Sept. 8, 1933 
Origin and Control of Apple-Blotch Cankers 
407 
scars where infected petioles had been attached, and at 38, or 77 per cent, 
of the scars where the petiole lesion had been less than 3 mm. distant from 
the abscission layer. One canker developed where the petiole lesion 
was 4 mm. distant, and one where this distance was 7 mm. A total of 
48 leaf scar cankers developed, of which 45, or 94 per cent, appeared 
where an infected petiole had been attached. Two of the other 3 cank¬ 
ers developed at scars where the leaf had fallen before September 27. 
It has been observed that leaves with basal petiole lesions frequently 
fall prematurely. 
It has also been noted that there is some correlation between the per¬ 
centage of petiole infection in different trees and the number of cankers 
per twig observed the next year. For example, in one tree showing 15 
per cent petiole infection in 1920, there later appeared 18 cankers per 
100 twigs, and in another tree showing 24 per cent petiole infection, there 
later appeared 30 cankers per 100 twigs. One tree showing 15 per cent 
petiole infection in 1921 later developed 54 cankers per 100 twigs, another 
tree with 41 per cent petiole infection developed 137 cankers per 100 
twigs, and a third showing 72 per cent petiole infection developed 400 
cankers per 100 twigs. 
These observations show that there is a close correlation between basal 
petiole infection and the development of leaf-scar cankers. 
INTERNAL INVASION OF LEAF SCAR 
While the petiole lesion may be so close to the abscission laygr that it 
visibily extends across to the twig, usually this is not the case (PI. 1, G, H), 
and the question at once arises as to the exact manner in which the 
fungus gains entrance to the twig. 
Cultural tests in the fall of 1921 with a number of Northwestern twigs 
bearing leaves with petiole lesions have indicated how this invasion occurs. 
The clasping bases of 36 petioles bearing basal lesions were cut off 1 or 
2 mm. below the margin of the lesion, sterilized a few minutes in a mer¬ 
curic chlorid solution, rinsed, and planted in poured plates of potato 
dextrose agar. The blotch fungus developed from 19, or 53 per cent, 
of these leaf bases. In one successful isolation the cut was 3 mm. below 
the margin of the lesion. In certain cases the fungus visibly grew out 
from both the cut end and the callous end of the leaf base. In another test 
a petiole segment cut off 1 mm. below a lesion yielded the fungus while 
the next segment, 3 mm. below the lesion, did not yield the fungus. 
These tests show that the fungus may progress downward inside of the 
petiole tissue to a distance of 1 to 3 mm. from the visible margin of the 
lesion. 
A number of leaf scars from which petioles with basal lesions had been 
removed were cut out of the twigs, sterilized a few minutes in a mercuric 
chlorid solution, rinsed, .and planted in agar plates. .Although there 
had been no visible indication of any infection about any of these scars, 
13, or 12.5 per cent, of the 104 thus tested yielded the blotch fungus in 
culture. In these cases the lower margin of the petiole lesion was within 
at least 2 mm. of the abscission layer. These tests, made the last of 
September, show that in a certain percentage of cases the mycelium from 
a basal petiole lesion grows downward inside of the petiole tissue, pene¬ 
trates the abscission layer, and invades the leaf-scar tissue of the twig 
before the leaf falls. 
