138 PLANT DISEASES 



branch to curve at the part attacked. The first indication 

 of the disease is the swelling of the branch, followed by a 

 longitudinal cracking and gaping of the bark, the wounded 

 surface soon becoming covered with a dark olive velvety 

 coating. 



Examined under the microscope, this velvet pile resolves 

 itself into myriads of upright, simple, or branched coni- 

 diophores, bearing conidia. These conidia are dispersed 

 by rain, wind, etc., and those that are located on the 

 tender bark of young shoots soon form the starting-point 

 of new knots. Later in the season the olive mould dis- 

 appears, and in its place a thick outgrowth of compact 

 fungus mycelium, called a stroma, is produced, hard, 

 nodulose, and black externally. During the winter numer- 

 ous cavities are formed in the stroma, each containing 

 many asci, enclosing eight spores each. In the spring the 

 ascospores are liberated, and in turn infect the host. 

 Other conidial forms of reproduction are known. 



Preventive Means. — Dr. Halsted says : 'The remedies 

 to be proposed are few in number, and easily applied. The 

 old one of removing all the knots with a knife and burning 

 them is highly recommended. However, when a tree is 

 thoroughly infested, it is not easy to cut far enough below 

 the excrescence to remove all the fungus. The writer has 

 frequently seen vigorous trees, highly prized by the owner, 

 so severely pruned as to leave only a few stubs in place of 

 the branches, and upon the tips of each the next season 

 would grow knots of considerable size. In all such cases 

 there seems only one thing to be done, and that is to 

 remove the whole tree and cast it into the consuming 

 burn-heap — the sooner the better.' 



All cultivated varieties are susceptible to the disease, 



