OAK DISEASES 259 



large and become hollow cavities with white margins. These 

 pockets become so abundant that the summer-wood is largely 

 converted into strings of white fibers leaving the wood brittle 

 and -easily broken. Further details concerning this white 

 pocket- or piped-rot will be found under chestnut diseases, 

 page 150. 



Straw-Colored Butt-Rot 



Caused by Polyporus frondosus Fries 



This rot of the heartwood of the base of oak and probably 

 of chestnut is found in eastern and central United States but 

 does not seem tO occur very commonly or destructively. It 

 develops only in the base of the trunk. The wood is not en- 

 tirely destroyed and the trees do not become hollow. 



Symptoms. 



In longitudinal section the upper advancing margin of the 

 decay is indicated by long, slender, white lines extending for 

 several inches upward into the sound wood. In advance of 

 the white lines, the wood is water-soaked and reddish in color. 

 The rotted wood is at first white and later tan- or straw-colored. 

 Most of the tissue is delignified but is firmly held together by 

 the less affected medullary-rays. The cut ends of the trunks of 

 felled trees become reddish brown after a month or two. 



The sporophores of the causal fungus arise from exposed or 

 biu-ied roots near the base of the tree. They are composed of 

 a fleshy slem which is much branched, the ends of the 

 branches forming small flat over-lapping shelf -like structures. 

 The whole fruiting-body is more or less globose. The upper 

 surfaces of the shelves are gray or drab and the under surfaces 

 white. 



Cause. 



The straw-colored butt-rot of oak is caused by Polyporus 

 frondosiis. The spores from inside the tubes on the under sides 



