224 



MANUAL OF TREE DISEASES 



maple are rarely if ever affected, even when growing among 

 red and silver maples which are severely infected. This sub- 

 stantiates the results of investigations in Europe which show 

 that there are various strains of the tar-spot fimgus. These 

 strains show preference for certain species of maples, but in 

 other respects are indistinguishable from one another. 



Symptoms. 



The first evidence of the developing spots on the maple leaves 

 are light green or yellowish areas. In the summer the spots 

 become black on the upper surface of the leaf due to the 

 black mycelium of the fungus. The surface of the tar-like 



spots is wrinkled 

 with anastomos- 

 ing furrows and 

 somewhat raised 

 above the surface 

 of the leaf. The 

 spots are usually 

 a quarter or half 

 inch across (Fig. 

 36). 



Cause. 



The tar leaf- 

 spot of maples is 

 caused by the 

 fungus Rhytisma 

 acerinum. Spores are borne free on the upper surface of the 

 black mass of fungous tissue as it develops on the leaf. It is 

 not known whether these spores infect other leaves or not. 

 But from the number of leaves affected on some trees, it 

 seems certain that these spores must account for the general 

 prevalence of the infections. When the spotted leaves fall 



Fig. 36. — Tar leaf-spot of maple. 



