This disease is variously known as the shoe-string fungus rot, crown Rot, mushroom Root Rot and Armillaria Root Rot. In addition to the apple many other trees and shrubs are known to be affected, some more seriously than others. The discussion is presented under Apple, inasmuch as this host is so generally distributed and because the disease is one of considerable importance to the apple-grower in some regions. In certain sections, however, Armillaria Root Rot is more prevalent on other plants. In the state of Washington the chief damage is done to prunes. In Oregon, prunes and apples are most severely affected. In these states the blackberry, raspberry, cherry, plum, gooseberry, peach, currant and loganberry are at times badly injured by this root disease. In Europe the trouble is found on forest trees, including the birch, beech, walnut, oak, chestnut, ash, pine, larch, alder, fir, willow and cotton - wood. These trees are liable to affliction in America, but as already intimated the disease in this country affects chiefly our more important fruits. In addition to the long lists of plants already given, the following may be added as minor hosts: olive, grape, crab, maple and potato. It is obvious, then, that a great number of widely related plants are likely to be affected by this disease. This fact complicates the matter of control, and is a factor in the economic importance of the disease.
It is believed that Armillaria Root Rot was observed in America as early as 1887 on grape near St. Louis, and later in the states of Texas and California. At present the regions most infested are those of the Central, Southwest and the Pacific Coast region. The disease has attracted particular attention along the Pacific Coast during the last fifteen years.
The damage done by this disease would be difficult to estimate. But bearing in mind the fact that all of the more common fruits are susceptible, and in many localities are severely injured, it is not impossible to gain a fair impression of the amount of losses incurred. In the state of Arkansas it is stated that the losses due to Root Rot exceed those of any other disease of the apple. In other localities doubtless a similar condition exists. As has already been pointed out, the disease in some states is more severe on the apple than on other fruits; again, the reverse is true. It may not be out of place to cite an example of the possible damage that this Root Rot may do. In a western prune orchard of about one thousand trees, an average of about eighty trees died each year for seven consecutive years as a result of Armillaria Root Rot. In seven years half the orchard was destroyed. In another case, an orchard of five thousand trees was reduced at the rate of two hundred and eighty trees per year for three consecutive years, the total trees killed being eight hundred and fifty. Losses of this kind are always keenly felt.
The disease assumes importance in one or more of the following ways: (1) the roots are killed and hence the trees die in one or more seasons; (2) the crown may be injured to such an h extent that the tree may be blown over under the strain of a heavy crop of fruit; (3) affected trees in bearing usually fail to mature their fruit, particularly in cases of severe infection; or the fruit matures poorly, is stunted and is of an inferior quality; (4) diseased trees often lack the normal amount of foliage; (5) affected plants make little or no growth.
Symptoms of Armillaria Root Rot. Evidence of this trouble varies with the part affected and there are no visible signs until considerable progress has been made by the pathogene. A striking characteristic is the marked localization and the slow development of the disease. The average observer will not suspect the presence of Root Rot until individuals or groups of trees die among many other apparently healthy ones. At some point it will then be found upon careful examination that the tree is girdled (Fig. 26) and that finally the top dies. Accompanying this phenomenon is a profuse development of string-like, hard, black, shiny, much-branched strands, 1/25 to 1/12 of an inch in diameter, in the soil at the base of the tree (Fig. 27). These strands have a peculiar mushroom odor and a tough texture. From these rhizomorphs, as they are called, numerous white-gilled, honey - colored mushrooms may arise in the autumn (Fig. 27). The mushrooms may be found at the base of the affected tree or at some distance along the roots.
Fig. 26. Armillaria Root Rot; note the girdling of the root.
The first external evidence of Armillaria Root Rot is that of a poor growth of the affected tree, accompanied by a yellowing or a wilting of the leaves in midsummer or later. In this stage the tree is beyond recovery, for infection in such a case occurred one to three years previously. Old and young trees are alike attacked by this Root Rot pathogene.
This Root Rot is caused by a mushroom known as Armillaria mellea. Sometimes the common names honey mushroom, or honey Agaric, are applied to it, on account of its light yellowish brown color. It occurs widely in the woods, orchards, berry patches and in newly cleared lands, and is extremely variable in form and appearance. The long list of plants which it may attack affords opportunities for an almost certain perpetuation. It may live as a saprophyte on buried wood, spreading through the soil by means of its rhizomorphs. In some fashion or other, the roots of the doomed tree are injured, and through such wounds the rhizomorphs (Fig. 27) of the pathogene enter the bark. Some contend that they enter roots through uninjured bark. The roots may be penetrated by way of the trunk. The rhizomorphs upon entering the root tissues spread out into the separate hyphae which make up the strand. The hyphse, or mycelial threads, grow between and into the root-cells, killing them, and finally the root decays. Destruction of the tree probably never proceeds more than four inches above the ground on account of a lack of moisture. In living or dead roots and root - stocks the pathogene may live for several years. As already mentioned under Symptoms, the mushroom itself develops from the rhizomorphs in the months of September, October and early November, and rarely in the spring. The mushroom obtains food from the rhizomorphs, and maturity is thus reached in a very short time. At full growth the mushroom varies from three to seven inches in height; the cap measures from two to six inches across; the cap is conical, yellowish at first but becoming dark with age; the gills are white, with reddish brown spots; the stem is swollen near the base, and possesses an annulus which is conspicuous in early stages of development but sometimes only slightly developed or even wanting. The mushrooms often grow in clusters, and, while edible, are not choice.
Fig. 27. Armillaria Root Rot fungus; rhizomorphs and young sporophores.
The system of rhizomorphs which attacks the roots actually travels through the soil from tree to tree. Thus infection occurs underground. From the gills of the mature fruiting bodies, or mushrooms, spores are liberated in great numbers. These spores are carried through the air for long distances and doubtless account for isolated cases of Armillaria Root Rot. The spores germinate in the soil, feeding on humus, and produce mycelial threads which unite to form the cord - like strands, or rhizomorphs.
If trees are once affected, there is little hope of saving or curing them. Nursery inspection is not reliable. The rhizomorphs may be broken off accidentally or intentionally, and thus there would be no certain evidence of the disease. In such a case, too, the tree concerned may have just been infected. In this way a diseased tree does not appear to be infected, and therefore it is certified. Thus the fungus is carried in nursery - stock.
As with a great many plant diseases, the elimination of the source of the inoculum offers some relief as a control measure. In selecting a site for orchard-planting ,cape should be taken to determine whether the fungus is in the soil Newly cleared land should be held in high suspicion. For the pathogene, as already pointed out, may live in the soil as a saprophyte on dead parts such as roots, chunks and stumps. It is a better policy to grow crops other than those listed as susceptible to Armillaria Root Rot for a period of at least three years. Likewise if a diseased tree is removed, another should not be planted in its place for three years. The same applies to a tree which is dead as a result of the attacks of this fungus. If it is desired to leave the tree in the soil, it may be isolated by digging a trench to prevent spread of the rhizomorphs to neighboring healthy trees. The trench should be dug near the ends of the roots and need not be more than one foot wide and two feet deep. Throw the dirt toward the tree, since such soil may contain rhizomorphs of the pathogene. In Oregon recent work shows that satisfactory results may be obtained in the control of this disease by . an "aeration method." By this method affected trees may be recovered. The roots are exposed about the crown of the tree; all diseased roots and bark are removed; the wounds disinfected; and the roots thus left exposed to the sun during the remainder of the summer. Finally all wounds are covered with some good wound - dressing and the roots are covered with soil the following winter. The application of chemicals to the soil offers some promise, although no experimental data are available on which to base reliable recommendations.
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