Paratylenchus spp. Micoletzky, 1922

California Pest Rating
Paratylenchus spp.  Micoletzky, 1922
Former Pest Rating: D
Current Pest Rating: C 
Initiating Event:

During the 1950-60s, several species of plant parasitic nematodes were given a ‘D’ rating as they were regarded as parasites, predators or organisms of little or no economic importance that did not require State enforced regulatory action.  However, these nematode species were inaccurately assigned a D rating as most, if not all, are plant parasitic and therefore, capable of damaging plant production and causing significant economic losses especially at the county and local residential/grower level.  Furthermore, the detection of plant parasitic nematodes in nursery stock may be an indication of contamination in violation of the State’s standard of pest cleanliness required for nurseries. Paratylenchus spp. were originally rated D.  The risk of infestation and permanent rating of this genus group are re-assessed here.

History & Status:

Background:  Generally, pest risk assessments and assignment of pest ratings are conducted per individual pest species and not per genus group primarily due to differing pest biologies, geographical distributions, host ranges, damage potentials, and risk mitigating requirements. However, an exception to this rule is made here for the genus Paratylenchus largely because of historical practice.  Over the past several decades, the genus, Paratylenchus, was seldom differentiated to species level by CDFA Nematologists mainly due to i) the common occurrence and wide distribution of member species within California, ii) no state enforced regulatory action required subsequent to their detection, and iii) greater demands of time involved in diagnosing high risk and other nematode species considered to be of greater economic importance than those belonging to Paratylenchus.

Members of the genus, Paratylenchus, are commonly known as ‘pin nematodes’.  Species reproduce producing eggs, four juvenile stages (J1, J2, J3, and J4) and adults.  The J1 juvenile stage is produced within the egg and undergoes a molt before the egg taches to release J2.  Males and J4 preadults do not feed. These nematodes inhabit the soil-root region of plants and feed on roots as obligate sedentary ectoparasites. Feeding by adults is limited to epidermal cells and the base of root hairs.  Juveniles may feed far out on the root hairs (Maggenti, 1981).  The nematodes move sluggishly and females become more sluggish after feeding.  The life cycle takes about 23 days for P. bukowinensis (Brzeski, 1976).  The non-feeding preadult of Paratylenchus is the most resistant stage that comprises about 90% of stages found in winter.  They can survive up to four years in the absence of a host.  Apparently, molting to the adult stage occurs in the presence of stimulatory substances emitted by suitable host roots (Mai et al., 1996; Maggenti, 1981).

Taxonomically, Paratylenchus spp. belong to family Paratylenchidae in the order Tylenchida and are characterized by their small body lengths ranging from 0.2 to 0.6 mm.  The genus comprises several species and in California, species diversity is high and yet to be determined.  Morphological identification of some species is difficult and therefore, diagnostics of Paratylenchus spp. can be based on rRNA gene sequences which are reliable genetic markers (Van den Berg et al., 2014).

Hosts: Paratylenchus spp. are associated with the roots of a wide range of herbaceous and woody plants including native plants, ornamentals, and many agricultural crops (Maggenti, 1981).  The host status of associated plants is not always known.  Dropkin (1989) lists host included in the families Cruciferae and Umbelliferae, such as carrot, celeriac, parsley and cabbage.  Other forage and woody plants include clover, grasses, grapes, and fruit trees.

SymptomsGeneral plant damage associated with Paratylenchus spp. is minimal except when nematode populations are high in number.  Under favorable conditions, nematode populations can increase to tremendous numbers and in greenhouse experiments damaging levels of 100-125 nematodes per gram of soil have been found (Maggenti, 1981).  In general, infection by large numbers results in overall reduction in top and root growth without obvious symptoms on roots.  Feeding results in brown necrotic regions in some hosts (Mai et al., 1996; Maggenti, 1981).

Damage Potential:  Generally, Paratylenchus spp. are common associates of several plants.  Few Paratylenchus species, such as P. bukowinensis and P. projectus have known economic importance.  However, plant damage caused by high populations of pin nematodes could be more significant in small-area plant productions and/or containerized crops in nursery, greenhouse, residential and local situations than in large acreages and environments such as, pastures, parks, and cultivated fields. Crop damage under field conditions may be difficult to assess as Paratylenchus spp. are often mixed with other genera and/or two or more stunt nematode species occurring together.

Spread:  The main mode of long and short distance spread through artificial means: infested plant roots, movement of nematode-contaminated soil, run-off and irrigation water, cultivation tools, equipment and any human activity that can move soils from infested to non-infested sites.

Worldwide Distribution: Paratylenchus spp. are distributed widely geographically (Mai, 1996).

