10 BULLETIN 120, U. S. DEPARTMENT OF AGRICULTURE. 



as are shown in Plate IV, 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. 1 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 (Eed 

 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 



1 Other investigators have reported a similar wintering over of the mycelium of apple 

 and other mildews, but its occurrence in the Tajaro Valley is particularly noticeable 

 because of its abundance and bearing on the problem of mildew control. 



