4 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 by the 

 writers, and further examples might be cited illustrating the striking 

 susceptibility of Pajaro 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 



