GRAPE DISEASES 



257 



*1 







(Fig. 69) very similar to that in the case of the black-rot disease. 

 They shrivel to a mummy as in black-rot, but have a slightly 

 more grayish aspect and the pustules are less numerous and 

 more scattered in dead-arm than in black-rot. 



Cause of dead-arm. 



It has been experimentally demonstrated that the fungus 

 Cryptosporella Viticola causes dead-arm. From vines which 

 were diseased the previous year pycnospores ooze forth, during 

 wet weather, about the time the buds burst in the spring. These 

 spores are spat- 

 tered promiscu- 

 ously, some of them 

 falling on young 

 shoots, either those 

 directly beneath 

 the old infected 

 cane or those some 

 few feet away. 

 After about thirty 

 days symptoms of 

 dead-arm begin to 

 appear. During 

 this period the 

 spores of the fungus 

 have germinated, 



the germtube has penetrated the part concerned, and infection 

 has resulted. Days of mist and fog favor spore-germination, 

 the process requiring not more than twenty-four hours. The 

 manner in which the fungus gets into the vine is unknown. 



In a great many cases the mycelium developed from the 

 germtube may grow from affected canes into the permanent 

 wood of the spur, arm or trunk, although in cases of less severe 

 infection this does not take place. The growth of the mycelium 

 in the canes is very slow, but it gradually getsinto the arms and 



FIG. 69. Rot of grapes caused by the dead-arm 

 pathogene. 



