336 



MANUAL OF TREE DISEASES 



slightly raised margin. Small black fruiting-bodies break 

 through the dead bark the spring following infection. After a 

 year or two the dead bark falls away, leaving the wood bare and 



surrounded by a 

 callus (Fig. 69). 

 The wounds are 

 then often healed 

 over and no great 

 damage results. 

 On the small twigs 

 which are killed 

 back, minute pim- 

 ples cover the yel- 

 low bark. The 

 repeated killing of 

 the twigs causes 

 abnormal branch- 

 ing and gnarled 

 growths which many times produce witches'-brooms. This 

 causes the tree to have a general irregular and scraggly appear- 

 ance. In the northeastern states, in some localities, it is often 

 hard to find trees that are not thus affected. 



Fig. 68. — Cankers on small sycamoie twigs. 



Cause. 



Leaf- and twig-blight of sycamore and oak is caused by the 

 fungus Gnomonia veneta. The perithecial stage occurs on the 

 fallen leaves and matures in the spring. The stage which de- 

 velops as the small cream-coloKred pustules, along the veins of 

 the blighted leaves while they hang on the tree, is known as 

 GlcBosporium neroisequum. Other names were given to the 

 spore stages developed on the twigs and cankers before their 

 identity as one fungus was proved. There are still many facts 

 about this disease to be learned. It is not known which spore 

 stage causes the first infection of new leaves in the spring. The 



