190 MASS. EXPERIMENT STATION BULLETIN 170. 



mining the health condition of a tree it is important that all the factors 

 in any way concerning it should be well understood; in other words, one 

 should be able to judge of the degree of vigor possessed by the tree. A 

 tree in a vigorous condition has a quite different appearance from one 

 that is less thrifty. In the former case the bark has a certain color and 

 other characteristics by which it is easily distinguished from those in a 

 less healthy tree. This is also true of the branches, twigs and leaves as 

 well as of the general habit of growth. 



Finally, in all tree work it is essential that as thorough knowledge as 

 possible should be secured of the structure and function of the tree, its 

 normal and abnormal characteristics, and the causes responsible for 

 health and disease. As a rule, tree workers have little idea of tree structure 

 and function; consequently their diagnoses are seldom correct. 



Fungous Diseases of Trees. 



There are troubles of a serious nature affecting trees which are not 

 associated with organisms; but by far the most numerous and trouble- 

 some diseases are caused by fungi, and occasionally by other types of 

 organisms. The fungi responsible for decayed cavities do the most 

 damage to trees. 



There are a great number of leaf spots — Septoria, Cercospora, Phyl- 

 lostida and other genera — which affect both our native and introduced 

 trees and shrubs, and mildews are found on almost every tree and shrub. 

 Much careful investigation has been given to the control of plant diseases 

 in general, and valuable results have been ol^tained from spraying and 

 other methods of treatment. (See Treatment.) The fungous diseases of 

 our agricultural crops have been thoroughly studied, and most of them 

 are of enough importance to warrant systematic treatment every year; 

 but a large number of the leaf spots affecting shade trees are not common 

 enough to do any particular harm, and at least during the past many of 

 them have not been considered worth serious study from the viewpoint 

 of treatment. 



Most of the fungi affecting leaves and branches are parasitic; a few 

 are saprophytic, i.e., attacking only dead tissue; while still other forms 

 flourish either as parasites or saprophytes. The root-like mycelia of 

 parasites in most cases penetrate the cells and rob them of nutriment. 

 Often fungi cause distortion of the tissues so that galls and other abnormal 

 growths are formed. They also have acquired the peculiar habit of 

 secreting ferments that dissolve the cell walls. All fungi are capable of 

 producing some injury, but economically considered, treatment is neces- 

 sary only when the injury greatly retards the growth of the tree or serioush^ 

 impairs its appearance. 



Among some of the commoner forms of fungi that affect shade trees 

 may be mentioned the following: — 



