18 DISEASES OF DECIDUOUS FOREST TREES. 



ticularly true of Usnea barbata (L.) Fr. and Ramalina reticulata 

 (Noehd.) Krempelh. (68). Trelease (100) reports similar effects due 

 to tree mosses. Dense masses of lichens growing on the bark of trees 

 are described as injurious by Waugh (105, 106), Waite (104), and 

 others (44). While these epiphytes may be directly responsible for 

 injuries, and even death, it should be stated that the evidence in favor 

 of such an assumption is not very conclusive. It may be that these 

 plants grow on trees already weakened by other factors. No definite 

 proof has yet been brought forward which would indicate that leaves 

 and branches are killed because of the growth of these plants on the 

 branches. 



DISEASES CAUSED BY MISCELLANEOUS FUNGI. 



As indicated in the introduction, the fungi which cause disease may 

 be roughly classified into two groups : Those which grow on the living 

 parts of trees and those which grow on the dead parts. Of the forms 

 growing on living parts, one may distinguish in a general way between 

 such as attack growing leaves and those which attack the living 

 branches or wood. Leaf fungi usually attack leaves in spots and 

 produce local disturbances evident as discolored, shriveled spots, 

 which in time dry and break away entirely, leaving holes. In other 

 cases they may bring about malformations of the leaves, causing them 

 to become swollen or much curled and twisted. In many instances 

 the leaves attacked by various species of fungi are killed and prema- 

 turely shed. While it is not practicable to include a list of all the 

 forms of fungi which cause leaf diseases of deciduous trees, some of 

 the more important may be mentioned. 



Among those fungi which attack the surface of the leaf are the 

 various mildews caused by the fungi of the family Erysiphese. These 

 fungi are found on many species of deciduous forest trees and are 

 most common on the red oak (Quercus rubra L.), elm ( Ulmus ameri- 

 cana L.), silver maple (Acer saccharinum L.), sycamore (Platanus 

 occidentalis L.), and willow (Salix sp.). They usually appear during 

 the latter part of the summer, and because of their late appearance 

 do slight harm, except where they attack young forest trees and 

 nursery stock. The small, round, black fruiting bodies are dotted 

 here and there over the diseased surface of the leaf (102). 



Finely powdered sulphur dusted lightly over the leaves and young 

 twigs will hold these diseases in check. This is very useful where 

 small trees or nursery stock are to be treated. The standard Bor- 

 deaux mixture as well as any other of the efficient fungicides will 

 control this trouble, but sulphur is preferable unless treatment is 

 wished for other fungous troubles also. 



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