Taxonomy Class Secernentea Order Tylenchida Superfamily Tylenchoidea Family Pratylenchidae Genus Pratylenchus
The genus name is derived from the words pratum (Latin= meadow), tylos (Greek= knob) and enchos ( Greek=spear). Originally described as Tylenchus pratensis by De Man in 1880 from a meadow in England. Pratylenchus scribneri was reported from potato in Tennessee in 1889. Root-lesion nematodes of the genus Pratylenchus are recognised worldwide as major constraints of important economic crops, including banana, cereals, coffee, corn, legumes, peanut, potato and many fruits. Their economic importance in agriculture is due to their wide host range and their distribution in every terrestrial environment on the planet (Castillo and Vovlas, 2007). Plant‐parasitic nematodes of the genus Pratylenchus are among the top three most significant nematode pests of crop and horticultural plants worldwide. There are more than 70 described species, most are polyphagous with a wide range of host plants. Because they do not form obvious feeding patterns characteristic of sedentary endoparasites (e.g. galls or cysts), and all worm‐like stages are mobile and can enter and leave host roots, it is more difficult to recognise their presence and the damage they cause.
Morphology
There are more than 70 described species, fewer than half of them are known to have males. Morphological identification of Pratylenchus species is difficult, requiring considerable subjective evaluation of characters and overlapping morphomertrics. Nematodes in this genus are 0.4-0.5 mm long (under 0.8 mm). No sexual dimorphism in the anterior part of the body. Deirids absent. Lip area low, flattened anteriorly, not offset, or only weakly offset, from body contour. SEM reveals that lip area is characterized by fusion of labial disc with submedial lip sectors; lateral lip sectors not reduced. Esophageal glands overlapping intestine ventrally for a moderate distance. Esophago-intestinal valve not well developed. Phasmids located at mid-tail or slightly posterior. Female: Genital tract monovarial with posterior branch reduced to a post-uterine sac. Female is slender. Tail 2 to 3 times the anal body diameter, terminus rounded (rarely pointed). Males: Have caudal alae (bursa) enveloping tail. Gubernaculum plain, not protruding.
Life cycle
This genus has the typical nematode life cycle with four juvenile stages, and the adults in some species are parthenogenic and in others amphimictic. The juveniles of P. coffeae mature and differentiate within the root, and adult females deposit eggs singly or in small groups within the tissues of the root. Commonly, the duration of the Pratylenchus life cycle is given as about 35-42 days, but this, considerably varies with species, e.g. P. penetrans can range from 30-86 days (Corbett, 1973) while P. coffeae in sweetpotato is from 30-40 days at 25-30oC to 50-60 days at 20oC. The variability is due partly to the length of the life cycle being temperature dependent (Corbett, 1973; Townshend and Anderson, 1976).
Life cycle of the lesion nematode, Pratylenchus spp., which can cause root damage to pyrethrum.
Ecology
Lesion nematodes are migratory root endoparasites. They are also extremely diverse in their adaptive capacity. Individual species show special preferences for temperature. Some are less able than others to tolerate extremes, especially cold, or the extreme drought areas, where vegetable production is less common except under irrigation. The influence of moisture as rain is not as significant. However, P. penetrans moves best when soil water has drained so that 8-12% of the soil volume is air-filled and survives a shorter time in wet than in dry soil . Preferred soil types include coarse-textured sandy loam, silt loam or occasionally, organic soil (muck). More P. penetrans penetrate roots in a sandy loam than in silt loam or loam. They are less common in clays and similar fine-textured soils; adults and juveniles of P. penetrans move farther in coarse than in fine-textured soil. Pratylenchus may be disseminated by transportation of plant root parts or soil, and by surface or irrigation water. Economic importance
Pratylenchus is considered second only to Meloidogyne in terms of plant species parasitised and extent of crop damage and loss. The thresholds vary somewhat with species, climate, soil type and host crop but a density of 1-2 nematodes g-1 soil at planting is a reasonable guide; thresholds range from 0.5-1.8 g-1 soil.
In Japan the principal species is P. coffeae, which caused significant losses, while in the U.S., P. brachyurus, is the most common.
Pratylenchus nematodes, known collectively as root rot nematodes, are worldwide in their distribution, affecting over 500 plant species. Only in a few countries are they reported on the Solanaceae and the damage they cause varies from country to country. The effect of Pratylenchus spp. on tomatoes is little known and it is most likely negligible.