From the blossoms the bacteria work their way down the pedicel to the spur, killing the bark and causing the leaves as well as the blossoms to wither. The leaf-tissues are not usually invaded. From the spur the bacteria may pass to the other healthy pedicels of the cluster and finally enter the fruit through its base. Fruit blight may also arise by the bacteria being deposited in wounds made by the curculio and other insects (Figs. 90 and 91). The bacteria are similarly introduced into the growing tips of twigs and watersprouts by aphids, twig blight resulting. Affected twigs emit the ooze which serves as a source of inoculum for other twigs, shoots and blossoms. In most varieties of pears, twig infection, unless removed, is inclined to continue down the main limb and even into the body of the tree. Watersprouts frequently mark the center of a canker, indicating the manner of entrance of the bacteria.
Fig. 89. - Oozing Fire Blight canker. Note drops of exudate.
Suckers which arise from the crown, at or below the surface of the soil, are often blighted, allowing the bacteria entrance into the bark of the roots.
Trees may die from such a form of attack. Grafts are especially disposed to blight during the first year or so on account of their rapid and succulent growth. Wounds in the larger limbs or the body of the tree may serve as centers of cankers. Here the bacteria are carried by the bark - boring beetle and deposited in their borings. In these cankers and blighted limbs and twigs the bacteria pass the winter. With the return of the warm weather and rains of the spring the rise of sap encourages the growth and multiplication of the bacteria, which ooze out and afford the source of the inoculum for the opening blossoms.
Weather conditions should not be confused with the causal factor of Fire Blight. On the other hand, the weather is correlated to some extent with epiphytotics of the trouble. Late frosts may stop blight by killing certain of the disseminating agents, such as aphids. Should no frost occur and were the spring backward, the insects would multiply rapidly while blossoming would be retarded, hence at blossoming large numbers of insects are at hand to disseminate the bacteria rapidly. During the growing-period, immediately following blossoming, should the weather become hot and dry, the rapid growth is checked and the otherwise succulent tips become harder and more woody. Such shoots are less liable to blight - infection. On the contrary, muggy periods favor the disease in that the tissues become gorged with sap and thus offer less resistance to the invading bacteria.
Fig. 90. - Fire Blight on apple fruit; drops of bacterial ooze on the surface.
Fig. 91. - Fire Blight on pear fruit; healthy fruit left, diseased fruit right.
In attempting to control Fire Blight, the following important points should be borne in mind: (1) that the disease is caused by bacteria which gain entrance to the host tissues only through wounds, or punctures by insects, into succulent, rapidly growing tissues, or through the nectaries of the blossoms. (2) That insects of several kinds are the usual agents of inoculation. (3) That practically all pome fruit-growing sections in North America are infested, and therefore there is always a source from which the bacteria may be disseminated. (4) That all known varieties of the hosts, on which the blight-organism occurs, are more or less susceptible; while some show resistance, none are wholly immune. Therefore control consists chiefly in the elimination of the pathogene from the infected trees. This is accomplished by a strict application of the following operations: (a) inspect all pear trees in the autumn and again in the early spring before the blossoms open, and cut out and treat all cankers in the body and main limbs. With a sharp knife, or draw-shave, remove all the diseased tissue, wash the wound with corrosive sublimate (one tablet to one pint of water), and, when dry, paint the wound with coal-tar or lead paint, preferably the former. The wound-dressing will need renewal every year or so. (6) Throughout the summer, beginning with the fall of blossoms, make an inspection every few days of the young trees. Break out the blighted spurs and cut out diseased twigs, making the cut at least six inches below the diseased portion. Disinfect the cuts with corrosive sublimate, (c) Remove all watersprouts from the trees two or three-times during the season. (d) In the nursery remove the blossom-buds, particularly of the quinces. Here inspection must be frequent, particularly in susceptible stock, in order to keep the disease under control. It is often necessary to inspect certain blocks daily, the diseased twigs being cut out as soon as observed. When budded stock of the first year becomes affected, the trees should be dug out, since cutting below the diseased area causes the trunk of the young tree to be crooked and therefore not marketable. (e) Control the insects; those which are active in disseminating the blight bacteria have been enumerated (page 327). The real point of attack lies in this phase of the problem. (For discussions of Fire Blight on other fruits, see Apple, page 21; Apricot, page 159; Cherry, page 191; Plum, page 386; and Quince, page 387.)
Barrus, M. F. Diseases of pears. Fire blight. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79: 1039 - 1048. 1916.
Whetzel, H. H., and Stewart, V. B. Fire blight of pears, apples, quinces, etc. Cornell Univ. Agr. Exp. Sta. Bul. 272:31 - 51. 1909.
Jones, D. H. Bacterial blight of apple, pear, and quince trees. Ontario Agr. Col. Bul. 176: 1 - 63. 1909.
Stewart, V. B. The fire blight disease in nursery stock. Cornell Univ. Agr. Exp. Sta. Bul. 329: 328 - 371. 1913.
Sackett, W. G. Hold-over blight in the pear. Colorado Agr. Exp. Sta. Bul. 177:2 - 8. 1911.
Pickett, B. S. The blight of apples, pears and quinces. Illinois Agr. Exp. Sta. Circ. 172: 3 - 8. 1914.
Waite, M. B. The cause and prevention of pear blight. U. S. Agr. Dept. Yearbook 1895: 295 - 300. 1896.