24 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 
If necessary, the work may be started on a part of the orchard and 
additional portions or rows included each year. Meantime close 
attention must be given to spraying. 
CONCLUSIONS. 
The orchards of the Pajaro Valley in California suffer more from 
apple powdery mildew than do those of any other large apple- 
producing district in the United States. The disease attacks the 
foliage and also the bark of the young twigs, but does not directly 
injure the fruit. It is therefore impossible to estimate the amount of 
commercial damage done, but since as much as 90 per cent of the 
foliage on trees of susceptible varieties may become diseased it is 
evident that such trees must eventually have their capacity for pro- 
duction lowered, either as a result of the vitality of the trees being 
reduced, or, more directly, because the annual growth and consequent 
increase in bearing surface is less than normal. The importance of 
giving proper attention to control measures is still further em- 
phasized by the fact that the disease recurs regularly year after 
year and gradually acquires a stronger foothold if its progress is not 
checked, 
The climatic conditions of the Pajaro Valley are peculiarly differ- 
ent from those of other large apple-growing sections, and their bear- 
ing on the general problem of mildew control may be summarized as 
follows: 
(1) They are responsible for a peculiarly sensitive physiologic condition 
of the trees, {a@) which is manifest in the extreme sensitiveness of the foliage 
and fruit to spray injury of one form or another, and (0) which appears to 
be evidenced by the pronounced susceptibility of the foliage to mildew attack. 
(2) They influence directly the prevalence of the disease and the damage 
done by it. in that they furnish favorable conditions for the spread and devel- 
opment of the fungus. 
(83) They supply conditions favorable to the breaking down of many com- 
pounds that are employed in spray mixtures and at the same time furnish 
conditions for the solution and the absorption of those decomposition products 
by the foliage. Thus, indirectly, the weather furnishes extreme conditions for 
the development of certain types of spray injury. 
The fungus Podosphaera leucotricha, which causes apple powdery 
mildew in the Pajaro Valley, winters in the lateral and terminal 
buds of badly mildewed twigs. The shoots that develop during 
the following spring from these infected dormant buds soon become 
more or less covered with mildew, and spores are produced in enor- 
mous numbers. These spores give rise to the first infections of the 
healthy foliage. Therefore, one of the most important steps in the 
control of apple powdery mildew is the elimination of these early 
twig infections which develop from whe diseased dormant buds. Cut- 
