Читать книгу The Pears of New York - U. P. Hedrick - Страница 33

“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. (b) 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. The real point of attack lies in this phase of the problem.”

Scab (Venturia pyrina Aderh.), after blight, is the best-known and most prevalent disease of the pear in New York. Like blight, it is found wherever pears are grown in North America, and also wherever pears are grown in foreign countries. It attacks the pear at all ages from the youngest to the oldest plant. Twigs, leaves, flowers, and fruit suffer. A closely related and very similar fungus attacks the apple and causes the apple-scab, but the two fungi are not the same and do not spread from the one fruit to the other.

The name describes the disease at maturity so that all may know it. Black, canker-like lesions spot the fruit, leaf, and twig. These are most characteristic on the pear. The scabs first appear on the fruit as olive-green velvety spots; the young fruits may drop; if they persist, growth may cease, the skin crack, or the fruit be distorted; the fruit-stalk is often shriveled. The scab shows on the leaves much as on the fruit and usually attacks the lower surface. On the twigs the scab is not so conspicuous, but appears as a small round spot which may or may not slough off and be replaced by healthy bark. Young twigs are most often attacked, in which case the scabby spots suggest scale insects.

Pear-scab is caused by a fungus. The chief life events of this fungus must be known to control the disease. The organism passes the winter in leaves on the ground. In the spring, the spores which have matured in the spore-cases are forcibly discharged, and, being very light, are carried hither and thither by the wind so that some of them reach the opening flower and leaf-buds. If moisture and heat are sufficient, the spores germinate, and an infection is started. A foothold secured, the germ-tubes branch and form a dense mycelium—the velvety layer visible to the unaided eye. From these masses of mycelium spore-stalks arise in great numbers bearing countless spores which by one agent and another are carried to other leaves, twigs, or blossoms for new infections. New infections continue throughout the growing season. The black scab spots on fruit and leaf are corky layers of tissue formed to heal the wounds made by the fungus which has ceased to grow vigorously in these scabs. The fungus may pass the winter on the twigs as well as in fallen leaves.

Different varieties resist the scab-fungus differently. Flemish Beauty and Summer Doyenné are most susceptible and in seasons favorable to the fungus seldom present fruits with a clean cheek no matter how careful the treatment. Pruning off badly infected twigs and plowing under scabby leaves are good sanitary measures. In New York, two applications of lime and sulphur at the summer strength, if applied annually, are usually sufficient to control the fungus. The first of these applications should be made when the blossoms show color, a few days before they open. The second should be put on when most of the petals have fallen. In seasons favorable to the scab, a third application two weeks after the second may be the means of saving the crop. The spread of the disease is greatly favored by damp warm weather.

Pear-growers are plagued by two leaf-spots, one of which is also known as leaf-blight. The leaf-spot here to be discussed (Mycosphærella sentina (Fr.) Schroet.) is sometimes called the ashy leaf-spot. The disease is not often seriously troublesome in New York, but is capable of doing great damage in both the nursery and orchard. The spots which give name to the disease are conspicuous enough, but even when present in great numbers are often not seen by the pear-grower until there is a premature dropping of the leaves in August or earlier. The trees often put out new growths, with the result that the wood does not ripen and the tree is left in no condition to stand the cold of winter in this northern climate.

As with nearly all diseases of plants, some varieties suffer more than others. Sheldon, Seckel, and Flemish Beauty are more injured than Kieffer, Lawrence, and Mount Vernon. Nursery stock is more often injured the second than the first year set. Only the leaves suffer. The fungus first shows its work in minute purplish spots on the upper surface of the leaf. The mature spots measure about one-sixth of an inch in diameter, are angular in shape, with well-defined margins, and have an outer zone of brownish-purple, with a grayish center. Late in the season, dots, the spore-cases of the fungus, appear in the gray central area. The fungus passes the winter in diseased leaves which fall to the ground in late summer. From these leaves spores are discharged into the air to be carried to the leaves after growth begins in the spring. The disease is usually controlled by the sprays necessary every year to keep pear-scab in submission. In the nursery, two-year-old trees are sprayed just after the new leaves open and twice thereafter at two-week intervals. One-year-olds seldom need to be sprayed.

Leaf-blight (Fabræa maculata (Lev.) Atk.) is a common and destructive fungus in pear-nurseries in New York and is sometimes troublesome in orchards. The quince suffers even more than the pear from this fungus. In the nursery, leaves and twigs are attacked, and in the orchard the pears themselves sometimes suffer. The disease appears in the spring as minute, reddish-brown circular spots on the upper surface of the leaves, but the fungus penetrates through to the lower surface as the disease progresses. Eventually the color changes to dark brown, and later a coal-black, raised spot appears in the center. The spots sometimes run together. Young leaves shrivel under the attacks of the fungus; while old ones, if badly diseased, turn yellow and drop prematurely. Twigs and leaf-stalks are frequently girdled, and the lesions are more elongated. The spots are similar on the fruits to those on the leaves. The fungus spends the winter in fallen leaves. In the spring the spores are discharged from the fruiting organs of the fungi and are carried to the tender leaf or twig of the pear or quince. The parasite begins growth at once and in about a month a new crop of spores develop. This fungus grows on various other pome-fruits which complicates remedial measures. The treatment recommended for leaf-spot should control leaf-blight.

As are all tree-fruits in New York, the pear is attacked by crown-gall (Bacterium tumefaciens Smith & Townsend). This disease, however, is seldom a serious menace to orchard trees this far north, but the vigor of nursery stock is sapped when the galls girdle the tap-root or the stem at the collar. Moreover, trees affected by crown-gall are barred in most states by inspection laws so that nurserymen can ill afford to produce gall-infected trees. It is a wise precaution not to plant badly diseased trees. The galls are tumor-like structures on the roots of the plant, or often at the juncture of root and stem. They vary from the size of a pea to that of a large egg, forming at maturity rough, knotty, dark-colored masses. Another form of the disease appears as a dense tangle of hair-like roots arising from callous-like galls. This form passes under the name “hairy root.” Neither preventive nor cure is known. Orchard or nursery should not be planted on ground known to have been infected as the disease is highly contagious. The brambles, especially raspberries, are common carriers of crown-gall, and none of the brambles should be planted as inter-crops in pear-orchards.

Brown-blotch (Leptothyrium pomi (Mont. & Fr.) Sacc. var.) is another fungus which is sometimes troublesome. The fungus causes reddish blotches on the fruit which coalesce into rusty-brown patches often covering the whole surface of the pear. Here, again, the Kieffer suffers most although fruits of other varieties are often disfigured by the blotch. The disease is most common on heavy soils and in densely shaded trees. Pruning to let in the sun is usually sufficient to keep the fungus in check, but a late application of lime and sulphur is often necessary.

Black mold (Fumago vagans Fr.), a fungus which grows in the honey-dew exuded by the nymphs of the pear-psylla, sometimes causes a sooty covering of the pears which spoils their sale. Twigs and leaves are also covered with thin superficial growth of the fungus somewhat to the detriment of growth. The remedy is obvious—control the psylla.

Pink-rot (Cephalothecium roseum Cda.) sometimes does much damage to pears in common or cold storage. The fungus seems able to enter the skin of pears only through injuries, and when reasonable care is used in handling the fruit the rot does little damage. Not infrequently it is found on fruits unpicked, having entered the skin through ruptures made by pear-scab, black-spot, or other fungi. This, of course, seldom happens in well-sprayed orchards.