Below Ground Symptoms

These may be more useful than top symptoms for diagnosing many nematode problems. Galls, abbreviated roots, necrotic lesions in the root cortex, and root rotting may all help in diagnosing nematode problems.

The most distinctive nematode symptoms on roots are galls caused by Meloidogyne spp. They are small, individual, bead like or fusiform swellings in some hosts. In other plants, galls may be massive lumps of fleshy tissue more than 1 inch in diameter, containing dozens of nematodes. Some hosts, including many grasses, may not form any visible root swelling even though the nematodes successfully establish giant cells, mature, and reproduce. In such cases, an absence of galls does not necessarily prove that there are no root knot nematodes present or that the plant species in question is not a host for that root knot nematode. Other root swellings must not be mistaken for root knot galls. Some ectoparasitic nematodes may cause root tips to swell; by the time root knot galling is visible, atleast some of the galls are found distributed along the roots, away from the tip. Nitrogen fixing bacteria cause swellings on the roots of most legumes (such as clovers, peas and beans). These swellings, called nodules, are easily distinguished from root knot galls by differences in how they are attached to the root and their contents. Nitrogen nodules are loosely attached to the root, and can generally be very easily removed; root knot galls originate from infestion at the center of root, so they are an integral part of the root whose removal requires tearing the cortex apart. In addition, fresh nodules should have a milky pink to brown liquid inside them, while root knot galls have firmer tissues and contain female root knot nematodes (creamy white beads) inside the gall tissues, near the fibrous vascular tissues of the root.

Abbreviated Root Systems

Abbreviated root systems may be caused by several kinds of nematodes. Many ectoparasites which feed on root tips, such as awl, dagger, and stubby root nematodes, cause root elongation to stop. These root tips sometimes swell to greater diameters than usual when they stop growing in length, and often become much darker in color than uninjured root tips. Lateral roots often emerge from the root a short distance behind the injured main root tip; if a series of lateral roots are injured as they emerge, the end of the root may acquire a bunchy or bushy arrangement of very short roots that is very characteristic of awl nematode injury to grasses and some other plants. Migratory endoparasites can also cause abbreviated roots. When lesion, burrowing, or lance nematodes injure the fleshy cortex of host roots, fungi which ordinarily could not penetrate the intact root are often able to colonize the injured tissues and, from there, infest the entire root. Rotted mature tissues at the tip of the root are a clue that endoparasitic nematodes and/or root rot fungi, rather than ectoparasitic nematodes, may have shortened the roots.

Root Rot

Rot of large roots and storage organs (potatoes, etc.) are sometimes caused by nematodes alone. The potato rot nematode, Ditylenchus destructor and yam nematode, Scutellonema bradys are two examples. Burrowing nematodes often cause extensive necrosis, which looks much like dry rot of fleshy tissues of large roots such as those of bananas and many tropical foliage plants.

Cyst Nematodes

An experienced observer can often see cyst nematodes (Heterodera and Globodera spp.) on the roots of their hosts without magnification. The young adult females are visible as tiny white beads. After a female cyst nematode dies, the white body wall is tanned to a tough brown capsule containing several hundred eggs.