DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 523 



The mycelium develops upright vertical conidiophores which abstrict 

 off conidiospores in chains. These conidiospores no doubt account 

 for the rapid spread of the disease, which is never very serious to the 

 Hlac shrubs, but no doubt to some extent interferes with the normal 

 physiologic processes of the leaves. Subsequently perithecia are 

 formed which are spheric in shape, almost jet black in color, and which 

 are surrounded by a circlet of hyphae known as appendages, which are 

 curved or dichotomously hooked at the extremities. Each perithecium 

 produces 3 to 8 asci, and each ascus contains 4 to 8 relatively small 

 ascospores, which measure 18 to 23/x by 10 to 12/x (Fig. 54). 



Maple (Acer spp.) 



Decay {Fomes fomentarius (L. Fr.) (Fig. 188). — The sporophores 

 of this fungus are hoof-shaped and appear first as small rounded knobs 

 on the surface of the trunk, or at branch stubs. The upper surface is 

 smooth and more or less definitely marked by concentric ridges. The 

 older fruit bodies owing to the action of the weather are uniformly 

 gray and appear as if powdered. The lower surface is reddish-brown 

 in color and shows numerous, small round pores. The margin of the 

 new layer is grayish white and very soft and velvety. The sporo- 

 phores are found usually singly, althoughby proximity of two, or several, 

 they may appear grouped together. The decay produced in the wood 

 of deciduous trees by Fomes fomentarius begins in the outer alburnum 

 immediately beneath the barky layers, and extends inwardly, until 

 it reaches the pith of the tree. The rotten wood is distinguished by 

 a large number of irregular black fines outlining areas of sound wood. 

 Wholly decayed wood is extremely soft and spongy, fight yellow and 

 crumbles into numerous separate wood fibers when rubbed. The 

 tinder fungus, Fomes fomentarius, is found in the deciduous forests of 

 Michigan, Minnesota, New England, New York, Wisconsin and in 

 other states. It grows rapidly in dead wood and the mycefium will 

 form large masses if the infected timber is kept under moist conditions. 



Leaf-blotch (Rhytisma acerinum (Pers.), Fr.). — The tar spot of the 

 maple is found about Philadelphia usually on the silver maple to which 

 it does slight injury. The black irregular spots are, however, alwavs of 

 interest to the laymen and questions are asked frequently about their 

 cause. The spot begins, as a yellow thickened area, when the maple 



