Pratylenchus penetrans




Rev 08/01/2023

Cobb's Lesion Nematode Classification Hosts
Morphology and Anatomy Life Cycle
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           Pratylenchus penetrans (Cobb, 1917) Filipjev and Schuurmans-Stekhoven, 1941

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Morphology and Anatomy:

A,C,D,E,J = female; B,F,G,H = male

Pratylenchus penetrans: Female (left), Male (right), second-stage juvenile and egg (center).
Photograph by Dr. J. D. Eisenback, VPI, 2016 (compressed from the original).

Reported median body size for this species (Length mm; width micrometers; weight micrograms) - Click:

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Widely, mainly in temperate regions, sandy soils.

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Economic Importance:

C-rated pest in California Nematode Pest Rating System.

Considered most economic important plant-parasitic nematode in northeastern U.S. Readily spread in bulbs and rooted nursery stock via irrigation water.

An example that characterizes some of the issues regarding migratory endoparasitic nematodes is provided by the root lesion (Pratylenchus penetrans) nematode problem of Easter lily bulbs in a relatively small acreage of northwestern California and southwestern Oregon.   Bulblets, usually picked from a previous crop, are field-grown for 2 to 4 years until the bulbs are large enough to sell to greenhouse operations for production of flowering plants.  Feeding by Pratylenchus penetrans on the roots retards the rate of growth of bulbs and extends the time necessary for them to achieve the 20-cm circumference used as a basis for marketing.  If bulbs have not reached 20 cm circumference in two years, they are replanted for one or two more years (Hawkins, 1991; Westerdahl et al., 2003).  Further, nematodes in root tissues attached to bulbs are readily distributed throughout the state.

Avoidance of the lesion nematode problem in Easter lilies by crop rotation or planting site selection is difficult.  The nematode has a very wide host range and is supported by the pasture grasses that are commonly grown as an alternative to bulb production.  Many nematicides have been tested for nematode control in the region and   several have resulted in groundwater contamination.  The current standard soil treatment for management of P. penetrans in the region is a preplant application of 1,3-dichloropropane (Telone II) at 428 kg/ha and 9 kg/ha of Phorate applied at planting.  Preplant drip applications of metam sodium or emulsifiable 1,3-dichloropropene in combination with oxamyl or metalaxyl show promise as a means of reducing the total amount of chemical applied to soil and reducing the risk of groundwater contamination (Westerdahl, et al. 2003).

Potato Early Dying Disease is caused by Verticiliiunm dahliae and exacerbated through interaction with  P. penetrans.  The effect of the nematode may be to:

  1. provide entry points for the fungus by feeding and endoparasitic migration.

  2. reduce host defense mechanisms.

Although Verticillium and Pratylenchus are considered the main causal agents in some potato growing areas of North America, other pathogens such as the bacterium Erwinia and the fungus Colletotrichum coccodes are associated with symptoms similar to
Potato Early Dying Disease.

In the early 1960s in California, a nematode survey of pear orchards was conducted in response to the occurrence of pear decline. Of the several different species of Pratylenchus reported in the state, only P. vulnus and P. penetrans were recovered from pear roots (Chitambar et al., 2018).


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Migratory endoparasite, mainly in root cortex but may enter vascular tissues in later stages of infection.

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350+ hosts, apple and cherry orchards, fruit trees, conifers, roses.

Pest of lily bulbs (grown on 300 acres for bulb production) in Del Norte County, CA.

Lily bulbs are rotated with dairy pasture which is also a host to the nematode.

For an extensive host range list for this species, click

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Life Cycle:

Ecophysiological Parameters:

For Ecophysiological Parameters for this species, click If species level data are not available, click for genus level parameters


Like other Pratylenchus species, P. penetrans has six life stages: egg, four juvenile stages and adult. First-stage juveniles develop within the egg, followed by a first molt to the second stage juvenile that hatches from the egg.

All juvenile and adult stages are vermiform. All post-hatch stages are motile and can infect plants.

