GRAPE DISEASES 257 
(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 Fic. 69. — Rot of grapes caused by the dead-arm 
spores of the fungus pathogene. 
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 gets into the arms and 
8 
