DISEASES CAUSED BY WOUND FUNGI. 



41 



SOFT ROT OF OAKS CAUSED BY POLYPOKUS OBTUSUS. 



A number of species of the black oaks, notably Qucrcus marilandica 

 Muench. and Q. velutina Lam., are affected with a disease of the heart- 

 wood which has been determined by Spaulding (94) to be due to Poly- 

 porus obtusus Berk. Diseased trees have been found in the eastern 

 part of the United States, and notably in the central Mississippi Val- 

 lev ; a large number of trees are usually found affected in a locality. 



The spores of the fungus germinate in the burrows of an oak-boring 

 insect {Prionoxystus robiniae Peck.). The fungus grows in the borings 

 and follows the in- 

 sect burrow until it 

 reaches the heart wood 

 of the tree; it then 

 spreads out from this 

 point, both up and 

 down the trunk, and 

 gradually brings 

 about a form of soft 

 rot. (Fig. 5.) The dis- 

 eased wood is lighter 

 in color than the 

 heart wood of the 

 healthy tree, and in 

 its lasl stages turns 

 almost white. "The 

 diseased wood retains 

 its-fibrous appearance, 

 but breaks much eas- 

 ier than does the 

 healthy wood. It 

 does not have the 

 shrinkage cracks 

 which are so charac- 

 teristic of some of the wood rots; neither does the affected tissue 

 crumble between the fingers nor break very easily into small flakes " 

 (94). The fungus grows in the trees rapidly, extending up and down 

 in the heart wood and growing outward until it reaches the sap wood. 

 Affected trees are weakened to such an extent within one or two years 

 after their first attack that the slightest windstorm causes the trunks 

 to break. 



The sporophore of Polyporus obtusus is a very characteristic one. 

 It usually appears at the original insect burrow, where it forms a 

 thick, more or less hoof shaped shelf; in some cases two or three 

 shelves may form, one immediately above the other. The sporo- 



149 



Fig. 5.— A living black oak tree with a sporophore of Polyporus 

 obtusus growing out of the opening of an insect burrow. 



