246 MANUAL OF TREE DISEASES 



of callus. The tissue around the canker is irregularly swollen, 

 causing badly deformed trunks. 



The cankers gradually girdle the trunk and the trees are 

 either blown over or die. Suckers are formed in abundance 

 just below the cankers. On young smooth-barked trunks, 

 another type of canker is formed which is at first a light yellow- 

 ish raised area of bark and later develops into a sunken dead 

 surface which quickly . girdles the stem. When the bark is 

 peeled from these cankers, thin sheets of white or pale brownish 

 mycelium are exposed. 



On the surface of the affected bark of both types of cankers 

 are numerous small black nodules which are the fruiting-bodies 

 of the causal fungus. 



Caiise. 



This canker of oaks and chestnut is caused by the fungus 

 Strumella coryneoidea. No ascus stage has been found con- 

 nected with this species and it is known only by the conidial 

 fruiting-bodies. Inoculations have not been made with this 

 fungus to determine fully its pathogenicity. The fungus is found 

 invariably associated with the cankers and has been isalated 

 . and grown in pure culture. The black nodules on the cankers 

 which have not girdled the trunk do not produce spores and 

 are abortive. As soon as the trunk is girdled, however, nu- 

 merous brownish powdery pustules burst through the bark. 

 The spores of the fungus are borne free on the surface of these 

 pustules and are believed to be carried by the wind. The 

 mycelium penetrates the wood deeply and causes a weakening 

 of the trunk. Infection usually occurs through a small branch 

 stub and from this center the mycelium spreads in all directions. 



Control. 



No definite control measures are suggested for this canker. 

 The ordinary surgical methods of canker treatment,, however, 



