MISCELLANEOUS PARASITIC AND SAPROPHYTIC ORGANISMS. 19 



TAR-SPOT. 



The tar-spot disease of the maple, caused by Rhytisma acerinum 

 (P.) Fr. (34, 35, 102), shows as black, irregularly shaped spots on 

 the leaves of different species of maples in the latter part of the sum- 

 mer. These black, blister-like spots sometimes occur very thickly 

 scattered over the leaves, and in cases where the attack is severe 

 the foliage is shed prematurely and the trees weakened thereby. 

 The fungus causing the disease develops beneath the epidermis of 

 the leaf during the summer and forms a black mass of mycelium. 

 After the leaf falls to the ground the fungus continues its develop- 

 ment and the following spring produces immense numbers of spores, 

 which it is supposed are blown by the wind on to the newly formed 

 leaves of the second summer. 



A number of other species of Rhytisma infest other trees than the 

 maples. In some seasons considerable damage is done, especially 

 to nursery stock of various ages, in which case the appearance is 

 very badly marred, both by the premature falling of the leaves and 

 by the black spots upon them while still clinging to the tree. Methods 

 of prevention consist in carefully raking the leaves together and 

 burning them in the fall, thus preventing the fungus from attaining 

 maturity the succeeding spring. This treatment alone if carefully 

 done should prevent serious inroads from this disease. 



RUSTS. 



Deciduous forest trees are affected only to a limited degree by 

 rust fungi. Xow and then one finds a leaf-rust (Pucdnia fraxinata 

 (Lk.) Arthur) both on the white ash (Fraxinus a merit-ana L.) and 

 green ash (F. lanceolata Borkh.), the telial stage of which occurs on 

 Spartina cunosuroides (1). A similar disease is caused on the poplar, 

 willow, and birch by other closely related rusts, namely: Melamp- 

 sora populina (Jacq.) Wint. on poplar (PopuJus deltoid ts Marsh. 1 

 24), M. hetulina (Pers.) Wint. on birch, and M. saliciscaprae (Pers.) 

 Wint. on willow. These species of rust appear on the leaves in 

 early summer as very minute, bright yellow spots, which gradually 

 turn darker as the season advances, and in the autumn are almost 

 black. The dark-colored spores, which are formed last, are the 

 mature winter spores of the fungus which enable it to live over the 

 winter. The amount of damage done by these rusts is usually 

 insignificant on large trees, but in some seasons when the weather 

 conditions are favorable trees may be entirely defoliated. The 

 most extensive damage done by them is in young plantations, this 

 being especially true of the willow leaf-rust, which has occasioned 



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