Nematode Management
INTRODUCTION
Nematodes are animals. More specifically, nematodes are non-segmented roundworms, which separates them from their close relatives the segmented roundworms, more commonly known as earthworms. Adult nematodes can vary in length from 1/30 inch to nearly 9 feet. Nematodes are commonly found in soil or water, including oceans. They may be the most numerous multicellular organisms on earth. A shovelfull of garden soil typically includes more than 1 million nematodes.
The majority of nematode species are regarded as beneficial. They feed on bacteria, fungi, and other soil-inhabiting or aquatic animals. Some are quite specific in the types of foods they feed on; others are considered omnivores and potentially feed on a wide range of foods.
Some species of nematodes are parasites of plants and animals. The focus of this chapter will be plant-parasitic nematodes. Plant-parasitic nematodes share three common characteristics. First, they are all microscopic, with adults ranging in length from about 1/30 to 1/4 inch in length. Secondly, they are obligate parasites of plants, meaning they must have living plant tissue to feed on to grow and reproduce. Finally, they all possess stylets, which are structures similar to hypodermic needles that nematodes use to puncture plant cells and obtain the cell’s contents. All plant-parasitic nematodes spend at least part of their life cycles in soil, though many are principally found in root or leaf tissue.
PLANT-PARASITIC NEMATODES
Plant-parasitic nematodes are microscopic animals that attack plants. Every species of plant has at least one species of nematode that parasitizes it. The majority of plant-parasitic nematodes (about 95 percent of the described species) feed on roots, either within the root tissue as endoparasites or outside as ectoparasites. Some nematodes feed within leaves. Plant-parasitic nematodes must have living host tissue to feed on to grow and reproduce. If the host dies, nematodes will disperse and search for other plants to invade.
Feeding by plant-parasitic nematodes, in general, does not result in the development of characteristic symptoms. For this reason, nematode problems often go undiagnosed. Typical aboveground symptoms of nematode infections include stunting, yellowing, wilting, and, most importantly from an economic standpoint, reduced yields. A few types of nematodes do produce characteristic symptoms or signs; these will be discussed when specific nematodes are described.
Nematodes are similar to insects in that they possess an exoskeleton. This skin must be shed or molted for a nematode to grow. A typical plant-parasitic nematode life cycle consists of an egg, four preadult stages (referred to as juveniles) and an adult. Females are often more destructive; males typically do not feed. In many species of plantparasitic nematodes, males are rare or not known to exist. The life cycle of a plant-parasitic nematode may be completed in as little as two weeks or as long as two years, depending on the species and the temperature.
Because of their size, plant-parasitic nematodes do not move long distances on their own. They are usually transported over long distances on machinery, in nursery stock, on transplants or seed, or by animals. Anything that moves soil moves nematodes, including water and wind. Some nematodes are known to move a few feet vertically in the soil during a growing season when environmental conditions are adverse.
NEMATODES OF IMPORTANCE IN VEGETABLE PRODUCTION
Northern Root-Knot Nematode (Meloidogyne hapla)
HOST PLANTS: Very wide host range, including virtually all vegetables.
BIOLOGY: The northern root-knot nematode overwinters in the soil as eggs. As soil temperatures increase in the spring, second-stage juveniles emerge, migrate through the soil, and penetrate the roots of host plants. The nematodes establish feeding sites behind the root cap. As the infected root continues to grow, the vascular tissue slips in the area where the nematodes have fed.
Shortly after successfully establishing a feeding site, the second-stage juvenile begins to swell and soon molts to a third-stage juvenile. Eventually, following two more molts, it matures to become an adult female or male nematode. Females are round and incapable of movement. Males are worm-like and generally exit the root because they do not feed. Female NRKN produce large numbers of eggs, up to 3,000, in a gelatinous matrix secreted by the anus.
The northern root-knot nematode can complete its life cycle in a month at optimal soil temperatures. Therefore, the nematode can complete multiple generations per growing season.
