Fungi and Wood 



strength before it is appreciably sof- 

 tened or reduced in weight. In wood 

 to be used for paper, decay fungi re- 

 duce the quantity and quality of pulp, 

 depending on the fungus involved and 

 the process to be used in pulping. 



Plywood is generally quite as suscep- 

 tible to decay as solid wood of the 

 species from which it is made, although 

 some glues hinder the passing of fungi 

 from one layer of wood to another. 

 Wood or paper impregnated with a 

 high content of phenolic resin (as im- 

 preg, compreg, or papreg) strongly re- 

 sists decay, but impregnation with urea 

 resins has given less consistent protec- 

 tion in the tests so far made. 



MOST WOOD DECAY FUNGI Will grow 



rapidly only between 60 and 90 F. 

 They remain alive during long periods 

 below freezing, but can be quickly 

 killed by heat at temperatures about 

 150. 



The food requirements of the fungi 

 limit many of them. The fungi that 

 merely discolor are generally unable to 

 attack heartwood of any species of tree 

 because of its lack of the sugars or 

 other readily digested food materials 

 that they require. Some decay fungi 

 can attack the wood of the broadleaved 

 species only; others are limited to soft- 

 woods; some are even limited to a 

 particular genus of trees. 



Moisture is the factor most impor- 

 tant from a practical standpoint. Fungi 

 cannot grow in constantly air-dry 

 wood, even in the more humid parts 

 of the United States. Strictly speaking, 

 there is no such thing as dry rot. Wood 

 must contain moisture equaling more 

 than one-fifth of the weight of the 

 oven-dry wood before decay or stain- 

 ing fungi can develop in it. Decay fungi 

 progress rarely or slowly, if ever, at 

 moisture contents below 25 percent 

 (oven-dry basis). The molds that 

 grow on the sugars and other foods 

 present in sapwood or destroy the 

 starch or protein glues used in bonding 

 some wood or wood-fiber products, 

 however, can apparently work under 

 conditions somewhat less moist than 



would be required for decay of wood. 



Two of the decay fungi are espe- 

 cially dangerous to buildings because 

 they can conduct water from moist soil 

 or wood and thus attack wood parts of 

 buildings that otherwise would be too 

 dry but they depend just as much 

 on a source of moisture as other fungi. 

 Fortunately these two species are not 

 common in the United States. 



The oxygen requirement becomes a 

 limiting factor for the fungi in some 

 situations. Wood that is completely 

 waterlogged decays slightly, if at all. 

 No important decay occurs in wood 

 that is under water. 



The heartwood of naturally durable 

 species contains chemicals that limit 

 the growth of organisms. These are 

 nearly insoluble in cold water, but most 

 of them can be extracted in hot water. 

 Such woods as redwood, baldcypress, 

 black locust, pitch-soaked pine, and 

 several of the cedars commonly remain 

 free from attack for decades of ex- 

 posure to the conditions that favor 

 decay. Unfortunately, the second- 

 growth stands on which we now de- 

 pend largely for lumber contain a 

 larger proportion of sapwood, all of 

 which is decay-susceptible. Building 

 practices that were reasonably safe 

 with the lumber of the past century 

 may not be good enough with the lum- 

 ber we have now. 



The salt in ocean water also appears 

 to have some importance in hindering 

 the decay in the hulls of boats. Am- 

 monium salts in the amounts used in 

 wood as fire retardants have prevented 

 the decay of wood in laboratory trials, 

 although they favor the growth of some 

 of the relatively harmless mold fungi. 



FOR LUMBER ALREADY DISCOLORED as 



it comes from the tree, all that can be 

 done at present is to distinguish colors 

 that indicate decay from those that do 

 not, in order to avoid discarding harm- 

 less discolorations. 



To avoid discoloration from fungi 

 that develop in logs, the best measure is 

 to get the logs to the saw promptly. 

 Where this cannot be done, fungi can 



