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Root-Galls. Abnormal enlargements often appear on the roots of plants. These enlargements are much more frequent than is generally supposed, but from their position under ground are rarely observed. From an economic standpoint they have not received the attention that they merit.

Although the term root-gall is usually applied to the abnormal enlargement of roots due to insects and other animal organisms, it has a much wider application as used by most plant-growers. The presence of nodules or local enlargements on the roots of plants has been discussed by different authors under the names root- galls, root-knots, root-swellings, and the like. In cases in which the cause of the nodules of hypertrophied tissue is known, special names have been assigned to the enlargements. Thus the gall formed by the eel-worm (Heterodera radicicola) is known as the nematode root- gall (Fig. 3428); the enlargement on the roots of cabbage and related plants by the myxomycete (Plas- modiophora Brassicae) is called club-root; the swellings on the roots of the peach, apricot, and many other plants, which are of characteristic appearance and usually appear at the crown of the plant, are known as crown- gall. Root-tubercles are small gall-like bodies found on the roots of many leguminous plants. They are sym- bionic in nature, the organism causing them being helpful to the plant. See Legumes.

Abnormal root enlargements are due to the following causes: (1) animal parasites, as in the nematode root-gall (Fig 3428), the galls formed on the roots of the grape by the phylloxera, woolly aphis galls on apple tree roots, and the like; (2) vegetable parasites, as in the club-root and the crown - gall (Fig. 3429); (3) mechanical injury, causing excessive callous development, root - burls, and so on. In addition to these the  causes of these enlargements are oftentimes obscure or unknown.

Swellings on the roots of the mulberry are said to be due to the hypertrophy of the lenticels. Some investigators have attributed gall-like root-growths in some instances to the hypertrophy of adventitious buds.

The root-galls caused by the nematode (Heterodera radicicola) may usually be readily recognized from other forms of hypertrophied tissue by the numerous knotty enlargements on the smaller roots infested by the worms. By careful search, in most instances, the distended female worms may be found in the infested tissue, where they appear as small nearly spherical pearl-like bodies, readily seen with the unaided eye. This minute worm, commonly called eel-worm, feeds upon the roots of a great variety of cultivated plants and is particularly destructive in the South. It is usually injurious in the northern states only to plants growing under glass. However, ginseng and some other outdoor perennials often suffer severely as far north as Michigan and southern Canada. The most effective remedy in the case of field crops is the removal of all rubbish that would harbor the worms during the winter. In greenhouses steam can be forced through the infested soil. When potted plants are badly affected, they may be severely root-pruned and repotted in soil free from worms.

The root-swellings caused by the grape-vine gall louse (Phylloxera vaslalrix) may bo readily recognized from other root-galls by the presence of the insects. The young insects, by puncturing the epidermis of the roots and sucking the sap, cause the galls to develop. The insect is found on the diseased roots in all stages of development during the summer.

The most effective method of holding the insect in check appears to be in the use of resistant roots, i. e., the grafting of the more tender varieties on roots of those that are stronger and better able to resist the attack of the insect. Bisulfide of carbon in some instances has proved effective in killing the lice.

The crown-gall appears to be the most harmful of root diseases affecting cultivated plants in this country. These galls have been reported upon the roots of the peach, apricot, almond, prune, plum, apple, pear, walnut, grape, raspberry, blackberry, cherry, poplar, and chestnut, and without doubt further investigation will find them on other plants as well. The crown-gall disease is now known to be due to a distinct species of pathogenic bacteria (Bacterium tumefaciens). It appears to be the same organism in all the host-plants affected by this disease. Seedlings from one to six months old appear to be most susceptible to this disease; hence it is particularly serious in the case of nursery stock. When the galls appear on young trees, they almost always occur on the side of the main root a few inches below the surface of the soil, or in the region of the crown. With more mature trees they are likely to occur at greater depth on lateral roots. At first the gall has a uniform outer appearance, but later it becomes warty from unequal growth. The tissue of the developing gall is soft and succulent, with nodules of woody tissue scattered through it. The galls vary much in size and may reach a diameter of 10 inches.

But little is known as to remedies for crown-gall. As the disease is primarily a nursery disease, the most effective remedy is in securing stock for planting from a non-infested nursery. This disease is often destructive to the roots of roses and asters in the greenhouse benches. Here the destruction of all diseased plants followed by steam' sterilization of the soil is the only remedy. 
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