Читать книгу The Pears of New York - U. P. Hedrick - Страница 32

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The pear is attacked by a half dozen or more diseases in New York, of which two, at least, need treatment every year, in every orchard, and on nearly every variety. One, pear-blight, is about the most malignant of the diseases of the orchard, for which there is no antidote and no alleviation or preventive except by the most drastic sanitary measures. The other, pear-scab, is always present but not always destructive, although some varieties are always injured by it. The scab, however, is amenable to treatment and at its worst only destroys fruit and foliage, seldom endangering the life of the tree. The four or five other diseases of the pear in New York are of minor importance and are readily controlled by the treatment necessary to keep in check the scab-fungus. Pear-blight merits attention first.

Pear-blight is a malignant bacterial disease, very contagious, usually virulent and so terrible in its consequences as to warrant the common name fire-blight. No part of the tree is exempt from destruction by the malign bacterium that causes blight of the pear. Root, trunk, branch, leaf, flower, and fruit are all attacked, turn black and wither under the disease. Few plant diseases produce more disastrous results. The pear competes with the apple in importance in Europe where blight is unknown. In America it is a poor fourth to the apple, peach, and plum, and takes fourth place instead of second because of the ravages of blight. About the most important discovery to be made in pomology is a race of blight-resistant pears. Failing in this, if the pear-industry is to grow, or even continue in its present magnitude, blight-resistant stocks must be found.

The symptoms of pear-blight are so characteristic that the disease cannot be confounded with any other malady or condition of the tree. It appears earliest in the season on the blossoms causing blossom-blight. Attacked by blight, the blossoms wilt, and after the petals fall, fruit and spur show the characteristic blackening of the disease. Blossom-blight may escape the attention of the pear-grower, but twig-blight, a succeeding form of the disease, can escape no one who has the sense of sight. No other disease of the pear brings on such palpable destruction to the tree as twig-blight. No other disease causes such comfortless despair to the grower. Twig, branch, or tree, as the case may be, in all affected parts, turns black, the leaves droop, seeming to show the effects of fire. A marked symptom is, if there can be doubt of those given, that the blackened foliage clings most tenaciously to the dead branches. Twig-blight is the most common manifestation of the disease. Another form of the blight appears as a canker on the trunk and large branches—canker-blight or body-blight. These cankers are dark, smooth, and sunken, with definite margins marked by a crevasse in the winter; but as spring comes on the advancing margins become raised and more or less indefinite. Occasionally an opaque liquid oozes from lenticels newly attacked. On branches, the cankers usually surround a smaller offshoot, sucker, or spur. The disease spreads with great rapidity, by reason of which it is easily told from winter-killing. Injury from cold is also more general, and the foliage browns rather than blackens.

Pear-blight is an American disease, the history of which was briefly given on page 51. Until recently it was confined to regions east of the Rocky Mountains, but since about 1900 it has been a virulent epidemic on the Pacific slope as well, and is now found from coast to coast wherever pears are grown in North America. It seems not to be found in the pear regions of other continents. It attacks the apple, quince, and other pomes as well as the pear, and plant pathologists declare it to be the most destructive disease attacking the pome-fruits. Trees in the nursery suffer as well as those in the orchard. Every variety of the pear bearing edible fruit is attacked. Fortunately, some sorts are more immune than others. Kieffer, Seckel, Winter Nelis, and Duchesse d’Angoulême are most resistant of standard varieties, while Bartlett, Clapp Favorite, and Flemish Beauty are little resistant.

Pear-blight is caused by a bacterium, Bacillus amylovorous, the discovery of which by Burrill in 1877 as a cause of this disease is one of the landmarks in plant pathology. The organisms are dormant during the winter, which they pass in the margins of blight-cankers where moisture is sufficient to keep them alive. With the return of vegetative growth, some sort of fermentation seems to set in and drops of a thick, opaque liquid ooze out of the margins of blight-cankers. These contain countless numbers of the blight bacteria which may swarm into the healthy tissues adjoining, or be carried by any one of the great number of kinds of insects which visit trees at flowering time to the pear-blossoms, to growing tips, or to wounds in tender bark. The pruner with his tools may be an unwilling agent in carrying the bacteria from tree to tree. The organisms now multiply apace, killing tissues wherever they find entrance and causing the several manifestations of the disease described under symptoms. Were it not that the bacteria are killed by sunlight and even brief periods of drying, the life of the plants attacked would be the only limits of the disease unless checked by man.

Theoretically, pear-blight can be controlled. Practically, pear-growers fail to control it. Control consists in orchard sanitation whereby the bacterium causing the disease is kept out of the orchard. This proves all but impossible in the average orchard. Sometimes, without doubt, the virulency of the disease is lessened. Possibly, if all the recommendations of plant pathologists could be put in practice, pear-growers would more often succeed in keeping blight down, but the necessary sanitary measures require such watchful care and so great an expense that few pear-growers can carry out the program for controlling this disease. Of those who have studied methods of control and have given advice on the subject, Hesler and Whetzel[24] are as reliable as any and we quote herewith their recommendations: