10 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 
as are shown in Plate LV, figures 1 and 2, and Plate VI, figure 2, the 
mildew growing over the surface of the shoots has an excellent op- 
portunity to work its way in between the bud scales and penetrate 
both the lateral and terminal buds. This actually happens, and 
within these buds the mildew passes the winter in a dormant condi- 
tion. The following spring, as the infected buds begin to open. the 
mildew commences to grow and keeps pace with the development of 
the new twigs.t Hence, infected shoots appear all over the trees as 
soon as they leaf out in the spring. The mildew growing on the 
young leaf and twig tissue is particularly virulent and produces 
summer spores in great abundance. It is not surprising, therefore, 
that a rapid infection of the healthy foliage soon takes place and that 
eventually practically all the leaves on the tree become diseased. 
SUSCEPTIBILITY OF VARIETIES. 
No varieties of apples grown in the Pajaro Valley are immune 
from powdery mildew, but some are more seriously affected than 
others. The relative susceptibility of different varieties will prob- 
ably be found to vary in different apple-growing districts, depending, 
among other factors, upon the effect which the local climatic and 
other conditions have on the foliage vigor. In general, the varieties 
that produce strong, vigorous foliage are less susceptible than the 
more delicately growing ones. In this connection it is interesting to 
note that in the Pajaro Valley the Yellow Newtown, which is one of 
the most susceptible varieties, can apparently be made much less sus- 
ceptible if the vigor of the foliage be increased by stimulation such 
as comes from spraying with very finely divided forms of sulphur. 
A list of the most susceptible varieties grown in the Pajaro Valley 
includes the Yellow Newtown, Yellow Bellflower, Smith (Smith’s 
Cider). Missouri (Missouri Pippin), Esopus (Spitzenberg), and 
Gravenstein. The varieties that are less severely attacked are the 
White Pearmain (White Winter Pearmain), Winter Pearmain (Red 
Pearmain), Red Astrachan, Rhode Island Greening, and Langford. 
SPRAYING EXPERIMENTS. 
At the beginning of the writers’ investigations it was assumed that 
the winter spores were the important source of the first infections in 
the spring, and the significance of what they have called the dormant- 
bud infections was not realized. It appeared, therefore, that there 
were two phases of the spraying scheme: (1) The winter spraying, 
directed toward killing the winter spores on the twigs, and (2) the 
+ Other investigators have reported a similar wintering over of the mycelium of apple 
and other mildews, but its occurrence in the Pajaro Valley is particularly noticeable 
because of its abundance and bearing on the problem of mildew control. 
