APPLE POWDERY MILDEW. 9 



in 1915 that for the first time in the Wenatchee Valley systematic 

 spraying for a fungous disease was undertaken. 



In general, the methods followed were based upon the recommen- 

 dations of Ballard and Volck (12) for the Pajaro Valley of Cali- 

 fornia; but since the climatic conditions under which they found 

 soluble sulphids injurious to foliage were not prevalent in the 

 Wenatchee Valley, the somewhat laborious process of removal of 

 these compounds from the iron-sulphid spray was not attempted 

 and a modified formula was used. (See formula 7, p. 11.) A con- 

 siderable quantity of commercial spray preparations was also used, 

 according to a report of Mr. O. T. Clawson, then State horticul- 

 tural inspector for the Wenatchee district, the following quantities 

 of spray materials were sold for mildew spraying in 1915 : 



Commercial sulphur pastes 17, 850 pounds. 



Lime-sulphur solution 11, 600 gallons. 



Iron sulphate (for making iron-sulphid* spray ) 31, 000 pounds. 



SPRAY INJURY. 



It has not been difficult to control the mildew effectively with any 

 one of several of the sulphur spray materials commonly employed 

 for spraying, but their use during the intensely hot summer weather 

 in the arid districts of the Northwest has invariably been attended 

 with severe fruit burning and some foliage injury. 



The importance of climatic factors on spray practices in mildew 

 control has been pointed out by Ballard and Volck (12). However, 

 the difficulty experienced in the Pajaro Valley of California, i. e., 

 defoliation and the dropping of fruit caused by sulphur injury, is 

 seldom met with in the irrigated regions of the Northwest, appar- 

 ently because of the absence of the foggy weather which is so prev- 

 alent in the Pajaro Valley. Sulphur burning on the fruit is found 

 to be the chief reaction induced by climatic conditions in the irri- 

 gated sections of the Northwest. Fruit dropping following sulphur 

 injury is only occasionally met with, and foliage injury usually is of 

 minor importance when it occurs at all. However, sulphur spotting 

 of the apples has often resulted in more loss than the mildew would 

 have caused. Growers have therefore largely abandoned attempts to 

 spray for the disease until a safe spraying program could be developed. 



Sulphur spotting is always confined to apples on the south and 

 southwest sides of the trees and involves only those apples exposed 

 to the direct rays of the sun during the hottest part of the day. 

 Apples shaded by the foliage are never affected. 



In the Wenatchee Valley no injury has resulted from sulphur 



sprays applied before the middle of May. Sprays applied after this 



date have generally resulted in the fruit burning as soon as hot 



weather starts, and sprays applied early in June have invariably 



62859°— 18— Bull. 712 2 



