DISEASES CAUSED BY WOTTNI) FUNGI. 



11 



SOFT ROT OF OAKS CAUSED BY POLYPOR1 8 OBTUS1 S. 



A number of species of the black oaks, notably Qut reus mafUandica 

 Muench. smdQ. velutinaljSim., 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 centra] Mississippi Val- 

 ley; 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 robiniaeVeck.). The fungus grows in 1 he borings 

 and follows the in- 

 sect burrow until it 

 reaches the heartwood 

 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 

 heartwood of the 

 healthy tree, and in 

 its last 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 Fig. 5.— A living black oak i re- wiiii a sporophoreo 

 which are SO cliarac- obtusus growing out of the opening of an insect burrow. 



teristic of some of the wood rots; neither docs 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 heartwood and growing outward until it reaches the sapwood. 

 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 Poh/porus obtusus is a very characteristic one. 

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

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

 shelves may form, one immediately above the other. The spo 



149 



