28 MANUAL OF TREE DISEASES 



result, the affected tissue falls out entirely and the effect is 

 known as shot-hole. When the spots become numerous the 

 leaves may be killed and the tree is more or less defoliated. 



Various types of fruiting-bodies, of the fungi causing leaf- 

 spots, are developed on the dead tissue. These structures 

 appear to the unaided eye as minute brown or black dots 

 scattered over the area or grouped in clusters near the center of 

 the spot. Spores are produced in or on these structures. 



Cause. 



Leaf-spots are caused by many species of fungi, usually of 

 the genera Phyllosticta, Septoria, Cylindrosporium, Cercospora, 

 Marssonia and Gloeosporium. These genera represent groups 

 in which the leaf-spot fungi are placed according to the micro- 

 scopic characters of the fruiting-bodies and the spores, which are 

 formed on the spots. The fruiting-bodies are simple incbsures 

 (pycnidia) or spore-bearing layers of mycelium (acervuli). 

 Because of the simpUcity and variability of the structures, 

 these fungi have been given numerous names. The confusi m 

 in classification has caused several names to be applied to the 

 same fungus on the same tree and on different kinds of trees. 

 In addition to the fruiting-structures mentioned above, the 

 leaf-spot fungi in many cases develop perithecia and ascospores 

 in the dead leaves on the ground during the autumn and winter. 



Following is a list of parasitic fungi which have been described 

 as causing leaf-spots. In some cases more recent investigations 

 have proved that several of the names are synonymous. These 

 are indicated when known : — 



Alder 



Cylindrosporium vermiforme Davis 

 Septoria alni Sacc. 

 Septoria alnicola Cooke 

 Septoria alnifolia B. and E. 

 Septoria maculans B. and C 



= Rhabdospora maculans (B. and C.) Saco. 



