BEST METHOD OF DEALING WITH THE DISEASE, 197 



By no amount of special pleading can it be made to appear that 

 an infectious disease should be tolerated on nursery trees offered 

 for sale simply because it is rather prevalent and is inconvenient 

 to deal with. Before the nurseryman can be allowed to sell such 

 trees without restriction he must establish conclusively that it is 

 not injurious, and not transmissible to susceptible species. 



We are disposed to include apple trees also in this recommenda- 

 tion. While these seem to be less subject to crown-gall in a serious 

 form than some other plants, frequently they do not make good 

 trees, and our cross-inoculations suggest, at least, that they may 

 serve to carry the disease to other plants and into localities previ- 

 ously free from it. Moreover, even when the apple gall does not 

 itself seriously injure the tree it may serve, as we have seen, for the 

 entrance of other parasites. 



In some cases the inspector will be in doubt whether to con- 

 demn stock or pass it, particularly when the trees have been care- 

 lessly grafted and show more than the ordinary amount of callus. 

 He may then either refer the specimens to some more experienced 

 pathologist or refuse to take chances. Until we know to the con- 

 trary excessive callus should be regarded as incipient gall. Ordi- 

 narily there will be no difficulty in determining whether or not a 

 given lot of trees has crown-gall or hairy-root, except when the 

 nursery stock has been dishonestly pruned before shipment to remove 

 signs of the disease, and then usually some traces will be left. In 

 case trees are improperly condemned there is always a remedy at law. 



SYNOPSIS OF CONCLUSIONS RESPECTING CROWN-GALL. 



(1) Crown-gall is a disease common in nurseries on the roots and 

 shoots of various plants, and likely to continue on the plants when 

 they are removed to orchards, vineyards, gardens, and hothouses. 

 It also occurs on various field crops. This name is used for the 

 disease whatever the situation of the galls on the plant. 



(2) When we began our studies the cause of crown-gall was 

 unknown, and by them it has been determined. 



(3) Bacteria were seen in crown-galls of the daisy in 1904, and the 

 studies then undertaken have been pursued continuously to date, 

 and are here first offered in complete form. 



(4) Tlie first successful isolations and infections were obtained 

 in 1906, and the biology of the bacterial organism derived from the 

 daisy has been determined more carefully than that from galls on 

 other hosts. 



(5) Hundreds of pure-culture inoculations on daisy have removed 

 the subject from the domain of speculation and shown that the galls 



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