
parisons were made between a-naphthaleneacetic acid and a-naphtha-— 
leneacetamide, it was found that these materials were so close in their 
degree of effectiveness that it was difficult to determine that any real ~ 
difference existed. Subsequent comparisons at Beltsville (2) and else- — 
where have substantiated this conclusion. Hoffman (72, 73) has 
made comparative tests on several varieties of apples with both com- — 
pounds as well as with the sodium salt of «-naphthaleneacetic acid. 
His results showed no essential difference between the three materials. 
Greve, Kadow, and Guy (8) likewise found no significant difference in — 
the effectiveness of these three chemicals under Delaware conditions, 
Enzie and Schneider (6), working with the Stayman Winesap variety — 
in New Mexico, seemed to find «-naphthaleneacetic acid shghtly more 
effective than the acetamide, while Murneek (77), working with the 
same variety, obtained somewhat better results with o«-naphthalene- 
acetamide. 
In experiments with harvest sprays trees receiving the same treat- 
ment commonly vary greatly in fruit drop. Even with relatively large 
numbers of trees per treatment it is necessary to obtain differences of © 
considerable magnitude before it can be concluded with any degree of — 
certainty that real differences exist between treatments. Until a more — 
accurate measure of results than counting fruits dropped is evolved it — 
seems reasonable to assume that a-naphthaleneacetic acid, «-naptha- 
leneacetamide, and the sodium salt of o«-naphthaleneacetic acid are © 
about equally effective in retarding fruit drop. : 
2 Acreage estimates for the various years are based on the sales of harvest-spray materials 
by the various manufacturing companies. 
