THE FIELD TESTING OF COPPER-SPRAY COATINGS. 3 



Due care must be taken to make the leaf samples representative. 

 About three samples of 200 grams each should be taken at various 

 parts of a sprayed planting. Under ordinary conditions collections 

 every three or four days will give satisfactory indication of the 

 persistence of the spray residue. If in the case of any variety of leaf 

 there is a tendency toward discoloration of the wash water from dis- 

 solution of organic compounds in such a way as to interfere seriously 

 with the color comparisons, one may titrate a measured quantity 

 of the wash water with a solution of potassium ferrocyanid previously 

 standardized against a copper solution of known strength, using a 

 weak ferric-chlorid solution as the end-point indicator, in small drops 

 on a white porcelain plate. Tests on many kinds of crop plants 

 indicate that this titration method will be seldom needed. Per- 

 mitting the weighed leaves to dry or to ''heat" may interfere with 

 accurate testing. 



The coating on the foliage will usually prove to be a fair index to 

 the thoroughness of application to -other parts. For convenience, 

 the samples are based on weight, but the spray coating is propor- 

 tional to area. The different ratios between these, due to difference 

 in species or age of leaves, when considerable, should be given due 

 weight in making comparisons. Obviously, it is possible to secure as 

 high tests from strong sprays unevenly applied as from moderate 

 strengths more evenly and effectively used, but it is believed that 

 the histories of such cases •will guard against incorrect interpretations. 



The amount of spray residue necessary for adequate protection 

 against any specific disease would doubtless vary with a complex set 

 of factors. There would thus exist a critical transition zone with 

 maximum and minimum limits above which protection would be 

 secured and below which protection would be insufficient. These 

 limits can be determined only from investigations covering a period 

 of years. The best spray practice should aim at keeping the spray 

 coating always above the maximum limit, and ordinarily good prac- 

 tice should never take the risk of falling below the minimum limit. 



FIELD TRIALS. 



During the two seasons that this method has been in use, something 

 like 75 schedules have been tested by the writers. The following 

 results are selected as being representative. 



APPLE LEAVES AT CROZET, VA. 



Winesap apple leaves were tested at Crozet, Va., June 23 to 

 July 22, 1917. The orchard was about 5 acres in extent and was 

 sprayed with 3-4-50 Bordeaux mixture on the date of the first collec- 

 tion. Each sample was divided, one portion being sent to the Bureau 

 of Chemistry, United States Department of Agriculture, for exact 

 analysis, and the other tested by the field method outlined above. 