Generally, root lesion nematodes have a life cycle of 45-65 days; they survive the winter in infected roots or soil as eggs, juveniles, or adults. In the spring, when plant growth is active, eggs hatch, nematodes are attracted to the plant roots and begin to feed and continue their life cycle within roots or in rhizosphere soil. Within the root, the nematode feeds on cortical tissue causing necrosis of cortical cells, cell breakdown, and formation of cavities. Necrosis is apparent as lesions that expand as the nematodes move lengthwise within the infected roots. Some nematodes may leave the root, enter soil, and re-enter the root at a different site causing a new infection (Chitambar et al., 2018).

Pratylenchus penetrans is sexually reproducing, males and females are present.

Chromosome number 2n=12, n=6.

Eggs deposited in roots and soil.

Wolbachia endosymbiont:

The endosymbiont bacterium Wolbachia has been detected in Pratylenchus penetrans. Ribosomal rRNA 16S similarity and presence of homologs to all Wolbachia marker genes clearly placed the bacterium from P. penetrans within the genus Wolbachia  while phylogenomicanalysis placed it at the base of the tree, suggesting that plant-parasitic nematodes were the first hosts for Wolbachia.

The study suggests an earlier transition to mutualism before the transition of Wolbachia to filarial nematode hosts. It is speculated that mutualism of Wolbachia endosymbionts arosee through horizontal gene transfer..

Pratylenchus penetrans appears to obligately depend on its Wolbachia endosymbions for survival, suggesting that mutualism is and ancestral trait.

A question that remains is how obligate mutualism of Wolbacia arose in filarial nematode hosts.

Nematode Virus

To date, few viruses have been identified within representatives of the phylum Nematoda. In 2019, Vieira and Nemchinov reported the root lesion nematode virus (RLNV1) associated with P. penetrans. Using moleculat techniques, te virus has been found to be widespread in North American populations of P. penetrans (Viera et al., 2020). The virus was detected in nematodes collected from a wide range of crop species spanning the breadth of the host range of this nematode.  Effects of the virus on the fitness or life-span of the nematode have not been determined. However, viruses associated with C. elegans (Orsay virus) and C. briggsae aknown to infect intestinal cells, to be  horizontally transmitted, and to slow activity of progent of infected nematodes (Viera et al., 2020).


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Necrotic lesions on peach roots after 24 hours. Symptoms of debilitation above ground, including chlorosis and twig die-back. Damage is increased by interaction with fungi.

Peach roots contain amygdalin (a cyanophoric ß-glucoside - essentially a complex sugar with a cyanide molecule attached). Amygdalin is a common constituent of Prunus spp., and is thought to be hydrolyzed during a series of molecular events initiated by the plant in response to invasion.  The hydrolysis products of amygdalin are hydrogen cyanide and benzaldehyde which are phytotoxic and result in cell death.

In a classical study, Mountain and Patrick (1959) found no amygdalin in root lesions associated with P. penetrans; they concluded that it is hydralyzed as either the plant invokes the defense mechanism against P. penetrans or that the nematode initiates the hydrolysis.  They also found that necrosis and cell death in the root tissue occurs rapidly following root invasion by the nematode and that it occurs in advance of the invading nematode.  An interpretation of thes observations in light of current understanding of plant defense mechanisms is that the plant is recognizing damage associated moleccular patterns (DAMPs) or parasite/pathogen-associated molecular patterns (PAMPs) and initiating defenses that result in cell death in advance of the invading nematode.  In their studies, Mountain and Patrick (1959) found that the necrosis could be initiated by the nematode alone and did not require the presence of fungi or bacteria, but that an imprtant contributor to peach replant problems by the nematode was that root degeneration was accelerated by the infection courts for fungi and bacteria that were provided by the damage.

In roots of pea, dill and alfalfa, male nematodes incited fewer and smaller lesions in the root cortex than female nematodes (Saikai and MacGuidwin, 2020).


Effect of different population densities of Pratylenchus penetrans on growth of onions (0, 666, 2,000, 6,000, 18,000 nematodes/Kg soil) (Canada).

