Helicotylenchus spp. Steiner, 1945

helicotylenchus spp.
California Pest Rating for
Helicotylenchus spp.  Steiner, 1945
Former Pest Rating: D
FINAL  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. Helicotylenchus 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 Helicotylenchus largely because of historical practice.  Over the past several decades, the genus, Helicotylenchus, 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 Helicotylenchus.  Alternately, certain species of the genus  that are capable of invading plant roots (e.g., H. multicinctus B-rated) may be assessed and rated individually as they can be more economically damaging to crop production and trade markets than non-root invading species.

Members of the genus, Helicotylenchus, are commonly known as ‘spiral nematodes’ since they assume a spiral form when relaxed with gentle heat.  Species reproduce mainly by parthenogenesis producing eggs, three juvenile stages, and adults.  These nematodes usually inhabit the soil-root region of plants and feed as obligate ectoparasites with their stylet (sword-like hollow tooth) inserted into the root. All motile juvenile and adult stages of Helicotylenchus feed. All species are parasitic on roots and other underground parts of plants (Siddiqi, 1972).

Hosts: Helicotylenchus spp. can attack a wide range of plants including agricultural crops, fruit trees, ornamentals, nursery stock, forest trees and shrubs, desert shrubs, grasses, and weeds. Species have been associated with different plants in soil around the root zone, however, the host status of associated plants is not always known.

SymptomsFeeding of Helicotylenchus spp. results in production of small discolored lesions in the root cortex and other underground parts.  Local lesions in the cortex result in death of cells on which the nematodes feed.  Feeding of high population levels of Helicotylenchus can severely damage roots by causing them to become slightly swollen, spongy, and discolored.  Eventually the cortex of such roots is sloughed off (Maggenti, 1981; Mai et al., 1960). Above ground symptoms may express yellowing of foliage, mild stunting, wilt and defoliation – depending on the population level of spiral nematodes present.

Damage Potential:  Plant damage caused by high populations of spiral nematodes may be more significant in small-area plant productions and/or containerized crops in nursery, residential and local situations than in large acreages and environments such as, pastures, parks, and cultivated fields.  Crop damage under field conditions is difficult to assess as Helicotylenchus spp. are often mixed with other genera and/or two or more spiral nematode species occurring together (Norton, 1984).  Helicotylenchus spp. are considered mild plant pathogens.  Crop losses under field conditions are not reported, however, under experimental conditions, reductions in root and total plant weight have been observed in cereals and grasses (Griffin, 1984).

Spread:  The main mode of long and short distance spread is through artificial means: 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: Helicotylenchus spp. are distributed worldwide.

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

California DistributionHelicotylenchus spp. are distributed throughout California.

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

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

Consequences of Introduction:

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– 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.

Risk is High (3) Helicotylenchus spp. are able to establish throughout the State.

2) Known Pest Host Range: Evaluate the host range of the pest. Score:

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

– Medium (2) has a moderate host range.

High (3) has a wide host range.

Risk is High (3) – Helicotylenchus spp. are known to be associated with several diverse plant species, however, the host status of associated plants is not always known.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score:

– 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.

Risk is High (3) – The main mode of long and short distance spread is through artificial means: 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.

4) Economic Impact: Evaluate the economic impact of the pest to California using the criteria below. Score:

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.

– 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.

Risk is Low (1) – Generally, Helicotylenchus spp. are considered mild pathogens of plants.  However, under high population levels in residential, nurseries and other small-area plantings, Helicotylenchus spp. infections could result in lowered crop yield.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

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. Score:

– 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.

Risk is Medium (2) – The impact of Helicotylenchus 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 spiral nematodes could affect home/urban gardening.

Consequences of Introduction to California for Helicotylenchus spp.:

Add up the total score and include it here. (Score)

-Low = 5-8 points

Medium = 9-12 points

-High = 13-15 points

Total points obtained on evaluation of consequences of introduction of Helicotylenchus spp. to California = (12).

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)Helicotylenchus spp. are widely spread 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 spiral nematodes, Helicotylenchus spp., is C.

References:

Griffin, G. D.  1984.  Nematode parasites of alfalfa, cereals, and grasses.  In Plant and Insect Nematodes, edited by W. R. Nickle.  Marcel Dekker, Inc.  New York and Basel.  243-321 pp.

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.

Norton, D. C.  1984.  Nematode parasites of corn. In Plant and Insect Nematodes, edited by W. R. Nickle.  Marcel Dekker, Inc.  New York and Basel. 61-94 pp.

Siddiqi, M. R.  1972.  Helicotylenchus dihystera.  Commonwealth Institute of Helminthology (C. I. H.) descriptions of plant-parasitic nematodes. Set 1, No. 9.

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


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

The 45-day comment period opened on Jun 2, 2016 and closed on Jul 17, 2016.


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Pest Rating: C