SYMPTOMS: The northern root-knot nematode, like many other nematode species that feed on vegetables, does not cause characteristic foliage symptoms. Typical symptoms are stunting, yellowing, and reduced yields. Severely infested plants usually wilt during periods of hot, dry weather because the nematodes disrupt the plant tissue.
Invasion of roots by northern root-knot nematode will result in the production of small swellings on the roots called galls. Galls will vary in size, depending on the numbers of nematodes feeding within them. Carrots are highly susceptible to the northern root-knot nematode, which causes losses due to forking or stubbing of the taproots.
Lesion Nematode (Pratylenchus penetrans)
HOST PLANTS: Virtually all species of cultivated plants.
BIOLOGY: Lesion nematodes overwinter as juveniles and adults within roots or in soil. These nematodes penetrate young roots. Once inside the root, they migrate between and through cells, often killing them. Lesion nematode females lay eggs singly in root tissue or in soil. Females typically produce fewer than 100 eggs. Life cycles can be completed in three to four weeks, depending on soil temperatures. They can complete multiple generations per growing season.
SYMPTOMS: Aboveground symptoms are virtually the same as those produced by the NRKN. Penetration of roots by lesion nematodes results in very small lesions. These wounds create a point of entry for other soil pathogens, such as the fungi Verticillium, Cylindrocarpon, Rhizoctonia, Colletotrichum, and possibly others. Lesion nematode-infected plants typically have reduced root volumes and weights. Feeding and migration by these organisms kills cells. Feeder roots are usually destroyed.
Distinctive brown lesions on chickpea roots indicate root lesion nematode infection
Cyst Nematode (Heterodera spp.)
HOST PLANTS: Four species of cyst nematodes feed on vegetable crops in Michigan. The carrot cyst nematode, Heterodera carotae, has carrot as its only host. The soybean cyst nematode, H. glycines, feeds on green beans and peas. The sugar beet cyst nematode, H. schachtii, feeds on a variety of vegetables, including beets, broccoli, cauliflower, cabbage, Brussels sprouts, turnips and spinach. The clover cyst nematode, H. trifolii, is reported to feed on some vegetables but is not regarded as a serious pest of these crops in Michigan.
BIOLOGY: Cyst nematodes overwinter as eggs within cysts in the soil. Cysts are the dead remains of female nematodes. They are lemon-shaped and about the size of a pinhead. Eggs hatch and second-stage juveniles emerge from the cysts, migrate through the soil, and enter roots to feed. They swell as they grow, eventually becoming large enough that they rupture the root and are exposed to the soil. If cyst nematode-infected roots are examined closely after gently removing the soil, these females can be seen with the naked eye. They are white or yellow depending on age. Males are worm-like and usually exit the root to mate with females.
Female cyst nematodes produce approximately 50 to 100 eggs in a gelatinous matrix outside their bodies, and many more eggs remain within their bodies. The eggs produced in the matrices typically hatch soon after production, whereas the ones contained within the females may not hatch for 10 years or more. The numbers of eggs produced by cyst nematodes vary by species.
SYMPTOMS: Feeding by cyst nematodes results in areas of stunted and yellow plants. Yields can also be significantly reduced. The severity of the symptoms varies according to host and the population densities of the cyst nematodes.
Female cyst nematodes can be observed with an unaided eye on infected plants during the growing season. Females are white or yellow and roughly the size of the period at the end of this sentence. Cyst nematodeinfected plants often have reduced root systems.
Healthy potato plant on the left, damaged plant on the right.
MANAGEMENT OF PLANT-PARASITIC NEMATODES
The best defense against nematodes is to avoid them. Once fields or plant tissues are infected with nematodes, eradication is usually possible. Nematodes are usually transported over long distances by machinery, in plant material, on animals, or by water or wind. Natural disasters such as floods are uncontrollable, but the patterns in which machinery is moved and the sanitation of this equipment can be controlled. These tactics should be considered when trying to avoid nematodes. The bottom line is that anything that moves soil moves nematodes.
Often, fields do become infested with nematodes. If samples indicate the presence of pest nematodes at damage threshold levels, then steps should be taken to reduce the nematode population.
(Source: MSU)
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