

DISEASE OF BLACK OAKS. 



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the tree is so weakened that it breaks over or dies outright. 

 All gradations in the severity of the disease were found 

 between those where the rot extended only a very short 

 distance from the burrow itself (plate 18, figure l)and 

 those with the tree nearly or quite dead, and having practi- 

 cally all of the wood rotted (plate 19, figure 1). As the 

 disease progresses in the heart wood the sap wood becomes 

 affected, in that it constantly becomes thinner and thinner 

 until the death of the tree, which latter event is usually 

 caused by the sap wood becoming so thin that it cannot 

 any longer hold the weight of the upper parts of the tree, 

 or the sap wood becomes so affected that it can no longer 

 carry on its proper functions in supplying water to the 

 upper portions of the diseased tree. As above mentioned, 

 the rot in the early stages extended only a few inches each 

 way from the original infection, but in the late stages the 

 whole tree trunk may become affected for its whole length. 

 From what could be gathered by observation, it seemed 

 that the fungus works remarkably fast in causing the death 

 of the attacked trees. 



The heart wood of the black oaks is naturally rather 

 dark in color but when affected by Polyporus obtusus it 

 turns a light yellowish tint and in the last stages becomes 

 almost white. The diseased wood retains its fibrous 

 appearance but breaks much easier than does the healthy 

 wood. It does not have the shrinkage cracks which are 

 so characteristic of some of the wood rots. Neither does 

 the affected tissue crumble between the fingers nor break 

 very easily into small flakes. Wherever there is room 

 enough for such formations to occur, strings and sheets of 

 mycelium are formed, which are nearly or quite white in 

 color. , In trees which are not yet badly affected the color 

 turns darker as we go from the rotted portion toward the 

 bark of the tree until we reach the unaffected sap wood. 

 In quite badly diseased trees there is a more or less distinct 



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