May 12, 1923 Injury to Foliage by Arsenical Spray Mixtures 
507 
From about three years after its introduction, London purple was 
generally considered to be nearly equal in efficiency to Paris green, and 
it is so considered to-day. 
ZINC ARS^NITE 
In 1903, an investigation of arsenicals and spray injury was begun by 
the California Experiment Station. In the course of this investigation, 
foliage tests were made with zinc arsenite in the summer of 1906. In 
1907, about 5 acres of apple trees were sprayed during the blooming 
season without injury, and the results indicated that this material was 
a promising insecticide. In 1909, the first commercial material was 
prepared by the California Spray Chemical Company, Watsonville, Calif. 
PRlBPARATION 
The process of manufacture in general consists in boiling together in 
water, in the presence of ammonia, zinc oxid and arsenious oxid, in 
approximately the proportion of their combining weights. The boiling 
is continued until the oxids combine, which is indicated by a marked 
thickening of the mass, and until the filtrate shows only traces of arsen¬ 
ious oxid. 
us:e AS AN INSECTICIDE 
At the present time zinc arsenite is used extensively in certain localities 
for the spraying of apple trees just after full bloom, at the rate of 3 pounds 
of the powder to 100 gallons of water. It is not used for later spraying 
on account of its tendency to injure foliage; but from recent tests it is 
possible that it will become an important insecticide for truck crops. 
INJURY TO FOLIAGE 
Luther (letter of May 20, 1910) advocated the use of zinc arsenite on 
apple, pear, bean, and potato, but not on delicate foliage like peach. 
Volck (55) pointed out what he believed to be a fallacy in accepting the 
foliage of any one plant as a reliable index to the injurious action of all 
arsenicals. He found that either bean or peach foliage was a suitable 
indicator for testing arsenate of lead that is intended for use on apples. 
Because of the great ease of obtaining bean foliage, it was adopted as a 
standard testing medium. Later he says: 
Arsenite of zinc may prove entirely neutral to bean foliage and yet when applied to 
peaches do marked injury. Samples which injure peach will later prove injurious 
to apples if applied in sufficient quantities, and bean foliage is not suitable for testing 
samples. 
Luther (17) in speaking of investigations in the Pajaro Valley states: 
On apples it [zinc arsenite] has been sprayed as thick as whitewash without the least 
bit of injury. On small field crops, such as beans, potatoes, etc., it has given no 
injury, but on the peach, which is supposed to be more hardy than the bean, the 
injury was severe. 
Woodworth (jd), in speaking of spraying conditions in the Pajaro 
Valley, says: 
This [zinc arsenite] has proven to be the safest of the arsenicals that can be procured 
in the form of dry powder. It is not so safe, of course, as the neutral lead arsenates, 
but has been used without very serious evidence of burning in the orchards where 
dusting has been adopted instead of spraying, . . . There is no doubt that the zinc 
arsenite stands foremost at the present time among the available arsenicals with high 
arsenic content. 
