230 



FUNGOUS DISEASES OF PLANTS 



It is difficult to explain the absence of the perithecial stage 

 for so long a period in Europe, and even now when many 

 observers would be alert to the presence of such a form, it is 

 certain that the occurrence of perithecia is exceptional. For a 

 long time it was supposed that the disease was imported to 

 Europe from America, as were other grape diseases, but since 

 the fungus is also found in Asia, there is no special reason for 

 this assumption. The use of the more common fungicides has 

 not been so successful in preventing the attacks of this fungus 

 as the simple sulfur dust treatment. 



XXIX. POWDERY MILDEW OF WILLOW AND POPLAR 



Untinula Salicis (De C.) Wint. 



This species is apparently limited in host plants to the two 

 genera Salix and Populus, but it occurs upon many species of 



these throughout their 

 distribution. The my- 

 celium occurs on both 

 surfaces, frequently 

 evanescent on poplars, 

 while often persistent 

 and in patch-like areas 

 on willow, or covering 

 the entire surface (Fig. 

 94). On the latter the 

 perithecia are also gen- 

 erally aggregated. 

 They average 1 3 5 yu. in 

 diameter, and are pro- 

 vided with numerous 

 (often more than 100) 

 hyaline appendages, 

 the tips of which are 

 distinctly uncinate. 

 The asci are generally 

 about 10, with 4-6 

 spores, measuring 

 20-26 x 10-15 /*■ 



Fig. 94. Powdery Mildew op Willow. 

 graph by Geo. F. Atkinson) 



(Photo- 



