2 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 
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 sufii- 
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 
Newtowns 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 dificult 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 IT 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? 
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 | 
+ 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., 1889. | 
