CROWN-EOT OF FRUIT TREES: FIELD STUDIES.* 



J. G. GROSSENBACHER. 



SUMMARY. 



Field studies from 1909 to 1912 have shown that in winter the 

 bark on the trunks of fruit and other trees is often loosened or in- 

 jured, near or below the surface of the ground; and observations 

 made at intervals throughout the year show that the most severely 

 affected portions of bark die and decay during the subsequent vege- 

 tative season, thus giving rise to crown-rot. It was also found that 

 similar bark injuries, resulting in cankered areas or the death of 

 the affected bark, occur in the bark of trunks some distance above 

 ground — at crotches and on parts of branches which appear to be 

 most subject to bending or strain in time of strong winds. 



Neither the factors which induce the initial bark-injury nor those 

 causing the subsequent death and decay have been determined ex- 

 perimentally, but were selected from the environment of affected 

 trees in widely separated locaUties chiefly by elimination and deduc- 

 tion; however, the amount of accumulated circumstantial evidence 

 is so large that there seems to be no doubt about the main causes 

 involved and the time during which they are operative. From the 

 data at hand it appears that the factors which are impUcated in the 

 production of the initial injuries are: 



ist. An unusually large increase in the diameter of tree trunks 

 during a vegetative season, necessitating an enormous increase in 

 the area of the bark and resulting in high bark tension toward the 

 end of the growing period, or the premature cessation in the process 

 of differentiation of bark tissues owing to some adverse condition 

 of the environment ; 



2nd. Low temperature and the resulting contraction of the bark 

 with a consequent increase of bark tension ; and 



3rd. A wind-exposed location so that the bark at the crown or 

 other places of bending of tree trunks or branches is subjected to 

 great strain and consequent excessive evaporation during strong 

 winds. 



Such conditions result in loosening or injuring patches of bark, 

 especially on the windward side of trunks and branches of trees, 

 which enter the dormant season with incompleted bark-growth or 

 with high bark-tension in regions of bending and strain. 



The death of injured or loosened patches of bark seems to be due 

 to isolation and drying out, and it is often immediately followed by 

 further disorganization and decay caused by the entrance of bark 

 fungi like Sphaeropsis and Cytospora. The wood underlying 



* A reprint of Technical Bulletin No. 23, Sejjtember, 1912. 



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