While this disease has not been observed with certainty on plums in the United States, it is known on apple, and its prevalence and destructive nature in Canada and elsewhere would seem to warrant an account here. The disease is known in Canada from Nova Scotia to Vancouver Island, and in Germany, France, England, South Africa and New Zealand. It has been the subject of observation and investigation since 1885, when it was described in France. Outside of the United States, the disease is said to be one of the most widely distributed of plant diseases. While Silver Leaf occurs chiefly on the apple in Canada, it is primarily a plum trouble in other regions where it prevails. The disease, known also as silver blight and silver disease, affects, in addition to the plum and apple, many other fruit - trees and bushes, as well as certain nut and forest trees. Among these may be noted: peach, pear, cherry (wild and cultivated), currant, gooseberry, almond, chestnut and ash.
Fig. 107. - Black Spot on plums; developmental stages.
Of all the fruit-trees the plum is said to suffer most. Among the plums it is quite probable that all varieties are equally susceptible to the spread of the pathogene, once the tree is infected. It is a common notion, however, that soft-wooded varieties are more commonly affected. In many instances this may be explained on the grounds that such varieties are in the majority in a given orchard or locality. On the other hand, if soft-wooded varieties are more susceptible, the explanation may lie in the fact that they are more susceptible to injuries, through which the attack is made. Reports have it that the Victoria, Gibson, Wales, Oullins, Early Rivers, Flemish, Czar, Monarch, Orleans, Washington and Damson suffer more than other varieties. Those said to be free from the disease in orchards where Silver Leaf occurs are: Diamond, Jefferson, Reine Claude, Den - niston, Mirabelle, White Bullace, Sultan, Wales, Englebert and Early Rivers. It will be perceived that certain varieties have been observed by different individuals to be in one case susceptible, and in another, resistant. This is not surprising, since so many factors must be taken into account in the consideration of susceptibility and resistance of varieties to any given disease.
In the beginning the disease is confined to a single branch or twig; but from year to year other branches are affected until the whole tree is involved. Very frequently one or more twigs are killed before the trouble extends to the rest of the tree. In many cases three to six years elapse before the tree is wholly killed.
The external signs of the disease are confined to the leaves. Affected foliage generally remains normal as to size and form, but the surfaces, instead of exhibiting the normal green color, show a peculiar ashen gray luster; the color more nearly approaches that of lead than any other which has been suggested (Fig. 108). This peculiarity is noticeable at a great distance from the trees as well as upon closer examination. After affected trees have died, fruiting bodies of the causal pathogene appear (Fig. 108). The limbs and roots are affected, but give no external evidences of the disease. . But on cutting into these parts the wood will be found to exhibit a brownish discoloration. Frequently discolored hearts are not accompanied by silvery foliage, and vice versa. In certain cases the discoloration of the wood is due to some other cause than the Silver Leaf pathogene. As a general rule only about 33 per cent of the affected trees recover from the effects of the disease, although cases are on record where a tree once entirely silvered finally recovered.
Fig. 108. - Silver Leaf: leaves on upper shoot silvered; leaves on lower shoot healthy. Twigs at right showing fruiting bodies of the causal fungus.
Although the Silver Leaf disease has been known for many years, the true cause was not definitely determined until 1902, when the fungus Stereum purpureum was discovered in this role. Some had held that bacteria were the causal factor involved, while others dismissed the question with the statement that the silvering of plum leaves was the result of some disturbance in the nutrition. Other more or less fantastic speculations regarding the cause of the disease are encountered in a review of the notions held on the subject.
Trees become inoculated above and below the ground; that is, the fungus may enter the tree through the branches or trunks, or through the roots. In general the trunks are the more liable to attack. Spores are blown to the aerial woody parts, where they germinate and penetrate through a wound of some sort. The fungus is not carried by pruning tools; it is not disseminated by means of buds or scions from a diseased tree. However, it should be noted in connection with the latter point that scions from healthy trees when grafted on diseased stock become diseased, as evidenced by their silvered leaves. The spores, which are carried to the trunks and branches, may come from any of the long list of plants which happen to be dead as a result of the attacks of Stereum purpureum. The fruit - bodies (Fig.
108) develop only on dead trees, either standing or in the brush-heap. They also occur on stumps of trees once silvered. So far as root-infection is concerned, the mycelial strands are easily carried in cultivation; they are torn from the roots of affected trees, transferred to roots of neighboring trees, where entrance is gained through an injury made by the cultivator. As intimated above, leaves are never directly affected. Trees infected in the fall show signs of the disease in the spring of the following year. Those infected earlier in the spring or summer may show silvering within one or two weeks, or in some cases only after two months. In all cases of branch-infection the leaves above the infection-court show more silvering than those below the point of attack. And only those leaves on the same side as the infection-court show the disease, those off a straight line from the point of inoculation to the tip remaining normal. It appears then that the real disturbing factor concerned in the silvering of the foliage is conducted rapidly in the sap. This is further supported by the fact that the mycelium of the fungus has never been observed within the leaf-tissues. The hyphae are found, however, in the roots, trunks and branches, in the xylem vessels. Here the walls of the vessels and of the medullary ray-cells are brown and their lumina are filled with a brownish red substance. The cause of the silvering in the leaves is thought to be due to some poisonous substance secreted by the mycelium in the woody portions. There is no alteration of the chloroplastids in the leaf-cells; the normal structure of the leaf is not markedly changed. The epidermis is enlarged, and raised, the elevation resulting in the formation of air spaces between the epidermal and mesophyl layers. Thus the chlorophyl cannot show through the surface; instead, the air spaces give to the leaf surface a lead - colored or silvery appearance.
As previously noted, most trees die, after which the fungus develops its fruit-bodies. These are long - lived, being able to withstand at least 13 months of dry weather. On moistening again, mature spores are ejected.
The customary measures of fruit disease control do not readily apply to Silver Leaf. As a matter of fact, no specific schedule for the control of Silver Leaf, based upon experimentation, is at hand. Certain precautionary measures should be taken:
(1) affected limbs, trunks and stumps should be wholly destroyed, and brush - piles should never be allowed to accumulate;
(2) avoid the use of the limbs or trunks of any of the previously-mentioned fruit - trees as props and posts; (3) avoid injuries to the roots, and to the trunks and branches; (4) avoid planting young trees showing brown hearts.
Gussow, H. T. Der Milchglanz der Obstbaume. Zeitschr. furflanzenkr. 22: 385 - 401. 1912. Gussow, H. T. Preliminary note on "silver leaf" disease of fruit trees.
Phytopath. 1: 177-179. 1911. Percival, John. "Silver Leaf" disease. Linn. Soc. Bot. Journ. 35:
390 - 395. 1902. Aderhold, Rud. Notizen uber einige im vorigen Sommer beobachtete Pflanzenkrankheiten. 4. Milchglanz des Steinobstes. Zeitschr.
fur Pflanzenkr. 5: 86-90. 1895. The Duke of Bedford and Pickering, S. U. Silver Leaf disease. Woburn Exper. Fruit Farm Rept. 12: 1-34. 1910. The Duke of Bedford and Pickering, S. U. Silver Leaf. Woburn Exper. Fruit Farm Rept. 6: 210-224; 234-235. 1906. Brooks, F. T. Silver Leaf disease. Journ. Agr. Sci. 4:133-144. 1911. Brooks, F. T. Silver Leaf disease (II). Journ. Agr. Sci. 5:288 - 308.1913.