Nematoid Worms
(Anguillulidae). The importance of this group of microscopic animals, as giving rise to disease in both wild and cultivated plants, is becoming ever more fully recognised with the extension of careful researches into the nature of diseases that were formerly ill-understood. They differ much from the earthworms, and other true worms, and exhibit a much lower type of structure. They are minute, white or translucent, and usually so small as to resemble short, slender, pieces of hair, even when visible at all without a magnifying glass. All those kinds that cause disease in plants are very minute, and live in the interior of the parts they attack, so that these parts must be cut into, or opened, before worms can be discovered. When seen through a microscope, they appear slender, tapering both ways, but the front end, in which is the mouth, is rather blunt, the hinder end, or tail, is usually long, and tapers gradually, or it may bear an extension of the skin along one or both sides. There is no head; nor are there limbs or organs of sense of any kind visible. The mouth opens in front; on the gullet and intestines there are usually two swollen muscular bodies, which serve for helping to prepare the food in its passage downwards. The intestine opens below in the anus, some distance from the end of the body, the tail, of varying length, lying behind it. The characters of the genera and species are recognisable usually in the mature animals alone. The situation of the sexual openings, and, in the males, two horny outgrowths, connected with reproduction, assist in supplying distinctive characters, as do also peculiarities in the internal organs, which can be made out, with no great difficulty under the microscope, in the living animals. The Anguillulidae are very numerous in species, and they show considerable diversity in habits and modes of life. A few live as parasites in the intestines of animals, e.g., Oxyuris vermicularis in man. Many, probably most, live free in damp earth, or in mud in which there is abundant decaying organic matter, or in decaying plant-remains under water. A certain number live in plants; some in the roots, producing galls or swellings; others in the green vegetative organs (e.g., in leaves of Plantago, of some grasses, such as Agrostis alba), or in the ovary (e.g., in Wheat). Only the plant parasites are here dealt with. They chiefly belong to a genus called Tylenchus, though much attention has been drawn to the ravages of another Nematode, known as Heterodera Schachtii, because of the injury done by it, in Germany, to the roots of the Sugar Beet, as well as to other plants--both wild and cultivated. The species of Anguillulidae seem to be very widely diffused over the world; though the difficulties of identifying the species are so great as to render unreliable the conclusions arrived at by anyone save an expert. The plant parasites usually give rise to a much-increased growth of cellular tissue, thus producing galls of a characteristic aspect. On microscopic examination of sections of the galls, the cells in them are generally found to be much larger than in healthy tissues, and less regular in arrangement and form; and they leave large inter-cellular spaces, or else one large, irregular central space, in which are found numerous young animals, together with oval or cylindrical eggs, the walls of which are so thin as to allow the young animal coiled up in each to be easily seen. Less frequently, along with these may be seen one or two mature individuals. The galls have usually a peculiar colour, due to sap in the cells near the surface. As a rule, there is no trace of an opening to be seen in the mature galls. The young animals seem to be dispersed chiefly by the galls decaying and setting free the eggs or young in the soil, to be blown about with the dust in dry weather. Several of the species--notably that which causes "bunt" in wheat (Tylenchus Tritici)--have a curious faculty of reviving in moisture, after having been dried up for months, or even years. They also seem capable of enduring extremes of cold and heat, up to 125deg. Fahr., without being destroyed; and they are not susceptible to vegetable poisons, though they perish in solutions of acids and of most metallic compounds. In pure water, they survive for a long time.
Cereals are very liable to be attacked, Tylenchus Tritici galling the ovaries, while T. devastatrix, Kuhn (T. dipsaci, J. Kuhn), attacks the stalk, and the inflorescences of the Rye, of various other grasses, and of many other plants, e.g., the Teasel (Dipsacus Fullonum), Clovers, Buckwheat, &c., causing them to remain stunted, with short internodes, and to become yellow. The former species is not troublesome to gardeners; the second one is injurious to many cultivated, as well as to uncultivated, plants. Both species of Tylenchus live entirely hidden from sight, as already described. The males of the genus Tylenchus have usually a thin skin or free membrane attached along one side of the hinder part of the body and the tail. Many other plants suffer through attacks of animals of the same genus; but the hurtful species are not fully known, and considerable doubt exists as to their true number as well as to their relationship to one another. During the past few years, their presence in a good many garden plants has been recorded in the "Gardeners' Chronicle," and other horticultural journals. The diseases produced by them often seriously damage, if they do not even kill, the plants. Not less dangerous than the above are the Nematodes that gall the roots of plants. The swellings usually are to be seen on the root tips, though often they occur higher up the root. They vary greatly in size, from little larger than a pin's head, on fine roots, to the size of a hazel-nut, or even larger. They are usually nearly round, or oval, though irregular forms are also common. The swelling is due to increase in the cellular tissue alone, the fibro-vascular tissue being hardly altered in any way. They are known to occur on a very large number of plants, both native and exotic, wild and cultivated, from grasses to Clover, and even on Coffee-trees, in South America. Among the most valuable plants injured by them must be specially mentioned the Sugar Beet, in its various forms, and also occasionally the Grape Vine.
In the galled roots of many plants a Nematode is met with, characterised by the fertile female becoming swollen, so as to resemble a lemon in form, with traces of the head and tail at the two ends. The female's body becomes united to the surrounding tissues, so as ultimately to look like a large cell merely, in which lie eggs, or young larvae newly emerged from the eggs. This Nematode has received the name of Heterodera Schachtii. Attempts have been made to show that almost all root galls of Nematodes are the work of this species; but in Scotland the writer has never met with Heterodera in rootgalls, but only with true species of Tylenchus. The animals, in autumn, pass from the food-plants, when these die and decay, into the surrounding soil, ready to make their way the following spring to the new crop of food-plants, should there be any such within their reach. Hence, the best mode of prevention consists in changing the crops on any piece of ground, so as to prevent suitable food for the Nematodes being afforded during the ensuing season. On infested soil, therefore, plants liable to attack should be followed by others unsuitable as food for the Worms. It is well, also, to use only clean seed, from a crop in which the disease has not appeared; and farmyard manure is somewhat dangerous, as liable to spread the Worms from diseased to healthy plants, either in the garden or the field.
If any plants show signs of being severely attacked, they should be at once burnt, to prevent extension of the disease from them to others that are healthy.