2 BULLETIN 712, U. S. DEPAETMENT OF AGKICULTUEE. 



dustry is highly specialized. In the Wenatchee Valley, Washington, 

 where the experimental work herein reported was carried on, the 

 arable land is almost entirely taken up with apple orchards; and, 

 having a high valuation, it is cut up into small holdings, the average 

 orchard area being somewhat less than 10 acres for each owner. 

 Great attention is given to every detail of the orchard operations; 

 pests and diseases are vigorously combated, and every effort is made 

 to produce a maximum amount of the perfect high-colored fruit 

 which has made this region notable. Imperfections and markings 

 affecting only the appearance of the apples cause a loss of grade 

 and consequent financial loss to the grower. If, as in the case of 

 apple powdery mildew, there is added to this effect a lessened pro- 

 duction, every effort to remove the cause is justified. 



The seriousness of apple powdery mildew is greatly modified by 

 the character of the season in any given year. The spread of in- 

 fection is greatly facilitated by a rainy spring. During seasons 

 when conditions for spore germination are most favorable the 

 disease assumes epidemic proportions and is the cause of serious loss. 

 It not only attacks the foliage and tender wood growth, but also 

 destroys fruit buds and directly attacks the fruit. In the Wenatchee 

 Valley the russeting of the fruit is considered one of the most serious 

 effects of the disease. 



REVIEW OF THE LITERATURE. 



The first mention of the disease in this country was made by 

 Bessey (1)"^ in 1877. He reported a serious outbreak of powdery 

 mildew {Podosphaera humei) on seedling apples and cherries in 

 the college nursery in 1871. 



The fungus was first described as Sphaerotheca leucotricha by 

 Ellis and Everhart (2) in 1888. 



Galloway (3) carried on the first extensive experimental work and 

 published the first recommendations for control. In 1889 he re- 

 ported that the disease occurred abundantly through all the region 

 east of the Mississippi. He found that it w^as confined to attacks 

 on young trees in the nursery, especially seedlings, the leaves becom- 

 ing dry and brittle and of so little use that the trees were rendered 

 worthless for budding. He stated that the disease is spread by 

 wind dissemination of spores, aided by insects, rain, and other agents. 

 He pointed out that the ascospores are of no practical importance 

 and that the fungus winters over in mycelial form. In a later publi- 

 cation (1) he reported the successful use of ammoniacal copper- 

 carbonate spray in controlling the disease. 



1 The serial numbers In parentheses refer to " Literature cited " at tiie end of tl:i« 

 bulletin. 



