DISEASES AND INSECT PESTS. 
63 
arresting' tlie proper passage of fluids, then eats its way into the wood, 
and finally utterly destroys the affected parts. There can hardly be a 
doubt that the seat of canker is generally iil the soil. The roots either 
absorb or touch something deleterious to the tree, and there is a stoppage 
or derangement of fluid, and perhaps a deposit of granular matter, and 
canker results. A change of soil, or rather the uplifting of the roots 
into new and different and warmer earth nearer to the surface, is 
generally the best remedy for canker; and this is the sooner effected if 
the cankered parts are reduced and dressed with oil or other oleaginous 
compound. 
Excessive growth may also produce canker, especially if it is prolonged 
late in the season. Gross wood hit by the frost before it is matured 
results in a sort of canker, very similar to that produced by a mere 
mechanical bruise or abrasion. The cold ruptures the cells when full of 
sap, and this immature wood becomes blotched and cankered. This 
sometimes happens on the best of soils, and especially in low-lying 
localities. When apples are grown in these care should be taken by 
root pruning, surface planting, and other expedients to check growth 
and hasten its maturity before the frost comes upon it. 
Another cause of canker is severe and careless pruning. It should 
never be forgotten that all pruning is unnatural, and, therefore, that 
as few wounds as possible should be made, and these of the smallest 
area. The fluid or sap of plants is apt to ooze out of all the fresh 
wounds made; and such diversions of sap not seldom form the nucleus 
of canker and other diseases. The mere interruption and accumulation 
of fluids in places where there is no outlet for them, one of the necessary 
results of pruning, often leads to gangrene or canker. The concentrated 
sap seems to undergo a species of decomposition and canker—that is, 
decayed, dying, or dead tissues of wood—^is the result. All pruning should 
be done as early in the life of stems and branches as possible. Pruning 
should also be done chiefly in the growing season, so that the wounds 
made may be quickly healed, and not left long in a raw state with the 
risk of degenerating into canker. Sudden changes of temperature, great 
extremes of dryness or moisture, and severe checks or arrestments of 
growth are also frequent producing causes of canker. 
During severe drought, the roots of apples often become parched, 
the sap almost viscid, the bark what is known as hide bound. Suddenly 
a rainy season sets in, the plants are flushed with sap, the vessels, being 
so long scantily filled, either burst or are disgorged, and canker follows. 
It is also possible that canker arises at times from American blight, 
scale, or other insect agencies. These injure or destroy the bark and 
woody tissue, there is a derangement of fluids, and canker is established. 
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