2 BULLETIN 120, U. S. DEPAETMENT OF AGEICULTUBE. 



Utah and New Mexico westward, and more particularly along the 

 Pacific coast, climatic conditions appear to favor its development into 

 a serious menace to successful apple growing. Throughout this whole 

 territory it is increasing in its distribution, and in those districts in 

 which it is already established it is gradually becoming a serious 

 orchard disease. It occurs more or less commonly throughout Wash- 

 ington and Oregon and in some districts has already acquired suffi- 

 cient importance to be given regular attention in the annual schedule 

 of spraying applications. At the present time the orchards of the 

 Pajaro Valley in California suffer more from apple powdery mildew 

 caused by Podosphaera leucotricha than do those of any other large 

 apple-growing district in the United States. It is true that in one 

 or two small coast sections in California the disease causes even 

 greater damage to the trees, but its commercial importance in those 

 districts is not comparable with that in the Pajaro Valley, where 

 the annual output of apples is about 3.500 carloads of packed fruit. 

 In that section more than 80 per cent of the apple acreage is in Yellow 

 Xewtowns and Yellow Bellflowers, both of which varieties are par- 

 ticularly susceptible to mildew attack. 



Throughout the western United States, according to the writers' 

 observations, apple powdery mildew attacks only the foliage and 

 young twigs and produces no direct injury of the fruit; therefore, it 

 is difficult to estimate the financial loss which the disease causes. 

 However, a comparison between the general appearance of a tree 

 badly attacked by mildew and one that has been kept relatively free 

 from the disease by spraying should readily convince one that such 

 unhealthy trees can not be expected to produce the kind of crops they 

 should and that their annual growth and increase in bearing surface 

 must be less than normal. Plate I, figures 1 and 2, and Plates II and 

 III show such a comparison between sprayed and unsprayed Yellow 

 Newtown apple trees in the same orchard. Badly diseased orchards 

 that are allowed to remain untreated become more and more seriously 

 infected each year. The cumulative effect of such a gradually 

 increasing general infection results in a decided decline in the vigor 

 and appearance of the orchard. 



The commercial importance of controlling apple powdery mildew 

 has long been recognized, and many investigators, both in America 

 and abroad, have given attention to the problem. As early as 1889 1 

 the Department of Agriculture conducted investigations and issued 

 spraying recommendations for the control of the disease on nursery 

 stock, and since that time numerous formulas for spray mixtures and 

 instructions for spraying have been published by various State ex- 

 periment stations. Meantime, similar investigations have been in 



1 Galloway. B. T. Experiments in the treatment of pear leaf-blight and the apple 

 powdery mildew. U. S. Dept. of Agr., Section of Vegetable Pathology, Cir. 8, 11 p., 1888. 



