460 
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 Root 
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- 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
go sets free the disease-producing bac- 
teria contained in them, and as & result 
the soil becomes infected. It ig 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. G G 
Hedgcock’s recommendations in the Bureau of 
Plant Industry, Bulletin 186, U. S. Department 
of Agriculture, Washington, D. C. 
