UNITED STATES DEPARTMENT OF AGRICULTURE 



BULLETIN No. 785 



Contribntion from the Boreaa 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. Winston, Assistant Pathologist, and H. R. Fxilton, Pathologist, Office of 



Fruit-Disease Investigations.^ 



CONTENTS. 



Page. Page. 



The persistence of sprays containing copper . . 1 



A practical method of field testing '2 



Field trials 3 



Apple leaves at Crozet, Va 3 



Bearing grapefruit trees at Winter Park, 



Fla 5 



Field trials— Continued. 



Citrus nursery stock at Bradentown, Fla. 6 



Celery at Bradentown, Fla 7 



Directions for use 8 



Conclusion 9 



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, Biu^au of Chemistry, United States Department of Agricultinre. 

 104322°— 19— Bull. 785 



