The Black Knot, or plum-wart, is a conspicuous disease of the plum and cherry, affecting both the wild and cultivated forms. It appears that, on the whole, plums suffer more than cherries. It sometimes happens that when wild plums and choke-cherries are growing in close proximity, even with their branches intermingled, the one may be affected with Black Knot, whereas the other shows no signs of the disease. This is explained on the grounds that the pathogene has become so adapted to growth on the plum, for example, that it is not capable of attacking and infecting the cherry, and vice versa. There has always been more or less discussion of the resistance and susceptibility of various varieties, but on the whole varieties within a species do not show marked differences in this respect. The Trifloras are said to be affected less than any other group of plums, and the Institias rank next in immunity, although the Damson is said to be very susceptible. On the other hand, the Domesticas are susceptible, except possibly the Middleburg and Palatine, which are relatively free from Black Knot. Further observations of the whole question of varietal susceptibility are desirable.
Black Knot is of native origin and has been the subject of horticultural and botanical writings since the beginning of the nineteenth century. It is not possible to state accurately just where the disease originated, but the first records show that many years ago Black Knot was particularly abundant along the northern half of the Atlantic seaboard. The evidence at hand indicates that the disease may have first affected cultivated forms of the plum and cherry in Massachusetts about 1800. It may be pointed out, however, that this portion of the East was first most thickly settled and consequently it was there first noticed. The disease was confined to the eastern United States until about 1879, when the pathogene spread westward, appearing in the vicinity of Cincinnati, Ohio. This invasion of the western states has continued uninterruptedly, and now the disease is found across the northern United States to the Pacific Coast. It occurs less commonly in large portions of the Southwest and Central West.
The disease is one of the most common of stone-fruit tree troubles in America. It is as destructive as it is common. It is of great economic importance because of its wide geographical range and on account of its prevalence for the past century. The loss in dollars incurred during this long period would be difficult to determine. But it is interesting to note that the destructive nature of Black Knot caused plums to be so scarce in New England in the autumn of 1875 that $2.50 was paid for a peck of Damsons in the city of Boston. At one time the raising of cherries was almost abandoned in the State* of Maine on account of Black Knot. The disease has been equally destructive in New York. In one plum orchard which, in 1884, netted $8000, thousands of trees were rooted out in 1885, and from the remaining ones the sum of but $250 was realized. Numerous similar cases might be cited, not only for New York, but for many other states within the geographical range of the disease. Destruction in any degree is a direct result of the death of affected twigs, limbs, and occasionally the trunk.
The disease affects only the woody parts of the host, and usually only the twigs, although the pathogene may spread from affected spurs into the larger limbs or the body of the tree. The normal form of the diseased part is strikingly changed (Figs. 100, 101, 102, 103 and 104). The knot is usually fusiform, but sometimes it may be the same diameter throughout its length, in which case the knot terminates abruptly. As the name suggests, black knots are produced. A knot begins at any of the following places: (1) near the tips of twigs (Fig. 100); (2) in the crotches of younger limbs at the union of the consecutive growths of two seasons; (3) on small spurs which commonly bear the fruits (Fig. 100); (4) near the axil of a leaf; and (5) in the crotches of limbs four or five years old. Knots vary from one-half of an inch to a foot or more in length and from a fraction of an inch to two inches in circumference. Usually the knots do not extend around the limb (Fig. 103), although in some cases they completely surround the affected part (Fig. 104). When their course is long, they tend to proceed spirally about the stem (Fig. 101). In the spring young knots are olive-green in color and at this time are solid but rather pulpy. As the season advances the knots become harder, more brittle, and their surfaces become black. Frequently older knots are attacked by insects which destroy the central part in the knot, leaving the black outer shell. On plums the interior of an old knot is honey - combed. Final stages in the development of the knot show the exudation of gum and the growth on the surface of a pink mold.
Fig. 100. - Small Black Knot swellings about buds.
