460 



ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 



at the end of the season, there is often 

 little or no disintegration and the gall in- 

 creases in size as new growth is added 

 each season. In time, however, the sur- 

 face of the hard gall usually suffers dis- 

 integration to some extent. Between the 

 softest and the hardest galls there are 

 many transitional forms and soft galls of- 

 ten develop a woody structure and become 

 persistent. 



Hairy Boot 

 In the apple, and in some other plants, 

 there is a very common trouble known as 

 hairy root. This is produced by the same 

 organism that causes other forms of crown 

 gall. In typical hairy root there is no 

 large gall formation, but there appear on 

 the main roots or on the crown clusters of 

 numerous, succulent, abnormally fleshy 

 rootlets which generally project nearly at 

 right angles. When dry these rootlets 

 shrink and become hairy in appearance. 

 Bacteria are not found in the abnormal 

 rootlets but in the flattened and often in- 

 conspicuous tumor from which they arise. 

 Hairy root is not found to be entirely dis- 

 tinct from other types of crown gall. The 

 tumors on certain plants frequently pro- 

 duce abnormal rootlets and the aerial 

 hard gall of the apple will often develop 

 roots when subjected to moist conditions. 

 Furthermore, the organism isolated from 

 hairy root is capable of inducing the for- 

 mation of typical galls as well as hairy 

 root when inoculated into different plants. 



Tumor Strands and Secondary Galls 



The interesting discovery has recently 

 been made that in certain plants strands 

 of tumor cells may push out from the 

 primary galls, and working through cer- 

 tain easily penetrated tissues, may reach 

 distant parts where by rapid multiplica- 

 tion of the tumor cells new galls are 

 formed which burst out from the parts 

 of the plant thus invaded. Much of the 

 severe breaking out of galls along the 

 canes of some of our small fruits and of 

 grapes may eventually prove to be second- 

 ary growths of this nature. 



Modes of Infection 



It seems probable that the disintegra- 

 tion which the crown galls usually under- 



go sets free the disease-producing bac- 

 teria contained in them, and as a result 

 the soil becomes infected. It is com- 

 mon experience that soil infection is 

 responsible for large nursery losses every 

 year. Nursery conditions are particular- 

 ly favorable to the transmission of the 

 disease. Trees are planted close together. 

 They are in the earliest stages of rapid 

 growth and hence in the most tender and 

 susceptible condition possible. The dis- 

 ease develops principally during the first 

 year or two and is said to gain entrance 

 almost entirely through wounds or as 

 a result of root-grafting or budding. 

 Budded trees are not so liable to infec- 

 tion as grafted trees, while root-grafts 

 made in a careful manner* result in less 

 gall formation than grafts carelessly 

 made. Ground infected by the presence 

 of diseased trees will prove capable of 

 transmitting the disease to young trees 

 set later in the same ground and cultiva- 

 tion, spreads the trouble along the rows, 

 while careless cultivation causes wounds 

 which afford opportunity for the entrance 

 of the bacteria. A large amount of infec- 

 tion is also possible where trees are heel- 

 ed into soil that has previously covered 

 galled nursery stock. 



Trees may also become infected from 

 the soil after being set out into the or- 

 chard. This frequently occurs with the 

 stone fruits but not very commonly in 

 the case of the apple. Most of the crown 

 gall in our orchards, however, is due to 

 the infection of the trees while in the 

 nursery. There seems to be little evi- 

 dence that infections spread from one 

 tree to another in most apple orchards. 

 Where berries, however, are set close to- 

 gether, serious spreading of the disease 

 from plant to plant, has sometimes been 

 recorded. 



Cross Infection 



While at present the limits of cross in-' 

 fection with different strains of the crown 

 gall organism are not definitely known, 

 yet we do know that crown gall bacteria 



* For suggestions regarding methods of graft- 

 ing and nursery practice refer to Dr. O. G- 

 lledgcnelt's recommendations in the Bureau of 

 Plant Industry, Bulletin l.SC, U. S. Department 

 of Agriculture, Washington, D. C. 



