470 University of California Puhlications in Botany [Vol. 7 



6. METHODS OF INFECTION UNDER NATURAL CONDITIONS 



There have been many expressions of opinion in published papers 

 concerning the manner in which living trees become infected with tlie 

 mycelium of Schizophyllum. Infection takes place in three more or 

 less distinct ways: (1) by entering through surfaces exposed by 

 mechanical injury, (2) by attacking parts of trees weakened through 

 certain physiological causes, and (3) by gaining admission through 

 tissues first injured or killed by other organisms. 



As regards mechanical injury, in Stevens and Hall (1910) it is 

 stated that "apparently this disease starts in roots injured by tools 

 during cultivation." Professor Home of the University of California 

 has observed an infection beneath an apple tree graft (fig. 1, pi. 58) 

 that had not been properly sealed with wax. An infection at a crotch 

 split in a peach tree (fig. 2, pi. 58) was reported by A. F. Camp, a 

 student in the University of California. Any woody part exposed 

 by injury forms a possible place of entrance for the fungus. 



"Weakening of the trees by excess water, or lack of proper drain- 

 age, was decided by Guegen (1901) to be a contributory cause of 

 infection in horse chestnut trees. Stone (1910) found that sun scald 

 and scorch of maple trees was followed by Schizophyllum and other 

 fungi. The writer has seen the sunburned parts of California Bay 

 trees covered with sporophores. It is evident that trees weakened by 

 certain physiological agents fall prey to this fungus, and probably 

 any loss of vitality on the part of the tree makes it susceptible to the 

 attacks of Schizophyllum. 



Wilson (1912) found that the sporophores issued through the 

 burrows made by the shot hole borer {Xylehorus dispar Fabricus). 

 Griffin (1911) stated that cherry trees weakened by bacterial gum- 

 mosis are frequently attacked and killed by Schizophyllum commune. 



No evidence has been brought forward in available literature to 

 show that Schizophyllum is able to infect healthy trees, or those not 

 injured or weakened in some way. It seems likely that infection can 

 be prevented by using care in cultivation, by painting wounds made 

 in pruning, by preventing crotch splitting, by protecting the trees from 

 sunburning, and by keeping them free from other diseases. 



