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. EKach 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. 
