— ee ee 
SIE a a - — —— —- $$ 
‘ 
COMMUNICABILITY OF THE DISEASE. 55 
increases considerably from the first to the second year, and but 
slightly the third year. | 
The disappearance of crown-gall is considered to be due to two 
causes: (1) Some of the hard galls throw out roots and develop 
into woolly-knot, while (2) a much larger number of galls gradually 
disappear as the tree increases in size, and such trees must recover 
from the disease. 
In closing the discussion of this subject it should be noted that the 
results given here are from seven parallel sets of experiments in six 
localities, or forty-two parallel experiments in five States, and include 
data on 31,886 trees. The results might not have been the same in all 
localities in the United States, but certainly fairly represent typical 
nursery conditions in the heart of the apple-growing belt in the central 
States of the Mississippi Valley. 
Pruning and grafting knives are not responsible for much of the 
disease.—In view of the fact that it has been asserted that crown- 
gall is largely communicated by the use of pruning and grafting 
knives, 100 carefully selected seedlings from the nursery were 
washed, thoroughly dried, and planted in sterilized soil in a green- 
house (p. 77). Before being planted a downward cut on the root 
near the ground line was made with a grafting knife which had in 
each instance just been used in making a cut in a live soft apple gall. 
The greatest care was exercised to see that the knife used in making 
the cuts in the seedlings was in each case passed back and forth 
through a fresh crown-gall before the cut was made. For compari- 
son, 100 healthy seedlings from the same stock were planted after 
being carefully washed and dried. Before planting, a cut was made 
in each of these seedlings as before, except that in each instance 
the knife blade was dipped in a one-tenth per cent corrosive-sub- 
limate solution before making the cut. At the end of the growing 
season 4 per cent of the control seedlings had developed crown-gall 
and only 2 per cent of those that had been wounded with the gall- 
infected knife were similarly diseased. This experiment fails to 
confirm the belief that the grafting knife is responsible for much of 
the crown-gall present in root-grafted stock in the nursery. 
Crown-gall not readily communicated to dormant trees. stored in 
bundles.—The communicability of crown-gall by contact of healthy 
trees with diseased ones while tied in the same bundle has been the 
subject of much dispute, especially between nurserymen and nursery 
inspectors. 
The experiments conducted in cooperative plat 6 in 1905 may 
throw some light on this subject. Two sets of seedling apple trees, 
100 in each, were inoculated with chips of living soft galls from 
186 
