Scutellonema spp. Cobb, 1913

California Pest Rating for

Scutellonema spp. Cobb, 1913

Pest Rating: C

PEST RATING PROFILE

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. Scutellonema 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 Scutellonema largely because of historical practice. Over the past several decades, the genus, Scutellonema, 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 Scutellonema.

Members of the genus, Scutellonema, are commonly known as ‘spiral nematodes’. Apparently, species reproduce mainly by amphimixis (fertilization by female and male) or parthenogenesis where males are rare or absent, producing eggs, three juvenile stages, and adults. These nematodes usually inhabit the soil-root region of plants and feed primarily as obligate migratory ectoparasites, while some species feed as endoparasites of roots. All motile juvenile and adult stages feed and all species of the genus are considered to be plant parasites. The life cycle takes about one month. The nematode may survive in dry soil by anhydrobiosis in a coiled state (CABI, 2016; Mai et al., 1996; Maggenti, 1981).

Hosts: Scutellonema spp. are associated with the roots of a wide range of agricultural crops, fruit trees, ornamentals, nursery stock, forest trees and shrubs, desert shrubs, grasses, and weeds. Agricultural crops include alfalfa, cotton, tobacco, potato, sweet potato, corn, coconut, banana, yams, and several other crops. The host status of associated plants is not always known.

Symptoms: General plant damage associated with Scutellonema spp. is commonly exhibited in roots as numerous small, brown necrotic lesion produced as a result of feeding by the nematode. Internally, isolated root cavities are produced by the nematodes. Above ground symptoms may include leaf stunting and chlorosis, and reduced growth. On yams, large numbers of Scutellonema (S. bradys) feed as endoparasites and cause extensive dry rot damage. Shallow lesion caused by the feeding of the nematode become avenues for secondary invaders, namely bacteria, fungi and mites (CABI, 2016; Maggenti, 1981; Van Den Berg, et al., 2013).

Damage Potential: Generally, Scutellonema spp. are common associates of several plants. Few Scutellonema species have known economic importance as agricultural pests while other species cause no known or little economic damage to crops (Van Den Berg, et al., 2013). However, plant damage caused by high populations of spiral nematodes could 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 may be difficult to assess as Scutellonema 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 plants, including tubers, 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: Scutellonema spp. are widely distributed in tropical and subtropical regions of the world, with the most number of species reported from Africa (Sher, 1964).

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

California Distribution: Scutellonema spp. are widely distributed in California.

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

The risk Scutellonema 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) –Scutellonema 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) –Scutellonema 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 through artificial means: Infested plant roots and subterranean storage plant parts (e.g., tubers, bulbs), 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, Scutellonema spp. are considered mild and common pathogens of plants. However, under high population levels in residential, nurseries and other small-area plantings, Scutellonema 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 Scutellonema 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 Scutellonema 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 Scutellonema 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). Scutellonema 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 spiral nematodes, Scutellonema spp., is C.

References:

CABI, 2016. Scutellonema brachyurus full datasheet. http://www.cabi.org/cpc/datasheet/49316 .

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.

Sher, S. A. 1964. Revision of the Hoplolaiminae (Nematoda) III. Scutellonema Andrássy, 1958. Nematologica 9: 421-443.

Siddiqi, M. R. 1976. Commonwealth Institute of Helminthology (C. I. H.) descriptions of plant-parasitic nematodes. Set 6, No. 85.

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, D. L. Coyne, A. T. Ploeg, J. A. Navas-Cortés, P. A. Roberts, G. W. Yeates, and S. A. Subbotin. 2013. Morphological and molecular characterization and diagnostics of some species of Scutellonema Andrássy, 1958 (Tylenchida: Hoplolaimidae) with a molecular phylogeny of the genus. Nematology, 15: 716-745.

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