THE MONTHLY BULLETIN. 207 



Galls are not so often found on tlio black root, Juglans califoDiica, 

 wiiieli is now used as a rootstoek for tlie grafted varietias of English 

 walnut. The disease on the walnut first causes the formation of a gall, 

 which later decays and leaves the tree with the appearance of being 

 eaten off at or below the surface of the ground. Black walnuts and 

 English walnuts have been artificially inoculated. 



The pecan has been found having the crown gall and has been arti- 

 ficially inoculated from pure cultures. Just how serious the disease 

 may be in the pecan is not known as the tree is not connnercially grown 

 in California. 



Quinces as grown in California have an aerial form of the gall, 

 called black knot. This has been proven by Dr. Erwin F. Smith to be 

 caused by a bacterial organism that is probably identical with that of 

 crown gall as found on other trees. The disease, in most cases, does 

 not seriously affect tlie tree, although hardly a quince tree in California 

 is free from it. 



CONTROL. 



The control of crown gall will be briefly considered under the fol- 

 lowing: (1) Resistant stock as shown by artificial inoculation experi- 

 ments on different species and varieties of Pninus; (2) Careful inspec- 

 tion of the tree before planting, followed up for a time by yearly 

 inspection; (3) Sterilization of roots of small trees l)efore planting 

 with a germicide; (4) Tree surgery. 



Stock Resistance. 



Grow, if possible, stock having some natural resistance to crown gall. 

 Grape stocks are often diseased. The European varieties are more 

 susceptible than the American species. It has been shown by Dr. Geo. 

 G. Hedgcock^ that the Rupestris St. George variety shows resistance to 

 crown gall as well as resistance to phylloxera. He also shows certain 

 varieties of apples, such as Northern Spy, Ben Davis and Yellow Bell- 

 flower, to be strongly resistant to crown gall and hairy root disease. 



Much attention has been given to the study of resistance in stocks that 

 should be adapted to the various stone fruits. The plan has been to 

 actuall}^ inoculate the various sorts of stock under experimentation with 

 pure culture of the crown gall organism. At the end of the growing 

 season a record was made of the percentage of gall that developed. 

 While one season was usually sufficient, it was occasionally necessary to 

 make another observation the following year. 



The following paragraphs summarize this work and show the per- 

 centage of successful infections on several of the different species and 

 varieties studied : 



Degree of Resistance. 



Apricots: Prunus Armenica (Mikado), 27 per cent; P. dasycarpa 

 (purple), 40 per cent; P. Mandschurica (Chinese), 42 per cent; 

 P. Mume (Japanese), 91 per cent; P. Armenica (Royal), 07 per cent; 

 miscellaneous (California varieties), 60 to 90 per cent. 



'Field .studies of the crown gall and hairy root of the apple, Bulletin 186, Bureau 

 of Plant Industry: Field Studies of the crown sail of the grape, Bulletin 1S3, Bureau 

 of Plant Indu.stry, United States Department of Agricultuie. 