Damage to lily bulb in Del Norte County is a reduction in the rate of growth.  Consequently, bulbs require two years to reach marketable size rather than one year.  Further, harvested bulbs are infested with the endoparasitic nematode and will be transported to new sites.

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Nematicides are effective on soil populations, but have resulted in groundwater pollution in the sandy soils of Del Norte County.

Chemical dips have been used for rooted material.

Hot water treatment at 45.5 C for 15 to 20 min kills the nematode in apple roots, 48 C for 10 min in strawberry roots.   Hot water dips are also used to treat harvested lily bulbs, but the difference between temperatures that kill nematodes an temperatures that cause damage to flower bud primordia is quite small.  Temperature regulation is critical.

Crop residues and organic amendments reduce populations in apple orchards, either by breakdown products or by encouraging growth of nematode-trapping fungi.

Treatment Thresholds. Severity of Potato Early Dying is related to the inoculum levels of fungus and nematode prior to planting.  Damage threshold levels are Verticillium >  4 propagules /gm of dry soil in the absence of P. penetrans and 2 propagules/gm dry soil in the presence of P. penetrans

Host Plant Resistance, Non-hosts and Crop Rotation alternatives:

For plants reported to have some level of resistance to this species, click
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Brown, A.M.V., S.K.Wasala, D.K. Howe, A.B. Peetz, I. A. Zasada, D.R. Denver. 2016.Genomic evidence for plant-parasitic nematodes as the earliest Wolbachia hosts . Scientific Reports | 6:34955 | DOI: 10.1038/srep34955

Chitambar, J. J., Westerdahl, B. B., and Subbotin, S. A. 2018. Plant Parasitic Nematodes in California Agriculture. In Subbotin, S., Chitambar J., (eds) Plant Parasitic Nematodes in Sustainable Agriculture of North America. Sustainability in Plant and Crop Protection. Springer, Cham.

CIH Descriptions of Plant-parasitic Nematodes, Set 2, No. 25 (1973).

Hawkins, L. 1991. IPM options for Easter lily bulb production: An overview. California Department of Food and Agriculture. PM91-2.

Mountain, W.B. and Patrick, Z.A. 1959. The peach replant problem in Ontario: VII Pathogenticity of Pratylenchus penetrans (Cobb, 1917) Filip. & Stek. 1941.  Canadian Journal of Botany 37:459-470.

Roman, J., Triantaphyllou, A.C. 1969. Gametogenesis oand reproduction of seven species of Pratylenchus. J. Nematology 1:357-362.

Saikai, K. , MacGuidwin, A. 2020. Difference in lesion formation by male and female Pratylenchus penetrans. J. Nematology 52:| DOI: 10.21307/jofnem-2020-090

Scheck, H. 2023. California pest rating proposal for Pratylenchus penetrans (Cobb, 1917) Filipjev & Schuurmans Stekhoven, 1941 Cobb�s lesion nematode. CDFA, Sacramento.

Sher, S. A., and Allen, M. W. 1953. Revision of the genus Pratylenchus (Nematoda: Tylenchidae). University of California Publications in Zoology, 57, 441�447.

Vieira, P. and Nemchinov, L. G. 2019. A novel species of RNA virus associated with root lesion nematode Pratylenchus penetrans. Journal of General Virology 100: 704-708.

Viera, P., Peetz, A., Mirnee, B., Saikai, K., Mollov, D., MacGuidwin, A., Zasada, I., Nemchinov, L.G. 2020. Prevalence of the root lesion nematode virus (RLNV1) in populations of Pratylenchus penetrans from North America. J. Nematology 52: | DOI: 10.21307/jofnem-2020-045

Westerdahl, B.B., D. Giraud, S. Etter, L.J. Riddle, J.D. Radewald, C.A. Anderson, and J. Darso. 2003. Management options for Pratylenchus penetrans in Easter lily. Journal of Nematology 35:443-449.

Copyright © 1999 by Howard Ferris.
Revised: August 01, 2023.