DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 529 



The water-soaked heartwood becomes tawny in color when dry. 

 Light-colored, isolated areas now appear in the discolored wood and 

 these areas originate the pockets. The rot spreads in all directions into 

 the surrounding tissue, but more rapidly in the summer wood of the 

 annual ring of the preceding year, so that the bulk of the pocket lies 

 in the summer wood of one year and the spring wood of the succeeding 

 year. Delignification now follows in which delignified wood fibers 

 appear in patches in the light-colored areas, and this delignification 

 spreads rapidly until white, oval to circular pockets are formed. 

 These lens-shaped pockets are at first filled with white cellulose, which 

 is later absorbed, leaving cavities. The diseased area increases in size 

 until the pockets reach a large medullary ray, which seems to check the 

 activity of the enzyme, so that the larger medullary rays become the 

 radial walls of the pockets. All the cellulose finally disappears, leaving 

 the pockets either (1) empty, (2) containing the shrunken white 

 membranes of the included vessels, or (3) more or less filled with myce- 

 lium. The last stage of the rot is characterized by the very light and 

 honeycombed nature of the wood. The pockets are longer than 

 they are broad, and all of the wood has disappeared, except the thin 

 walls around the pockets, which remain distinct and usually involve the 

 heartwood uniformly. The rotted wood is, therefore, in the shape of a 

 cylinder and there is a brownish discoloration of the heartwood on the 

 outer edges of the affected area. 



The growth of the mycelium seems to be preceded by the enzymes 

 which cause the disintegration of the wood. A few of the larger vessels 

 show hyphal threads and these become more numerous, as delignifi- 

 cation advances, until they become stuffed with small, intricately 

 branched, colorless hyphae. When the hyphce are exposed to the air, 

 they become brown in color. The sporophores are found on dead 

 trees, or the dead areas of living trees. The sporophores are thin 

 shelving bodies formed in the cracks of the bark, sometimes assuming 

 a conchate shape. They sometimes form in parallel lines, and range up 

 to 5 cm. in width. These sporophores may be formed on the dead tree 

 for a number of years. This fungus is widely distributed in the southern 

 states and ranges as far north as Ohio. The only method of control 

 is to prevent the infection of trees by eliminating forest fires, by pre- 

 venting the formation of the sporophores, and the destruction of all 

 diseased timber which has the rot. 



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