UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 785 4g 
Contribution from the Bureau of Plant Industry 
WM. A. TAYLOR, Chief 
Washington, D. C. PROFESSIONAL PAPER. March 13, 1919 
THE FIELD TESTING OF COPPER-SPRAY COATINGS. 
By J. R. Wixston, Assistant Pathologist, and H. R. Fuuron, Pathologist, Office of 
Fruit-Disease Investigations.' 
CONTENTS. 
Page. Page. 
The persistence of sprays containing copper. . 1 | Field trials—Continued. 
A practical method of field testing..-.-...-.-- 2 Citrus nursery stock at Bradentown, Fla- 6 
i AEG AT ONS Se Se See ct ae ae eee 3 Celery at Bradentown, Fla......--.--.---- 7 
Apple leaves at Crozet, Va.......-..-..-. Syl GDITeCTIONS fOnsUSC see eerie le uate ratreperete 8 
Bearing grapefruit trees at Winter Park, Conclusiony Ae 3:75. Su. couse fy aceite 9 
TOTES A EA ee eee 5 
THE PERSISTENCE OF SPRAYS CONTAINING COPPER. 
General experience in the use of copper-containing spray mixtures 
indicates a progressive decrease in the fungicidal effectiveness of the 
spray coatings which is to a large extent proportional to the relative 
amounts of spray residue present. A direct decrease in effectiveness 
is brought about by weathering factors, such as washing by rain or 
dew, mechanical abrasion by wind, and slow chemical change in the 
exposed spray coating. An indirect decrease in effectiveness may 
result from the growth extension of plant parts, either through en- 
largement of organs already sprayed or through the development of 
new organs subsequent to the spray application. In practice the 
interval between spray applications depends on the rates of both 
weathering and growth extension. The second can be judged by 
careful observation. In so far as the first is concerned, observations 
are often misleading. Present-day spray calendars are based only 
in a general way on the average practical and experimental experi- 
ence of past years. In any particular season or section or planting 
the actual requirements for adequate protection may vary widely 
from such average, necessitating greater or less frequency of applica- 
1 The writers wish to express their appreciation of the cooperation in connection with this work of the 
staff of the Miscellaneous Division, Bureau of Chemistry, United States Department of Agriculture, 
104322°—19—Bull. 785 
