374 Diseases of Economic Plants 



necessitating an annual replacement of some 500,000,000 

 in the United States, indicates the enormous money values 

 involved. 



Decay of timber is brought about by agencies similar, 

 sometimes identical, with those causing rot in living trees, 

 but cannot occur unless both moisture and oxygen are avail- 

 able. Decay can be prevented by impregnating the wood 

 with various antiseptics, as creosote or zinc chloride. *■*''' ^^^ 

 Merely removing the bark from posts increases their length 

 of utility. 



A few of the chief causes of the decay of dead wood are as 

 follows : 



Dry-rot (Merulius lacrymans (Jcq.) Fr.). — One of the 

 most common and destructive of all the rots of structural 

 timber, it renders the wood spongy and brownish. If very 

 moist, a profuse, superficial, pure white mycelium may 

 develop, at first loosely, later in dense sheets or strands. 

 The sporophores are flat, at first white, later red, and still 

 later yellow-brown. The spore-bearing surface carries 

 shallow pores penned between folds and wrinkles. 



Sap-rot ^^^ {Polyporus versicolor Fr.). — In addition to 

 its parasitic life upon the chestnut and catalpa, this fungus 

 grows as a saprophyte upon all kinds of deciduous woods, 

 and is regarded as the most serious of all wood-rotting 

 fungi which attack the dead wood of broad-leaf trees. It 

 destroys probably 75 per cent or more of the broad-leaf 

 species of timber used for tie purposes. Whenever such 

 species are used for fencing, for posts or poles, or for any pur- 

 pose where they come in contact with the soil, they are sure 

 to be attacked sooner or later by P. versicolor. P. pergame- 

 nus Fr is similar to P. versicolor, and also of great im- 

 portance, appearing upon cut timber or on large wounded 

 surfaces of standing trees, especially following fires. It 

 is found throughout the United States on all deciduous 

 woods. 



Numerous genera and species of fungi other than those 

 mentioned above can also bring about similar changes. 



