OIDIUM OR POWDERY MILDEW OF THE VINE. 323 



A mature perithecium consists of a nearly spherical case containing 

 from four to eight cell-sacks called asci. The walls of this case are 

 strong and resistant to cold and other unfavorable conditions which 

 destroy the summer forms of the fungus. They are made up of thick 

 polygonal cells and form a very efficient protection to the asci. An 

 ascus contains from four to six spores. (See figs. 8 and 5, IV.) 



Each penthecium is furnished with 10 to 25 hair-like appendages 

 of about the same thickness as the mycelial threads and from one-one 

 hundredths to one-fiftieth of an inch in length. These appendages are 

 divided by several cross partitions and are coiled at the free end. They 

 consist usually of a single thread, but occasionally branch. (See fig. 

 5, II.) 



The perithecia remain upon the canes^ and leaves or in the soil until 

 the return of warm weather in spring. As soon as the temperature 

 becomes sufficiently high, the walls of the perithecia .break open and 

 allow the asci to emit their spores. These spores, when they reach a 

 leaf or growing shoot, germinate in the same manner as the summer 

 spores and give rise to a new generation of Oidium. 



The perithecia do not all emit their spores at the same time owing 

 to the different conditions of the places in which they have passed the 

 winter. Some may even remain in the soil until the following spring. 

 Perithecia have been shown to contain spores capable of germination 

 eighteen months after their formation. 1 It is therefore impossible to 

 completely exterminate the fungus in a vineyard in one year, however 

 thorough the treatment. Burying the perithecia by plowing-in simply 

 aids in their preservation and they are ready to cause a new infection 

 two years later when brought to the surface again by later plowing. 

 Although the Oidium was introduced into Europe as early as 1845, it 

 was not until 1892 that the winter form of the fungus was found there. 

 It was for this reason uncertain until that time that our Oidium and 

 that of Europe were identical. 



It seems probable that the fungus, under some conditions, carl pass 

 the winter in the conidial or summer form. At all events, in certain 

 districts the perithecia have never been found and are therefore absent 

 or very rare. They are very rare in the interior valleys of California. 

 On the other hand, they are produced in immense numbers on vines 

 within the influence of the summer ocean fogs. At Berkeley it is com- 

 mon to find thousands of them on every leaf and cane of a vine. More 

 than 100,000 perithecia have been counted on the upper surface of a 



1 Revue de Viticulture, No. 655, p. 



