194 SHADE-TREES IN TOWNS AND CITIES 



trees, however, will reveal the disease, and they can be cut 

 down before they become dangerous. If the trees are not 

 blown down they usually die suddenly in early summer, the 

 foliage wilting completely. 



CANKER 



Diseases of the bark of trees manifest themselves by the 

 appearance of small protuberances and other malformations 

 commonly known as canker. The disease is due to fungi, 

 one of the most destructive of which is Nectria cinncibarina 

 (Tode) Fr. Its spores obtain entrance into wounds caused 

 by hailstones, insects, or breakages, and the mycelium grows 

 through the partially weakened wood, and ultimately pro- 

 duces small red clusters of fruiting bodies. A small area of 

 the bark is killed and a healing callus begins to form at the 

 edge of the wound. The callus of the first year is then 

 invaded by the fungus and killed, and a second layer of callus 

 starts to develop. The continuation of this process extends 

 the growth of the canker over a large area. 



Canker can be treated by cutting away all diseased por- 

 tions of wood and bark, washing the exposed surface with a 

 solution of copper sulfate, one pound to five gallons of 

 water, and then painting the same with coal-tar. 



The chestnut bark disease or canker {Diaporthe parasitica 

 Murrill) is now the most serious of all the fungous diseases 

 of trees. The spores enter the bark through an abrasion or 

 some other wound. From the point of infection the fungus 

 grows, kills a small patch of bark, and then spreads all 

 around the limb or trunk and girdles it. The disease then 

 becomes conspicuous by the brown color of the dying foli- 

 age. The branches are usually attacked first, and from 

 these the canker spreads through the entire tree and kills it. 



