This disease, troublesome for many years in Europe, was noted on apples in America about 1899. At this time it was found doing damage in Nova Scotia and in New York State. From time to time reports of the European canker on apple have come from various parts of United States and Canada, and now it is known to prevail over a wide area on this continent. It has gained most prominence, as an apple disease, in eastern Canada, New York, New England and the Pacific Coast. The disease is perhaps of European origin. It has been well - known there for scores of years, because of its destructiveness not only to the apple, but also to many forest trees. Previous to 1880, however, no profound investigations were made abroad; about this time considerable attention was given to it in German writings. Subsequently the disease has received more than usual consideration in Italy, Austria, Switzerland, France, Australia, New Zealand, Holland and England.
In Europe, this canker is the most destructive and most dangerous of all fruit diseases. Thousands of trees are killed in their prime. In many localities certain varieties can no longer be grown, and there are even districts in which apple-culture has become a real problem as a result of the ravages of this disease. Nor are the losses confined to the apple, nor to fruit trees. It will be less difficult to give an impression of the economic importance of this disease if brief reference to the plants affected is made. Among these are, in addition to the apple, the pear, quince, cherry, gooseberry, lime, beech, maple, ash, alder, hazel-nut, linden, plane-tree, oak, hornbeam, ironwood, dogwood and magnolia. In Germany, for example, beech- - stands are often ruined by it. And while the greatest damage done to forest trees is in foreign countries, its importance on the apple makes it a serious pest.
In America, its range and destructiveness have not reached proportions which even approach those in Europe. Yet in certain sections, already referred to, the canker is becoming troublesome; and it seems to be on the increase.
So far as varietal susceptibility is concerned, little information is at hand for American conditions. But German authorities say that growers have always spoken of cankerous varieties. It is held by some that thick barked varieties are more resistant than others. Again, the matter of whether a variety is susceptible or resistant is said to depend directly upon its susceptibility or resistance to Frost Injury. Certain writers say that varieties of apples that grow erect and open, remain free from this canker, whereas others near by suffer severely. In contradiction to these observations and opinions, it is held that all varieties suffer alike, and that so-called resistant sorts, when planted among diseased trees, soon exhibit a susceptibility equal to that of their neighbors. Any variety, it is said, which is exposed is just as liable to infection as any other. It is admitted by some that weak-growing varieties show larger cankers, because of their poor vigor, yet this is in most cases not worthy of practical notice. In Germany, susceptible varieties include the Canada Russet, Rheinischen Bohnapfels, Harbert's Russet, Norman Bitter - Sweet, Gray French Russet, Hubbardston, Red Cardinal, Canada Reinette, Red Fall Calvill, White Winter Calvill, Winter Goldpearmain and others. Those resistant are: Fursten, Schaffelder, Boiken, Gravenstein, Carpentin, Eiser and Purple Red Cousinot. In England, Warner's King is said to be notably less susceptible than other varieties.
In later stages it is not at all difficult to detect the presence of the European apple-tree canker, nor to distinguish it from other kinds of cankers (Fig. 33). In the younger stages, however, it is likely to be overlooked by the casual observer; and when noticed it is found to be not markedly different in appearance from young cankers of other kinds. The first intimation of the disease consists of a slight browning of the bark in a more or less circular area. This is quickly followed by an intense darkening of the affected tissue. The surface of the canker sinks, leaving the edges prominently raised. In a short time a crevice is evident along the margin of the healthy and diseased bark. The affected tissues are killed, become dry, and slough off, exposing the wood, and at the margin a callus develops. The enlargement of these cankers is slow; within a year the length of the lesions may not increase more than one - half of an inch. But they increase in size the second year, and even for as many as a score of years after, thus resulting in large cankers.
Older cankers are recognized as of two sorts: open cankers and closed cankers; and there are various intervening types. These names are very descriptive. In the first kind, the wound is open, the wood is exposed, and a series of dead callus-layers center about a common point (Fig. 33). While the edge of this type of canker is raised, there is no distinct swelling, and no gall-like appearance. In the closed canker, the whole presents the appearance of a rough gall. The edges are swollen to the extent that the wound is practically closed and the wood almost covered. It is said that open cankers prevail on those erect and more vigorous growing branches and twigs in the crown of the tree. On the other hand, the closed type predominates on the horizontal or hanging limbs. In the case of open cankers the margin continues to extend. The prevalence of this type of canker on erect branches is explained on the basis that such limbs lack the necessary food supply to overcome the progress of the pathogene. Here the pathogene appears to have the advantage as evidenced by the expansion of the lesion and the failure of the host to check the spread. In the closed cankers the host has the advantage. Along the margins of the older cankers, brilliant red fruiting bodies of the pathogene develop. These are easily seen with the naked eye, even though they are never larger than a pin - head.
Fig. 33. - European apple canker.
The lesions center about a wound, a bud, or the fork of two branches. Such wounds as those caused by hail, insects, pruning and frost are common seats of the injury. Very commonly a stub or twig may be found at the center of the canker.
The European canker is caused by the fungus Nectria galli-gena. As previously intimated the mycelium lives over from year to year in the diseased bark. In the spring and early summer red perithecia develop in the wound and under favorable conditions discharge their ascospores. Conidial tufts are developed at this time of year also, so that there are two kinds of spores for initiating primary infections. It has been shown that insects are highly important as agents of inoculation; the woolly aphis, for example, is very active in carrying the spores of the fungus. In Europe an outbreak of canker is said to follow closely an unusual prevalence of this insect. It has been estimated that in a single canker 300,000 ascospores are available for dissemination. The spores germinate in a few hours and the germtubes enter the bark through wounds or lenticels. Within a week the effects of the fungus are visible. The mycelium, developing from the germtube, permeates the bark, the wood and the pith. The attacks are confined chiefly, however, to the bark, where the cortical cells are killed by the fungus. As a result of death, the affected portion of the bark turns brown, the cells collapse, and the canker shows a sunken surface. If the atmosphere is continuously humid, conidial tufts arise from the mycelium. From these tufts, conidia are liberated; they are then carried to other points where new cankers are formed. The mycelium grows more rapidly parallel to the long axis of the limb and hence the canker is the longer in this direction. Where the wood is entered, the mycelium invades the sap - tubes in which it passes up and down. It is believed that at points above and below the canker the fungus again attacks the cortex, this time from within, thus forming a new canker without direct external inoculation. About a year after the canker starts to develop the mycelium forms the red perithecia. These may act as a means of carrying the fungus through the winter.
The chief measures to be used against the European canker are those of eradication as outlined for the Black Rot canker (page 52). The smaller limbs and the badly diseased larger branches should be wholly removed. Smaller cankers on large valuable limbs and on trunks should be cut out, the upper and lower ends of the wound pointed, and finally a wound-dressing, preferably coal - tar, should be applied.
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