CIRCULAR 3 7 6, U. S. DEPARTMENT OF AGRICULTURE 



significant conclusions : " The two regions where the formation of 

 calhis in root grafts was most abundant * * * coincide exactly 

 with the position of more than 90 per cent of the galls, * * * 

 indicating a close relation between the formation of callus and the 

 development of the disease." All of the writers' observations and 

 experiments furnish additional evidence to support these conclusions 

 of Hedgcock. 



It should also be noted that the failure to find a parasitic organism 

 or agency in diseased tissue at any given time does not necessarily 

 justify the conclusion that the organism or agency is not present or 

 that it has not been present at some previous time. Until conclusive 

 evidence is brought forth proving that malformations " supposedly 



crown gall " are not due 

 to the strongly pathogenic 

 organism Bacterhim tume- 

 faclens Sm. and Tn., the 

 present rigid inspection and 

 rejection of crown-gall trees 

 should be maintained. It is 

 considered that this partic- 

 ular question is of vital im- 

 portance to the orchardist. 



RECOMMENDATIONS FOR 

 CONTROL 



Seedling apple trees of 

 1-year's growth ordinarily 

 show a certain amount of 

 hairy root and crown gall. 

 These seedling trees with 

 either type of the disease 

 when lined out for budding 

 or when used for grafting 

 continue to carry the dis- 

 ease, in the great majority 

 of cases at least. The expe- 

 rience of the writers is that 

 the apple-seedling growlers 

 inspect the seedling stocks 

 rather critically. Only an 



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Fig. 6. — Lot of untreated Summer Rambo trees 

 grown as checks foa' those shown in Figui'e 5. 

 Reading from left to riglit : Galled trees (29), 

 doubtful (14), clean (57) 



occasional gall and a few hairy-root trees may escape their scrutiny. 

 However, when these diseased stocks are planted out or cut up into 

 pieces for piece-root grafting, the nurseryman is simply propagating 

 the disease. The best practice, therefore, is to inspect critically the 

 seedling stocks before using them for grafting and to burn all the 

 obviously infected ones. It has been generally accepted by both 

 pathologists and nurserymen, at least since the work of Hedgcock,. 

 that the selection of roots free from crown-gall and hairy-root infec- 

 tion is an important factor in the control of this disease. 



Since this disease is caused by a germ easily killed by germicides 

 when it can be reached and since this germ may be present over the 

 surface of the roots, the problem of killing the organisms present 

 on the surface and of disinfecting the material resolves itself into 



