May 6, 1918 Canker of Poplars and Willows 333 



slower growth. They usually originate from some wound in the trunk 

 or from a dead branch. The disease first forms a circular dead area on 

 the trunk around the base of the dead branch or around the wound 

 through which it entered. The tree then attempts to limit the disease 

 by developing a ring of callus around the infected area. The mycelium 

 of the fungus, however, gradually grows under this callus and kills a 

 new zone of tissue. This process is repeated year after year until there 

 is formed a large canker consisting of successive rings of dead tissue. 

 The old dead bark finally separates from the sapwood, leaving the dead 

 area more or less exposed. The surface of this dead sapwood shows the 

 concentric rings of dead callus which formed annually in the attempt to 

 check the disease. No fruiting bodies of any kind have been found asso- 

 ciated with these lesions, and it is very doubtful if this peculiar type of 

 canker is caused by the disease discussed in this article. 



Willows have also been found attacked by this canker disease, espe- 

 cially the weeping willow (Salix hahylonica) , which is often planted as 

 an ornamental and shade tree. The willows are killed in much the same 

 manner as the poplars 



THE FUNGUS 



This canker of poplars and willows is caused by a definite species of 

 fungus which grows as a parasite in the bark and to a limited extent 

 in the sapwood of infected trees. The causative organism is Cytospora 

 chrysosperma (Pers.) Fr., since inoculations of this fungus made into 

 healthy poplars have produced the typical cankers, and pure cultures of 

 this organism have been reisolated from the cankers thus produced. 



After the mycelium of the fungus has been growing for several weeks 

 in the bark, it forms fruiting pustules or pycnidia. These pycnidia pro- 

 duce a large number of curved, hyalin, i -celled spores which are extruded 

 from the pycnidium in the form of threadlike irregular coils. These are 

 called "spore horns" or "tendrils" (PI. 27, D). The development of the 

 spore horns is not limited to any special season, but may occur during 

 any month of the year. The production of pycnidia seems to be limited 

 to the bark, at least in the semiarid regions of the western United States, 

 since no evidence of fruiting bodies of the fungus has been found on 

 decorticated limbs, trunks, or stumps of affected trees. 



The pycnidia are not evident on the surface of the bark until after the 

 spore horns have been dissolved and washed away by rains; then the 

 dead bark shrinks and the mouths of the pycnidia become evident (Pi. 

 28, A). Only the pycnidial stage of this fungus is known. 



INVESTIGATIONS AS TO THE CAUSE OF THE CANKERS 



Pure cultures of Cytospora chrysosperma were made by means of single 

 spore colonies from spore horns obtained from Popidus wislizeni and 

 P. alha. Inoculations were made on small bushes of P. alba with material 



