The apple and pear particularly are likely to suffer from this root-trouble occurring in the South and Southwest. To growers in these regions it is probably best known as a cotton disease; however, it affects not only fruit-trees and cotton but also forest-trees, vegetables, forage crops and weeds. The range of host - plants is practically unlimited, a matter of no mean consideration from the standpoint of control.
Plants affected by this disease usually show threads of the causal fungus on the roots. These threads are at first whitish, then dirty - white or brown. If cotton shows the disease, then apples and other trees in close proximity are likely to become afflicted. The disease is recognized on cotton by a sudden wilting, usually during the latter part of June or early July, such symptoms being exhibited by isolated plants here and there in the field.
Apples affected with Ozonium Root Rot show a sudden wilting and death of the leaves; this is particularly characteristic of young trees. In case of old trees death is more gradual. In general affected trees have an abnormal, sickly appearance for a year or more prior to actual death. The causal fungus surrounds the tap root as well as the lateral roots; such roots die, decay, and thus cannot function, either in lending mechanical support to the tree or in furnishing the top with water and food.
Numerous theories have been advanced by growers to explain the causal relationships of the Ozonium Root Rot. But, as already intimated, the fungus Ozonium omnivorum creates the disturbance. It lives and spreads in the soil and seems to have a decided preference for the Houston clay or black waxy soils of the Southwest. The fungus grows best where soil aeration is the poorest. A high temperature and plenty of moisture are favorable to its development. Therefore the hot weather of the South favors it; and it flourishes where there is excessive water of irrigation, if other conditions are favorable.
Once in the soil the fungus is washed about, and finally a root is encountered. The bark and woody tissues are penetrated, their cells being killed as the mycelium proceeds; ultimately decay is the result. After its work is complete, the mycelium breaks up into segments which are washed away to other roots of the apple or other plants. It may also spread by growing through the soil on decaying material, or may be carried by tools used in cultivation. No special fruiting bodies are known.
To the grower who contemplates planting an orchard in the Southwest it is advised that the list of possible host-plants be carefully consulted before proceeding. If it is determined that the pathogene is present in the soil of the site proposed for the orchard, it is best to wait from three to five years before planting. Select trees from a nursery known to be clean. Orchard cultivation gives proper aeration for the roots and at the same time presents conditions unfavorable to the fungus. Deep fall plowing has been advised for cotton; this measure may assist not only the cotton-grower but also his neighbor who may wish to grow apples. In severe cases the fungus may be isolated by digging trenches about infected trees, going as deep as the roots penetrate into the soil. All weeds, most particularly the sida, should be destroyed. A tree once found diseased ordinarily cannot be saved; this is due to the fact that there is usually no external indication of the trouble until the whole root - system is practically destroyed. Diseased and dead trees should be removed and destroyed. If a stump is left, it is recommended that the dirt be removed from about the roots, allowing them to dry, then that the whole stump and its root system be burned.
Shear, C. L., and Miles, G. F. Miscellaneous papers. V. The control of Texas Root Rot of cotton. U. S. Agr. Dept. Plant Indus. Bur.
Bul. 102: 39-42. 1907. Pammel, L. H. Cotton Root Rot. Texas Agr. Exp. Sta. Bul. 7: 6 - 30.
(Also Rept. 2: 61-86.) 1890. Galloway, B. T., and Woods, A. F. Diseases of shade and ornamental trees. Root diseases. Southern Root Rot. U. S. Agr. Dept.
Yearbook, 1896: 248-249. 1897. Stevens, F. L. Ozonium omnivorum. In The Fungi which Cause Plant Disease, pp. 662-663. 1913. Duggar, B. M. Root rot of cotton and alfalfa. In Fungous Diseases of Plants, pp. 479 - 481. 1909.