Few know that nemas—also called nematodes, roundworms, threadworms—living free in the soil or parasitic on plants, are disease carriers and as such sometimes play an important rôle in the dissemination of bacterial and fungous pests of plants. These little animals, much smaller than earthworms and very different in structure, move about in the soil, often from plant to plant. Their method of entering the host plant varies. Some enter the roots, some climb the stems and enter the leaves and the flowers and thence the seed.

The small size of the nemas, most forms reaching a length of only one-sixtieth to one-eighth of an inch, excludes their traveling long distances. A few yards is all they are likely to cover in a lifetime. But this is enough to enable them to carry germs from an originally diseased plant to neighboring plants. Often hundreds, even thousands upon thousands, of these nemas may be present at a disease center, from which they may carry the germs in all directions. A common feature of such nema-carried diseases is the appearance in the growing crop of small diseased areas which become larger and larger.

However, nemas themselves often are carried long distances by other agents, such as water, wind, and animals, or by man himself, who may transport them with soil on footwear, tools, and other articles, or may inadvertently ship them on or in seeds, seedlings, or other agricultural products. Such passively transported nemas may carry disease germs; such instances are known. It is claimed that the pseudomonas disease of wheat was brought from East India to Egypt associated with the gall nema of wheat.

Nemas, as a rule, have a smooth surface, apparently unfit for the attachment of another organism; yet it has been observed that bacteria as well as fungous spores may become attached to them, sometimes on account of the "stickiness" of either the nemas or the other organism.

Nemas disseminate spores and bacteria by another method. Just as birds feeding upon berries pass the seeds through their intestinal tract unimpaired and often bring them to favorable new locations where they may germinate and grow, so the nemas spread fungi and bacteria. Bacteria may pass uninjured through the intestine, the nema digesting only the slimy mass that often surrounds the bacteria. Or a nema feeding on fungl may digest the mycelium but not the spores, which often have a thick protective covering. These spores are later voided with the feces, and if the nema meanwhile has moved to a new location the spores may find it a suitable place for development.

FIGURE 178.—A larval gall nema, Tylenchus tritici, of wheat carrying various spores (1-5) of the fungus Dilophospora alopecuri (partly after Atanasoff). The sexual organs of the nema are still undeveloped. Two kinds of spores are seen, so-called pycnospores 1 and 4 and secondary spores 2, 3, and 5. The spores are drawn on a somewhat larger scale than the nema

Many nemas have their optimum development in association with rot and decay, which usually means an accumulation of bacteria and fungi. Such a rot on a plant may be the result of an attack by a noxious fungus or bacterium. The nemas moving through this mass will very probably carry some of these germs away to neighboring plants. Because of this kind of association, these nemas are called saprophytic (that is, living on decaying plants).

In the soil nemas play somewhat the rôle that the fly plays in the spread of human diseases. It has been observed, for instance, that a disease of red clover caused by a fungus, Fusarium trifolii, is spread by such nemas from an originally infested plant to neighboring ones. But more remarkable cases of dissemination by nemas of bacterial and fungous plant diseases are known. In one instance the nema, itself a parasite, carries the disease germs not only right to the host plant, but even to a specific part where alone the disease can develop. The gall nema of wheat is claimed to be the carrier of the pseudomonas disease of wheat, a bacterial malady, and also of the dilophospora disease, due to a fungus. Both diseases are always associated with this gall nema, and it is asserted that neither can develop unless the young wheat plant is attacked by this nema. Seemingly the germs attach themselves to the nema (fig. 178) as it moves about in soil water containing them.