- BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 
BEARING OF CLIMATIC CONDITIONS ON THE SPRAYING PROBLEM. 
It is very probable that this cool, foggy climate with its lesser 
amount of sunshine is the factor which produces apple foliage that 
has a lower resistance to injurious spray materials and a higher 
susceptibility to powdery mildew attack than that grown in dis- 
tricts where more intense sunshine and higher temperatures obtain. 
Also, the presence of fog and dew moisture on the foliage tends 
to dissolve and decompose some spray materials after they have 
been applied. In fact, the conditions surrounding the problem of 
spray injury in the Pajaro Valley are distinctly different from those 
in the eastern United States, for instance, where the foliage is fre- 
quently subjected to washing rains. In the latter case the injurious 
substances liberated by the decomposition of the spray materials on © 
the foliage are, to a great extent, washed off as rapidly as they are 
formed. In the Pajaro Valley, on the other hand, no such wash- 
ing occurs, and the injurious substances remain on the leaves, to be 
dissolved night after night by the fog and dew and absorbed directly 
through the leaf surface or through abrasions, thereby producing 
foliage injury. This susceptibility of the foliage to spray injury 
has been especially noticeable in the case of arsenicals. Paris green, 
even of the best grade, can not be used, on account of the severe 
foliage injury which it produces. The ordinary type of lead arse- 
nate, known as the acid arsenate, that is used freely on apple foliage 
in most parts of the United States, is capable of causing serious 
burning and defoliation of the trees in the Pajaro Valley, and it was 
not until the much more stable so-called triplumbic, or neutral, lead 
arsenate was introduced that a safe arsenical was available. This 
tendency to decompose and cause burning is shown by other spray 
materials. However, injury from Bordeaux mixture, for instance, 
is not as severe in the Pajaro Valley as it is in the humid Eastern 
States. Possibly fog moisture is not as free a solvent of resistant 
copper compounds as is rain water. Nevertheless, Bordeaux mix- 
ture and other copper sprays are too injurious to permit of their re- 
peated use in this valley. The same is true of lime-sulphur solu- 
tion and other soluble sulphids which naturally suggest themselves 
as mildew sprays. Lime-sulphur solution of a strength commonly 
employed with success throughout the East for summer spraying 
can not possibly be used in the Pajaro Valley on account of the 
foliage injury which it produces. 
Extensive field tests of spray materials have been carried out ii the 
writers, and further examples might be cited illustrating the striking 
susceptibility of Paj aro Valley apple foliage to injury from sprays. 
Several years of investigations and observations have convinced the 
writers that the trees of that district are in a particularly sensitive 