Official Control:  Currently, Paratylenchus spp. are D rated pests in California (see ‘Initiating Event’).  Paratylenchus spp. are on the ‘Harmful Organism Lists” for Australia and Nauru (USDA-PCIT, 2016).

California DistributionParatylenchus spp. are widely distributed in California.

California Interceptions: For the past several decades, Paratylenchus spp. have been detected in several imported plant and soil shipments intercepted in California.

The risk Paratylenchus spp. would pose to California is evaluated below.

Consequences of Introduction: 

1) Climate/Host Interaction: Paratylenchus are able to establish throughout the State.

Evaluate if the pest would have suitable hosts and climate to establish in California.

Score: 3

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.

– Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Paratylenchus are known to be associated with several diverse plant species, however, the host status of associated plants is not always known.

Evaluate the host range of the pest.

Score: 3

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

High (3) has a wide host range.

3) Pest Dispersal Potential: The main mode of long and short distance spread through artificial means: Infested plant roots, movement of contaminated soil, run-off and irrigation water, cultivation tools, equipment and any human activity that can move soils from infested to non-infested sites.  Increase in reproduction rates depends on the plant species parasitized.

Evaluate the natural and artificial dispersal potential of the pest.

Score: 3

– Low (1) does not have high reproductive or dispersal potential.

– Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Generally, Paratylenchus are considered mild and common pathogens of plants.  However, under high population levels in residential, nurseries, greenhouse, and other small-area plantings, where high nematode populations can build up under favorable conditions, Paratylenchus spp. infections could result in lowered crop yield.

Evaluate the economic impact of the pest to California using the criteria below.

Economic Impact: A

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Economic Impact Score: 1

Low (1) causes 0 or 1 of these impacts.

– Medium (2) causes 2 of these impacts.

– High (3) causes 3 or more of these impacts.

5) Environmental Impact: The impact of  Paratylenchus spp. on natural environments is most likely not significant as the species is already widespread without causing apparent detriment to ecological balances and processes, however, heavy infestations of pin nematodes, particularly in potted plants, could affect home/urban gardening.

Evaluate the environmental impact of the pest on California using the criteria below.

Environmental Impact: E

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact.

Environmental Impact Score: 2

– Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

– High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Paratylenchus spp.: Medium (12)

Add up the total score and include it here.

-Low = 5-8 points

Medium = 9-12 points

-High = 13-15 points

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation is High (-3).  Paratylenchus spp. are widespread in several contiguous and non-contiguous climate and host regions throughout the state.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score:  Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 9

Uncertainty: 

None.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for the pin nematodes, Paratylenchus spp., is C.

References:

Brzeski, M.  1976.  Paratylenchus bukowinensis.  C.I.H. Descriptions of Plant-parasitic Nematodes Set 6, No. 79.

Mai, W. F., P. G. Mullin, H. H. Lyon, and K. Loeffler.  1996.  Plant parasitic nematodes – a pictorial key to genera.  Fifth Edition.  Comstock Publishing Associates a division of Cornell University Press, Ithaca and London.  277 p.

Maggenti, A.  1981.  General nematology.  Springer-Verlag New York Heidelberg Berlin.  372 p.

USDA-PCIT.  2016.  United States Department of Agriculture, Phytosanitary Certificate Issuance & Tracking System (PCIT). https://pcit.aphis.usda.gov/PExD/faces/ViewPExD.jsp .

Van den Berg, E., L. R. Tiedt, and S. A Subbotin.  2014.  Morphological and molecular characterization of several Paratylenchus Micoletzky, 1922 (Tylenchida: Paratylenchidae) species from South Africa and USA, together with some taxonomic notes.  Nematology 16: 323-358.


Responsible Party:

John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health@cdfa.ca.gov


Comment Period: CLOSED

1/11/2017 – 2/25/2017

Comment Format:

♦  Comments should refer to the appropriate California Pest Rating Proposal Form subsection(s) being commented on, as shown below.

Example Comment:
Consequences of Introduction:  1. Climate/Host Interaction: [Your comment that relates to “Climate/Host Interaction” here.]

♦  Posted comments will not be able to be viewed immediately.

♦  Comments may not be posted if they:

Contain inappropriate language which is not germane to the pest rating proposal;

Contains defamatory, false, inaccurate, abusive, obscene, pornographic, sexually oriented, threatening, racially offensive, discriminatory or illegal material;

Violates agency regulations prohibiting sexual harassment or other forms of discrimination;

Violates agency regulations prohibiting workplace violence, including threats.

♦  Comments may be edited prior to posting to ensure they are entirely germane.

♦  Posted comments shall be those which have been approved in content and posted to the website to be viewed, not just submitted.


Pest Rating: C