342 Journal of Agricultural Research voi. xm, no. 6 



of cotton produced by the pistillate trees of the valley cottonwood, 

 only the staminate or non-cotton-bearing trees should be planted. 



The Lombardy poplar is also resistant to this disease, but not as 

 much so as the valley cottonwood. 



Cyiospora chrysosperma is not an active vigorous parasite on the well- 

 cared -for, more resistant species of poplar. It therefore follows that in 

 controlling this disease the trees should be given plenty of water, and 

 care should be taken not to injure the bark of the tree in any way; 

 particularly should the cutting off of large branches be avoided. In 

 general, the most resistant species for planting should be selected, 

 the trees should be given a sufficient amount of water, and should be 

 protected against injury from lawn mowers, horses, etc. Trees thus 

 selected and taken care of will be practically immune to the disease. 



Since this disease is known to be a serious parasite in nurseries and 

 propagating beds and to be distributed by means of such diseased stock, 

 it is of the utmost importance that all nurseries which supply poplar 

 stock to the dry regions of the western United States should be inspected 

 for this and other serious diseases of poplars like Dothichiza populea} 

 All suspicious as well as plainly diseased stock should be destroyed by 

 burning. General precautions of this nature, if taken, will do much to 

 control the introduction of this disease into new territories. 



If a tree already has this disease, it may often be saved by cutting off 

 the infected branches at least 12 inches below where any signs of the 

 disease can be detected. The ends of the branch should be painted with 

 creosote or coal tar. If the tree is small and has a large canker on the 

 main stem involving more than one-third of the circumference of the 

 tree at that point, it would be best either to plant another tree or to cut 

 the tree back to the ground and let one of the most vigorous suckers or 

 sprouts grow. It would be impossible to cut out all of the diseased 

 tissue on the trunk of a small tree without practically girdling it. 



If the tree is 12 inches or more in diameter where infected, and the 

 lesion is small, the disease may possibly be eradicated by cutting out all 

 of the diseased bark and the discolored sapwood. A layer of sound bark 

 2 inches wide should then be cut from around the diseased area with a 

 thoroughly sterilized knife. AH of the surface exposed by the pruning 

 operation should be painted with a strong solution of shellac or coal tar 

 to prevent reinfection. 



SUMMARY 



(i) A serious canker of poplars and willows is prevalent throughout 

 the semiarid regions of the southwestern United States. 

 (2) This disease is caused by Cytospora chrysosperma. 



' Hedocock, G. G., and Hunt, N. R. dothichiza populea in the United States. In Mycologia, 

 V. 8, no. 6, 300-308. pi. 194-195, 1916. Literature cited, p. 308. 



