Sept. 8,1923 
Origin and Control of Apple-Blotch Cankers 
405 
6 leaf-scar cankers were visible, on April 27 there were 68, by May 20, 
95 had appeared, and by September 15, 109 were present on these twigs. 
Thus the three cankers between leaf scars appeared in the fall of 
the first season, and 103 out of 109, or 95 per cent, of the leaf-scar 
cankers did not appear until the second season. Fifty-seven per cent 
appeared between March 22 and April 27 and previous to the petal-fall 
period, 25 per cent between April 27 and May 20, and 12 per cent after 
May 20. 
On September 27, 1921, eight twigs on a badly cankered Northwestern 
tree at Knightstown were tagged and carefully examined. Four cankers 
were found in the fall, and by April 4, 1922, 6 were visible. By April 25 
only 1 more lesion had appeared, but between this date and May 25, 
39 cankers became visible and only 3 more appeared later, making a 
total of 49 cankers, of which 39, or 79 per cent, appeared between April 
25 and May 25. All but one, which appeared the first fall, were located 
at leaf scars. 
It is of interest to note that the cankers appeared about a month later 
in 1922 than those studied at Mooresville in 1921. This difference may 
be attributed in part to the fact that the spring season at Mooresville 
is always more advanced than at Knightstown, which is only 45 miles 
distant and 12 miles farther north. In 1920 the petals fell nine days 
later at Knightstown than at Mooresville. A more important reason 
is, however, that the spring of 1922 was later than that of 1921. The 
petals fell six days later at Knightstown than in 1921. March, 1922, 
averaged 7.4 0 F. cooler than March, 1921, and April, 1.4 0 cooler, according 
to the records of the Federal Weather Bureau. 
The appearance of the majority of these cankers early in the spring of 
the second season shows that infection must have occurred the previous 
season and that the mycelium must have been present in the cortex of 
the twig all winter. 
Since the early blotch sprays, as will be shown later, prevent infection, 
it seems that all spore infection must occur during a rather definite period 
soon after petal-fall. Consequently, the delayed appearance and the 
variation in date of appearance of the leaf-scar cankers is difficult to 
explain upon the basis of direct spore infection. On the other hand, these 
phenomena are readily explained upon the basis of mycelial invasion of 
the twig from basal petiole lesions because the mycelium might cross 
from the petiole to the cortex of the twig at any time during the season. 
The problem here is quite unlike that found by Wiltshire (17) in England 
in connection with the late fall and early spring spore infection of leaf 
scars by the apple-canker fungus (Nectria ditissima) which acts somewhat 
as a wound parasite. 
The cankers do not appear as gradually enlarging spots but are of 
considerable size when they first become visible, and the portion which 
thus makes its appearance is soon delimited by a fissure in the cortex 
(PI. 1, C, D, F). The mycelium apparently penetrates a considerable 
area of bark tissue before its effects become visible. 
PREVALENCE OF PETIOLE LESIONS 
Petiole lesions (Pl. 1, G, H, I,) are very abundant on the lower limbs of 
cankered trees. Observations made on unsprayed Northwestern trees in 
1920 showed that out of 4,937 leaves examined 1,633, OT 33 P er cent, bore 
petiole lesions. In 1921, 5,099 leaves on Northwestern trees were exam- 
