Pratylenchus is a genus of plant-parasitic nematode

Nematode

The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...

s responsible for root lesions on many plant hosts in temperate regions around the world. Lesion nematodes are migratory endoparasites that enter the host root for feeding and reproduction and move freely through or out of the root tissue. They do not become sedentary in the roots, as do the cyst or root-knot nematodes. Feeding is restricted almost entirely to the cortex of the root. Species differentiation is usually based on morphology; especially by variation in mouthparts known as stylets.

Hosts and symptoms

Root Lesion Nematodes infect a larger variety of hosts than any other type of nematode. Pratylenchus penetrans in itself has over 350 hosts. They can range from potatoes and corn, to bananas and wheat. While the range of hosts might be huge, there are specific hosts that Root Lesion Nematodes cause the most significant damage to. Pratylenchus is found to be associated with corn more than any other host. The symptoms are fairly similar throughout each of the different hosts. As the nematode enters the host’s root, it creates a hole…or lesion… in the root. This initial puncture of the root turns into brown or black necrotic lesions. As the root begins to die, you begin to see secondary symptoms above ground. These symptoms may include stunting, chlorosis in the leaves, wilt like symptoms, and may eventually lead to death. These symptoms can usually be seen in small patches throughout the field as nematodes tend to have a patchy distribution throughout the field. In serious cases, root lesion nematodes can cause significant yield loss.

Environment

Pratylenchus can endure a wide range of environmental conditions. Moist temperate soils are ideal conditions for breeding and migrating underground, but they can persist even in warm and dryer environments. When the nematodes are susceptible to drying they lie quiescent until moisture increases and plants resume growth.
Lesion nematodes remain inactive when soil temperatures are below 59°F (15°C); except for P. penetrans, there is little activity until temperatures rise above 68°F (20°C). Sandy soils and weed infestation are other factors that increase the likelihood of Pratylenchus prevalence. Several parameters influence the soil environment, and the optimal conditions may vary between Pratylenchus species. Some important factors that can alter Pratylenchus abundance include: soil moisture, mineral components, temperature, aeration, organic matter, and pH.

Management

While a normal distribution of Pratylenchus is patchy, serious cases can destroy an entire field of a crop. Seeing as complete destruction is devastating, management and control are extremely important. There are several different methods that can be used in order to help control or eliminate problems with root lesion nematodes. The genus Pratylenchus has several different species and each is controlled in a different manor.

In general there are a couple major ways to manage root lesion nematodes. The first includes soil fumigation and the use of nematicide

Nematicide

A nematicide is a type of chemical pesticide used to kill parasitic nematodes.One common nematicide is obtained from neem cake, the residue obtained after cold-pressing the fruit and kernels of the neem tree. Known by several names in the world, the tree was first cultivated in India in ancient...

s. Either injected into the soil, or sprayed onto the plants, both of these options introduce a nematode killing substance into the environment that the nematode lives in to hopefully decrease the nematode problem. While this may work with some species, it does not work with all of them and can get expensive. This is normally seen only in more expensive crops where the cost of the nematicides can be offset and well worth it. This can be used for P. brachyurus for example.

The next type of control involves water immersion. If infected hosts are transplanted, some may have successful management through a hot water bath. Transplants are immersed in 54˚C water bath for 30 minutes. This practice should be enough to kill the nematodes that are infecting the plant without damaging the plant itself. P. coffeae is one of the species that can be controlled by this treatment.

Another possibility includes crop rotation. Different hosts are susceptible to different species of Pratylenchus. If you have one species of Pratylenchus, planting a crop not susceptible to that specific species will eliminate the problem. Or for P. neglectus specifically, a 4-5 year crop rotation will allow the problem to subside.

Finally, the best, and sometimes only form of control with root lesion nematodes is resistance. By selecting plant genotypes that are resistant to affects of the nematodes, you don’t have to worry about any other form of control. In the case of P. vulnus there is no good way to control it other than to use host resistant varieties. Resistance has been found for several different hosts and species of Pratylenchus and more research is constantly being done to find even more.

Life/disease cycle

Root lesion nematodes are migratory endoparasites. This means that they usually feed on the roots internally versus ectoparasites that feed on the roots externally. The migratory endoparasite enters the root by puncturing a hole in an outside cell with its stylet. Once the nematode has access to the inside of the cell, it continues to migrate from one cell to the next feeding along its way. As it moves through the cells, and takes nutrients from them, the cells are left with tiny lesions that eventually become necrotic as the root begins to decay. As the nematode feeds on the cells in order get nutrients, the metacarpal pump (a part of a nematode) begins to “pump” nutrients are ingested into the nematode. Over the course of feeding on a specific root, female nematodes lay single eggs that can hatch in the root (or if the egg was laid in the soil, it hatches in the soil) and the process begins again with the juvenile nematode. Pratylenchus does not need both male and female to reproduce; in fact the females can and do lay eggs without the presence of males. For some species, the number of females drastically outnumbers the number of males.

Importance

The genus Pratylenchus is considered one of the most important among plant-parasitic nematodes because of its wide range of hosts and worldwide distribution. They may reduce root growth or inhibit root development by forming local lesions on young roots. The lesions may also lead to secondary infections by fungi or bacteria, increasing the plant's susceptibility. The overall root damage associated with Pratylenchus can cause poor growth, reduce crop yields, or kill off the plant completely. These nematodes hold economic importance if crop loss is drastic and profits decrease.