24 BULLETIN 120, U. S. DEPARTMENT OP AGRICULTURE. 



If necessary, the work may De 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 hearing 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 (&) 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. 



(3) 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 Jie diseased dormant buds. Cut- 



