COMMUNICABILITY OF THE DISEASE. 45 
with galls. I have failed to get a catch; but I have no trouble in making the inocu- 
lation on our common seedlings. They usually take hold on the seedlings at the graft 
union. F ; 
This would seem to indicate immunity of the Virginia crab. 
Howard (24) made a number of experiments to test the contagious 
nature of different galls, using apple grafts, and found that apple 
trees inoculated with apple galls were affected to the extent of 30 
per cent; apple trees inoculated with raspberry galls, 162 per cent; 
and apple trees inoculated with peach galls, 22 per cent. He, however, 
gives no comparative results with apple trees which had not been 
inoculated; hence, it is not certain that his results are due to inocu- 
lation. 
Alwood (2) states: 
Apparently crown-gall can be readily inoculated from a diseased plant into healthy 
ones; hence, diseased plants should not be allowed to remain among healthy ones in 
the orchard. 
In another publication Alwood says: 
Cultivating the orchard may possibly serve to spread the disease by carrying disease 
germs from one tree to another. It is very probable that infection occurs without 
assistance wherever parts of the diseased tissues remain in the ground occupied by 
apple trees. 
Alwood bases his conclusions in the first instance upon experiments 
containing a small number of trees. 
Norton (37) regards crown-gall as a communicable disease on the 
apple, and says that ‘‘in grafting, the disease may be communicated 
from diseased to healthy trees by means of knives, etc.” He does 
not, however, give experiments or cite authority for the statement. 
Butz (7) states that healthy trees set in infected land develop the 
disease within a year or more. 
The author (19) stated in a bulletin in 1905, concerning the hard 
form of crown-gall-on apple trees, that ‘‘the results of extensive inoc- 
ulations with this type have failed to prove that this disease is of a 
contagious nature.” And again, concerning the soft form of crown- 
gall: “Nor is there proof yet that they are of a contagious nature.” 
Similar statements were published in 1905 in Science (18) and in the 
National Nurseryman (18). These opinions were based upon a 
failure to secure positive results from inoculations of apple trees 
with pieces of hard galls in earlier experiments, disease occurring 
both in control and in inoculated plants. 
Later experiments by the author (22) in 1906 and 1907 and pub- 
lished in 1908 established the fact that crown-gall of the soft form 
on apple trees was contagious and that it was identical with crown- 
gall of the almond, apricot, blackberry, cherry, peach, plum, prune, 
chestnut, and walnut. These experiments also indicated that the 
disease in the hard form was either slightly or not at all contagious. 
186 
