APPLE POWDERY MILDEW AND ITS CONTROL. 5 



physiologic condition and that almost any artificial treatment given 

 them is liable to bring striking results, either beneficial or injurious. 

 In fact, precipitated sulphur may be applied as a spray in such a 

 manner as to produce two distinct physiologic effects, one markedly 

 beneficial and the other decidedly injurious. 



APPLE POWDERY MILDEW. 



Apple powdery mildew, as has been stated, may be caused by either 

 Podosphaera oxyacanthae or P. leucotricha, formerly called Sphaero- 

 theca mali (Duby) Burr. Oidium farinosum Cooke, which is fre- 

 quently reported as the cause of the disease, is probably, according 

 to Salmon, the conidial form of P. leucotricha, It is also probable 

 that in the absence of what is termed the perfect stage the fungus is 

 often assumed to be P. oxyacanthae when P. leucotricha is the form- 

 actually present, though both species exist on the cultivated apple in 

 the United States. The species occurring in the Pajaro Valley is 

 P. leucotricha. The identity of the fungus has been determined by 

 repeated examinations of the perfect stage, which occurs frequently 

 in that locality on the twigs, though not on the leaves. However, the 

 following description of P. leucotricha and its life history will apply 

 very well to P. oxyacanthae except for certain technical differences, 

 some of which will be mentioned. 



As has been stated, the disease attacks the foliage and current 

 year's twig growth. Infection of the young fruit is extremely rare, 

 and only occasionally does one^find a flower-cluster bud that has been 

 attacked. In the latter case the entire individual flowers are usually 

 involved, the floral organs are reduced in size and much deformed, 

 and the stems are short and thick. (PL IV, fig. 2.) 



Mildewed areas on the leaves occur most commonly on the under 

 sides. They are white or grayish, and the term " powdery " very 

 well describes their mealy appearance. The diseased spots may vary 

 in size from a point invisible to the naked eye to patches three- fourths 

 of an inch or more in diameter, and several of these may become 

 established on a single leaf. In a large percentage of cases the entire 

 upper and lower surfaces of the leaf become involved. Mildewed 

 leaves are crinkled and stunted and often very much narrowed, 

 owing to the fact that the growth and expansion of the leaf tissue are 

 checked in the areas covered by the fungus. Plate V, figure 2, illus- 

 trates this crinkling and stunting, and Plate IV, figure 1, illustrates 

 the narrowing. In the Pajaro Valley the disease makes such rapid 

 progress during the spring and summer that by the end of the leaf- 

 forming season it is difficult to find normal, healthy leaves in any 

 unsprayed Yellow Newtown or Yellow Bellflower orchard. 



