324 Minnesota Plant Diseases 



has been recommended, or the plants may be mowed in the fall, 

 removed and burned. Good results have also been obtained 

 by burning over the bed with the aid of a layer of straw. The 

 selection of varieties will also assist in the combating- of the dis- 

 ease, as certain varieties are much more susceptible than others. 



(Fig- 35-) 



Powdery mildew of strawberry (Sphaerotheca castagnei 

 Lev.}. This disease has not been reported from many localities 

 in the United States but seems to give promise of becoming a 

 destructive disease under favorable conditions. The leaves and 

 fruit are attacked though the former usually suffers most. On 

 the under surface of the leaf the fungus forms the characteristic 

 superficial mildew of this group of fungi and when the summer 

 spores are formed the powdery appearance is noticeable. 



The leaves curl up and may finally dry up. The winter 

 spores appear in sacs borne in the receptacles (sac capsules) 

 usual for this group of fungi. The exact identity of the form is 

 not known but it is probably a close relative of the powdery 

 mildew of hops. 



Powdered sulphur is usually recommended, or spraying with 

 ammoniacal copper carbonate. 



Powdery mildew of cucumbers (ErysiphecichoracearumDC.*). 

 This disease has been reported as destructive to cucumbers, 

 especially those grown in greenhouses. It is a typical powdery 

 mildew and appears to be identical with the exceedingly com- 

 mon powdery mildew of composites, which appears so abun- 

 dantly on a great variety of our wild plants. The mycelium ap- 

 pears on the leaves or stems of the cucumber as small white 

 spots, which soon produce the mealy powder of summer spores. 

 The spread of the infection may be rapid and the infected spots 

 increase in size, becoming yellow and then brown, and may final- 

 ly destroy the whole leaf or even the entire plant. The winter 

 spores are formed in sacs, found in small black capsules, com- 

 mon in the powdery mildews. A large number of sacs is 

 formed in the capsule and the appendages of the latter are sim- 

 ple and interwoven with the mycelium. When the capsules are 

 formed, the mycelium has become a greyish or dirty white coat 

 on the leaf surface. 



