Wherever this disease occurs it is known as yellows, or peach yellows. It is primarily a trouble of the peach and nectarine, although it has been observed on almonds, apricots and Japanese plums. Seedling peaches are said to be more sensitive than budded trees. Seedling trees derived from budded fruit are equally susceptible.
The origin of the trouble is unknown. Records of it date back to 1760, and in 1806 it was the subject of horticultural writings. The first discovery of peach yellows is thought to have been made in Pennsylvania, near Philadelphia. Outbreaks of the disease occurred in the East in 1791, 1806, 1807, 1817 to 1821, 1845 to 1858, 1874 to 1878, and in 1886 to 1888. It appeared in Michigan about 1869, or earlier. The disease is not known on any other continent than North America, and the early records coming from the eastern part of the United States would, therefore, seem to show that peach yellows is native to eastern United States. It occurs as far south as southern Virginia, west to Arkansas and northeastern Texas, north to Canada, and east to New England.
Peach yellows is conceded to be one of the most serious diseases of the peach in America. It has been extremely ruinous in the region outlined in the preceding paragraph. Soon after its discovery, hundreds of orchards along the Atlantic Coast were destroyed by the disease, causing growers in many localities to abandon peach-culture. In one county in Michigan the number of peach trees destroyed by yellows between the years 1874 and 1890 is estimated at more than one - half million. Symptoms.
The marked symptoms of peach yellows consist (1) in a premature ripening of the fruit (Fig. 76); (2) a red spotting on the surface of the fruit; (3) the development of secondary shoots in great numbers (Fig. 77), these being dwarfed and unhealthy in appearance; (4) the development of short shoot - axes with sickly foliage of a yellowish or reddish brown color, having a tendency to roll sidewise. These symptoms begin to develop in the middle of the summer. Subsequent developmental stages are observed at various seasons of the year.
Certain evidences of yellows are apparent the first year that the orchard is affected. (1) There is a premature ripening of the fruit (Fig. 76); this, will likely be the first noticeable symptom. This premature ripening may take place a few days to several weeks prior to the time of normal ripening. It may occur on but one or two branches, in which case the peaches on the rest of the tree ripen normally, or all peaches on an affected tree may exhibit this abnormality. This phenomenon should never be confused with the work of the peach-tree borer; trees severely injured by this insect may ripen their fruit prematurely, but the flesh is never red spotted and shoots are not put forth as in yellows. (2) Diseased fruits are always reddish or purplish spotted externally, and they show red streaks scattered through the flesh. The amount of redness varies; there may be only traces, or the whole fruit may become entirely crimson. The flavor of such fruit is usually insipid and sometimes bitter. (3) Abnormal dwarfed shoots come out on the trunks and limbs (Fig. 77). These may not show, however, until the second year, in which case the only signs of yellows the first year are those enumerated above. Such shoots bear small leaves of a pale - green, yellowish, reddish or whitish color, and there is a marked tendency toward repeated branching. Leaves on affected trees are notably more slender, but it should be borne in mind that certain varieties like the Elberta, Carman, Champion, Hill's Chili and others possess leaves which are characteristically long, narrow and straight, with a natural tendency to droop.
Fig. 76. - Peach - yellows; on the right small healthy peaches, on the left large prematurely ripened fruit.
Fig. 77. - Peach yellows; note the clusters of shoots.
The second year during which the orchard is affected with yellows certain characteristic symptoms are exhibited. (1) Abnormal dwarfed shoots may appear on the trunks or limbs; but, as already noted, these may develop during the first year. (2) Affected trees may be barren after the first year, or they may bear another crop of fruit which ripens prematurely as described for the first year. (3) Affected trees may die the second year, but ordinarily they succumb slowly from the top downward (Fig. 77). When the first symptoms of yellows appear in any part of a tree, it is thought that the whole tree is then diseased.
The true yellows of peach should never be confused with little leaf or California yellows. This disease is characterized by the development of spindling, yellow shoots on the new growth with small, narrow, yellow leaves. The foliage along such shoots drops prematurely, leaving tufts at the ends. The fruit fails to develop; it shrivels and finally falls. It is a trouble peculiar to trees from three to seven years of age, whereas true yellows affects older orchard trees.
Just what causes peach yellows is still unknown. It therefore falls into a class, from the causal standpoint at least, with rosette and little-peach. The cause of yellows has aroused much speculation because of the importance of the disease and on account of its obscure nature. Many theories have been advanced to explain the origin of the peculiar symptoms exhibited by diseased trees, but all of these have been certainly disproved. Some of the prominent theories regarding the cause of peach yellows follow: (1) severe winter injury; (2) excessive rainfall; (3) impoverished soil, that is, a deficiency in lime, potash and phosphoric-acid; (4) insects; (5) fungi; (6) bacteria; (7) crowding trees in the orchard; (8) excessive cultivation; (9) over - bearing. Many others might be added. It will be seen that these fall into one or the other of the categories: weather, soil, orchard management and parasites.
The prevailing opinion of the modern pathologists is that the disease is caused by a parasite which as yet has not been seen. It was long ago established that the disease is communicable, that is, the causal factor may be transferred from a diseased tree to a healthy one, and after a time the latter tree will show symptoms of yellows which in due time run their course. Buds from diseased trees convey the causal factor of yellows to the stock on which they are inserted. The inoculum, whatever its nature, is apparently carried in a few diseased cells which, if induced to unite with normal cells of the stock, will cause the stock to become diseased. Diseased seeds also carry the inoculum; and there are probably other ways in which it is carried.
The knowledge of remedial measures is about as meager as that of the causal relations. "Cures" are worthless and impossible. It has been shown that while fertilizers make the trees temporarily greener and apparently more vigorous, yet in the end they are of little or no value in the control of yellows. Trees have been treated with such materials as lime, wood ashes, kainit, muriate of potash, dissolved bone-black, bone-ash, nitrate of potash, nitrate of soda, sulfate of ammonia, tobacco dust, dried blood and stable - manure. Treated trees are just as likely to be attacked as those left untreated.
The prompt removal and destruction of affected trees has been recommended since 1828. The advice is still reliable. It has been practiced effectively in the State of Michigan in years past. The eradication of diseased trees must be an annual operation until no traces of yellows remain. Where no such efforts have been made to exterminate the disease, it has prevailed to such an extent that orchards have been destroyed and the culture of peaches abandoned. In order to accomplish general and effective eradication, laws have been enacted in many states. The first was enacted by the legislature of Michigan in 1875, which law made it a misdemeanor to neglect the destruction of diseased trees. This step toward legislation in general was followed by Ontario in 1881, and by New York in 1887. While laws exist in most states at the present time, they are ineffective because of a lack of enforcement; there is either a lack of personal responsibility or a lack of prosecution by authorities. Oftentimes many influential growers, being ignorant of the destructiveness of yellows, oppose the extermination of diseased trees. Frequently inspectors are not competent. In some states growers have no chance to appeal to the decision of inspectors. Healthy trees may be reset on the same ground without danger. At least such trees are no more likely to be affected than their neighbors. Trees should always be purchased from a reliable nursery.
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