This disease, which is known as Blister Canker, and Illinois canker, was first given attention in Illinois in 1902. Since that time it has been reported from the following states: Arkansas, Indiana, Iowa, Kansas, Missouri, Ohio, Oklahoma, Nebraska, New Hampshire, New York, Virginia and West Virginia. Besides its occurrence in the United States, the Blister Canker is found in Germany, Italy and Cuba, although it does not appear to have assumed great importance in these countries.
In certain states, as, for example, Missouri, and the southern portions of Illinois, Indiana and Ohio, this is the most injurious canker disease of the apple. It is said that within the last few years orchardists in Kansas, Missouri, Oklahoma and Nebraska have lost more than a million trees from it. The injury consists in the destruction of both large and small limbs. Not infrequently large trunks are affected, and trees are killed. The only available estimate upon the actual importance of this disease comes from Ohio. In the southern part of that state from 1 to 90 per cent of the trees are affected, amounting to an annual loss of thousands of dollars. In the summer of 1912 thousands of trees, and in many cases entire orchards, were practically ruined by Blister Canker in Iowa. The Ben Davis appears to suffer more than other varieties.
The later stages of the Blister Canker are very characteristic (Fig. 22). However, in the earlier stages the lesions are distinguished from those of other cankers with some difficulty. At first the spots are not conspicuous, and would be easily overlooked by the casual observer. The bark takes on a dingy, brown appearance, the size of the cankers varying at this early age. The lesion enlarges most rapidly in the direction of the long axis of the limb. The interior of the generally diseased area shows healthy areas of bark scattered through the whole affected portion. This gives a peculiar mottled appearance to the canker. The margin of the diseased bark is sharply delimited from the healthy tissues, sometimes a crevice marking the boundary of the canker. The dead area is usually a little depressed, due to a dying and shrinking of the bark. Frequently branches are girdled, hence the parts above wither and die. It is no uncommon sight to see dead fruits and leaves clinging to limbs which have just been girdled by the Blister Canker pathogene. In late summer and autumn small tan-colored cushions, fruiting structures of the pathogene, appear under the bark at or near the margin of the canker for that season. Later the cankers become blackened and appear burned. The bark becomes very dry and brittle, and irregular patches fall, exposing the dead wood (Fig. 22, at the top). Sometimes whole trees are barked; and frequently one side of a tree is thus devoid of its bark, the other side remaining normal. Cankers sometimes extend from the roots to the limbs ten feet above ground. Ordinarily, however, the cankers extend three feet or less, and are confined to the larger limbs. Older cankers show, in place of the cushions mentioned above, dark fruiting cushions, called "nail - heads" by growers (Pig. 22). The presence of these cushions gives a blistered appearance to the canker, and they form the most pronounced distinguishing feature of this disease (Fig. 22).
Fig. 22. - Blister Canker of apple. Old canker, showing "nail - heads," or "blisters."
The pathogene, Nummularia discreta, was described from eastern America in 1834, but at that time it was not recognized as the causal factor in Blister Canker. Now there is no doubt as to the ability of this fungus to cause this disease.
The fungus is in no strange habitat when found on dead wood. And it appears that it is unable to establish itself without the presence of an injury of some kind in the bark. It may live about the orchard indefinitely on dead limbs of the apple; it also easily lives on Sorbus, Cercis, magnolia and elm. Hence these trees may be regarded as possible sources of trouble wherever Blister Canker prevails.
On the surface of the old fruiting pustules - the stromata - ascospores can be seen in the spring exuding from the sunken perithecia. These spores are washed and blown to infection-courts, chiefly wounds, where they germinate and their germ-tubes establish a food relationship with the bark. It has been observed that one of the most common seats of Blister Canker injury is in old stubs which have been left by careless pruning. The fungus now in the bark extends into the wood where it grows even more rapidly than in the bark. Some limbs, if carefully examined, show brown, discolored wood at points several feet away from the canker. It is probable that the fungus can gain an exit where the bark is injured, at which places new cankers would be formed. Wherever the mycelium penetrates the bark the tissue is killed, and finally is disintegrated. For some reason the spread of the fungus within a given area is not complete at first, but here and there sound bits of bark are left unaffected for a time. This explains the mottled appearance,of the younger cankers. Finally, however, these spots are also killed. The growth and spread of the hyphae within the bark is so rapid that no callus is laid down at the advancing margin as in many other canker diseases. The only attempt made on the part of the tree to inhibit the advance of the fungus is the formation in the summer of a layer of cork. It seems that growth of the pathogene in the tissues continues as long as there is sufficient oxygen and moisture present. Its attack on heart - wood and sapwood appears to be conditioned by these factors. The supply of air and water must be in proper proportions.
In July and August the fungus develops its stromata by a massing of the mycelium in certain portions of the diseased bark. As they increase in size the bark is ruptured, exposing their light, tan-colored surfaces. On these disc-shaped stromata are developed, on erect conidiophores, honey - colored conidia. It is believed that these conidia may be carried to the limbs of the apple and other hosts where infection results. Just when these conidia function as inoculum is not clear, although possibly as soon as developed. Whether they are capable of overwintering is not known.
As the stromata grow older a ring of black stromatic tissue is formed beneath the disc. The ring, or cylinder, extends into the wood of the limb. The connection of the stromata with the wood allows them to persist for as long as ten years, even after the bark has long since fallen away.
In the upper portion of the stromata there are formed, from April to June of the following year, flask - shaped cavities, with long necks opening at the surface; these are the perithecia. These contain asci and ascospores; the ascospores are discharged and come to lie in small, black heaps on the surfaces of the stromata. This expulsion is said to occur in the spring, but ascospores may be found on the discs at any time. It is probable that they cling there for a year or more; at least their vitality is retained for several months. There is some indication that the woolly aphis carries the spores.
The Blister Canker is one of the most difficult diseases to combat. The rapid growth of the fungus in the heart- wood renders a cure practically impossible. If the canker is found in its early stages, however, it may be profitable to cut away the injured bark and wood and cover the wound with coal-tar or some good dressing. (For methods of cutting-out cankers, see page 54.) Limbs showing older and more extended cankered areas should be removed entirely. Since the fungus enters the tree through stubs left by careless pruning, a step toward the control of this disease consists in avoiding such indifferent procedure. It should always be remembered that the fungus can get into the bark only through injured tissue. In those regions where Blister Canker is especially troublesome, resistant varieties may be sought with profit.
Hasselbring, H. Canker of apple trees. Illinois Agr. Exp. Sta. Bul.
70: 225 - 239. 1902. Gloyer, W. O. Apple blister canker and methods of treatment.
Ohio Agr. Exp. Sta. Circ. 125: 149 - 161. 1912. Pammel, L. H., and King, C. M. Four new fungous diseases in Iowa.
The blister canker of the apple or Illinois canker (Nummularia discreta Tul.). Iowa Agr. Exp. Sta. Bul. 131: 212 - 215. 1912.