B. P. I.— 203. 



II-THE WRAPPING OF APPLE GRAFTS AND FFS 

 RELATION TO THE CROWN-GALL DISEASE. 



By Heemann yon Schrenk, Special Agent in Charge of the Mississippi Valley Laboratory, 

 and Geoege G. Hedgcock, Assistant in Pathology. 



INTRODUCTION. 



The crown-gall disease of apple trees, as indicated in a recent pub- 

 lication of the Bureau of Plant Industry/' appears in three distinct 

 tj^pes — the hard crown-gall, the soft crown-gall, and the hairy-root 

 forms. While the hairy-root disease of apples has usually been con- 

 sidered a type of the crown-gall disease, it in reality is an entirely 

 different trouble, with manifestations which are not in the least like 

 those met with in the true crown-gall disease. The soft type of crown- 

 gall on the apple has not A^et been clearly differentiated from the hard 

 type, and in the following discussion the two are considered as one. 



The knots which characterize the crown-gall disease of the apple 

 usually appear at some point, either on the root piece or on the scion, 

 where the two are united; that is, the gall may form at the end of the 

 tongue of either the scion or root piece, or at an}^ point where either 

 piece has been wounded. This fact is one commonl}^ recognized by 

 all nurser^^men and scientific workers who have studied this disease. 

 About 90 per cent of these knots will appear on the end of the scion 

 piece. The exact cause of the formation of the gall is as yet somewhat 

 uncertain. It would seem, however, that, whatever the cause, the 

 point most exposed to the disturbing factor, whether it be due to a 

 fungus, to bacteria, or to soil or atmospheric conditions, is the junction 

 of the scion and root piece. 



When the newly made grafts are laid awa}^ in the grafting cellar 

 either in sawdust, excelsior, moss, or other bedding material, callous 

 tissue begins to form on the cut surfaces of both scion and root piece. 

 This callous tissue from the two pieces will in time fill the intervening 

 spaces between the surfaces of the scion and the root piece, and ulti- 

 mately the root callus and the scion callus will join. Where the root 

 piece and the scion are of exactly the same size and where they are 

 united so as to fit exactly^ a very perfect union will take place. Where 



« Bulletin No. 90, Part II, Bureau of Plant Industry, U. S. Dept. of Agriculture, 

 '■' The Crown-Gall and Hairy-Root Diseases of the Apple Tree," 1905. 



100—11 13 



W 



