APPLE DISEASES 
ant on slender stems. The galls may 
also interfere with sapflow and the decay 
of soft galls usually involves adjacent 
healthy tissues producing serious wounds 
or in some cases girdling the stem with 
death of the plant as a result. One of the 
worst effects of the disease is the frequent 
prevention of normal root development 
resulting in the fa1lure of a young tree to 
establish itself or in the retarding of 
its growth. 
Cause 
Crown gall has been Known and recog: 
nized as a serious disease for many years. 
Until comparatively recently, however, 
the cause was unknown. The careful 
experiments begun in 1904 by Smuth, 
Brown and Townsend, of the United 
States Bureau of Plant Industry, have 
proved conclusively that the disease is 
caused by a bacterium, under the name 
Bacterium tumefacrens. 
The bacterium causing crown gall is 
an organism which can exist in the soil. 
It seems to be widely scattered in many 
soils, but appears especially abundant 
in nurseries and in land where plants af- 
fected with crown gall have previously 
been grown. Plants in a young, tender 
and rapidly growing condition, are most 
subject to infection, whereas older and 
more mature plants are not so frequently 
attacked. The wounds made in root- 
grafting and budding offer a favorable 
point of entrance for the germs, and in- 
juries to the underground parts of plants 
by cultivation, attacks of borers, etc, 
render them susceptible to infection. In 
fact, the organism may be considered a 
wound parasite. 
The Effect of the Organism 
In order to cause infection the bacteria 
must enter into some part of the plant 
where new cells are in the process of 
formation. They make their way into the 
living cells and stimulate them to abnor- 
mal and very rapid multiplication. Cells 
near those containing bacteria may per- 
haps be stimulated in the same way. The 
number of bacteria in any one cell is not 
large. They multiply slowly and do not 
appear to injure the cell to any great ex- 
tent, but merely to stimulate rapid di- 
459 
vision. When a cell containing bacteria 
divides into two, each of the daughter 
cells contains some of the bacteria and 
hence will be stimulated to rapid multi- 
plication. Thus, as a result of the pres- 
ence of the bacteria, there is formed an 
abnormal mass of rapidly growing tissue 
in which the elements become distorted 
and twisted. It is an unnatural, wasting 
growth, in no way adapted to the needs 
of the plant. Such galls or tumors vary 
greatly in form, size and consistency, de- 
pending apparently upon the virulence of 
the particular germ causing the infection, 
the kind of plant or variety which has 
been attacked, the particular tissues 
which were first infected, etc. Even in- 
dividual plants of the same variety show 
great differences in susceptibility to this 
disease. 
The Soft Gall 
One of the commonest forms of crown 
gall especially on herbaceous plants and 
cane fruits, is the fleshy form in which 
the outgrowth is somewhat soft. These 
grow rapidly and are not protected by a 
bark or corky layer. Growth usually 
commences in the spring (sometimes 
earlier). After a few months of develop- 
ment the outer layers of the gall begin 
to die and are attacked by various bac- 
teria and fungi. This generally ends in 
the decay and sloughing off of nearly 
the whole tumor by the end of the sea- 
son. The margin of the gall, however, 
usually remains alive and during the 
next season the tumors grow out again, 
followed as before by decay. When the 
gall dies some of the healthy substance 
of the plant is usually killed and a 
wound is formed which renders the en- 
trance of other diseases easy. Galls are 
known to die off completely in some cases 
at the end of the first or second season, 
while the plant recovers; but commonly 
the tumors reappear from year to year. 
The Hard Gall 
Not all galls are of this soft type, but 
many are very hard and woody, in which 
case the growth is apt to be slower and 
the gall persists longer—often for many 
years. The exterior may develop a bark- 
like covering and instead of a rapid decay 
