Blister Canker 
65 
ress is made longitudinally thru the tracheae. Radial 
progress is comparatively slow and is most rapid in the medul- 
lary rays. 
10 Hyphae are always found closely associated with the 
characteristic bro\^Ti discoloration of the wood. The wood 
cells are often killed several cm. in advance of the invading 
hyphae. 
11 The fungous growth is usually slightly faster below 
than above the inoculation in parts of the tree above ground. 
In the roots, the reverse is true. 
12 There is a great deal of difference in the susceptibility 
of different varieties of apple trees to blister canker. Of the 
common varieties Ben Davis, Gano, and Delicious appear to 
be the most susceptible while Oldenburg and Wealthy appear 
to be the most resistant. 
13 Susceptibility is greatly influenced by the general 
vigor of the tree, by the supply of water and soil nutrients, and 
by the season of the year in which the inoculation occurs. 
Trees are rendered more susceptible when too little water is 
available, or when they are under-nourished. The greatest 
amount of infection is found in trees making less than normal 
growth. Trees appear to be least susceptible to infection dur- 
ing the period in early spring and summer when they are mak- 
ing the most rapid growth. The addition of fertilizers to the 
soil not deficient in any element has been of little or no benefit. 
14 Both anatomical and physiological factors contribute 
to cause a variation in resistance of apple trees to blister 
canker. It is possible that chemical factors may also play an 
important role. 
15 Control measures m.ust be preventive rather than 
curative. A diseased tree can be cured only by removing all 
infection. This is obviously impracticable except in a very few 
cases. 
16 The heavy pruning of infected trees aggravates the 
disease. 
17 Both ascospores and conidia may be inhibited by 
spraying or painting the cankers with either copper sulphate 
or lime sulphur. 
18 Infection may be prevented to a great extent by dis- 
infecting and covering all pruning wounds. 