An interesting response is shown in the case of twigs affected near their tips. When the knot so develops, the twig becomes bent so that a right angle will be made from the knotted side. Sometimes, in the case of affected branches which are not killed, a swelling is produced just above and below the knot.
The Black Knot disease is caused by the fungus Plowrightia morbosa. Its action stimulates the tissues of the twigs and limbs to form the characteristic galls or knots. The fungus begins its work by disseminating its spores in the spring. This process is promiscuous, but some of the spores find lodgment upon the plum or cherry. At various points, already enumerated, the knots have their beginning. With the growth of the spore there develops a system of vegetative threads which pervade the bark and which very soon attack the growing tissue (the cambium) between the bark and the wood. The fungus irritates this region and the tree responds by forming not only an excessive amount, but also an irregular arrangement of the bark tissues (Figs. 103 and 104). This irregularity of development in the affected region proceeds to such an extent that the normal form of the limb is strikingly changed (Fig. 102). The newly forming knot can often be detected in the fall, when it appears as a slight swelling (Fig. 100). It is, however, more conspicuous in the spring; at this time it enlarges and the bark is ruptured, thus exposing a yellowish surface (Fig. 101). This color does not prevail long, but the fungus grows to the exterior and thereon develops its summer spores which give to the knot a velvety olivaceous appearance. These spores are disseminated in April, May and June, being carried by the wind to suitable places for initiating a new knot. As the season advances the gall gradually changes color; by the first of September black dots appear over the surface, and within another month the whole knot is perfectly black and presents the appearance which is so commonly seen (Fig. 102). If one examines such a knot closely, it will be observed that it is then covered by innumerable small elevations which project from its irregular surface (Fig. 102). Each of these elevations constitutes a winter condition of the fungus - a perithecium in which a second kind of spores develops from January to June, depending upon the locality, and which are discharged upon maturity. The further history of these spores is similar to that of the summer spores previously described; they propagate the parasite. The original portion of the knot is thus matured, but the fungus in the bark may continue to grow at either end, thereby increasing the length of the old knot. In this manner a single knot may in time extend a long distance on a branch, its course tending to proceed spirally about the stem. Control. Eradication of the causal fungus is the recognized method of most practical value in the control of Black Knot. In following this principle, it should be remembered that (1) the summer spores are produced abundantly during the late spring and early summer, (2) the winter spores develop from midwinter to spring, and (3) there are many kinds of plums and cherries attacked by these spores. Therefore, prune out the knots before either kind of spores is matured, that is, in the fall or early winter, before January. Look for them in the wild as well as in the cultivated species of plums and cherries. An annual fall inspection of the trees and the removal and destruction of all the knots is the most satisfactory program of eradication. There is no objection to more frequent inspections, but a single annual inspection, if carefully done, should prove sufficient. It is necessary to destroy these knots, otherwise the spores contained therein are likely to be liberated and start the trouble again. This method of control has not given the satisfaction that it should, the difficulty apparently lying in the carelessness of the inspector and the failure of the neighbor to cooperate. It is obvious that if but a few knots are overlooked, the fungus may still operate successfully. And the growers of a given region must make the plan one of joint - action; one individual cannot exterminate the fungus in a locality. If the existing laws were enforced, the extent of injury from this disease would be greatly diminished. The reward for any such eradication measures will be reaped in accordance with the vigilance of the local orchardists.
Fig. 101. - Black Knot; one-year - old knot with a brownish velvety surface.
Fig. 102. - Black Knot; two-year - old knots with black roughened surface.
Fig. 103. - Black Knot; cross-section of a knotted twig - The enlargement has not completely encircled the twig.
Fig. 104. - Black Knot; cross - section of a diseased twig. The knot has completely encircled the twig.
As a measure purely supplemental to the one just outlined the trees should be sprayed with bordeaux mixture, the number of applications depending upon the severity of the case in hand. In cases of a threatening nature the sprayings should begin late in March; five or six subsequent applications at intervals of two or three weeks should be made.
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