198 TREE WOUNDS AND DISEASES 



Si/ver fir canker does much damage to the 

 stem of the tree by forming excrescences which 

 swell and burst the bark, thereby inducing 

 timber wounds. On damp soils the disease is 

 most prevalent, though occasionally on high- 

 lying, sandy land it is both rapid and deadly. 

 Cutting and burning affected trees is the only 

 sure remedy. 



Spruce Fir. Apt to get uprooted when 

 growing on shallow soil, and to die out on that 

 of a dry, gravelly nature. Subject to the 

 attacks of both insects and fungi, the depreda- 

 tions of the well-known spruce gall aphis 

 extending all over the country. The timber 

 on certain soils gets affected by rot. 



Sycamore in a young state is a healthy tree 

 and liable to few diseases ; but as it advances in 

 age hollow trunk is frequently to be met with. 

 Around London especially, the foliage of the 

 sycamore is liable to be damaged by a fungus 

 disease due to Rhytisma acerinum^ in which the 

 leaves are covered with irregular-shaped black 

 spots. These, however, do not appear to 

 affect the health of the tree, but, as they spread 

 rapidly and are by no means ornamental, 



