Tylenchulus semipenetrans		Contents		Rev 11/13/2012
Citrus Nematode		Classification		Hosts
		Morphology and Anatomy		Life Cycle
Return to Tylenchulus Menu		Economic Importance		Damage
		Distribution		Management
Return to Tylenchulidae Menu		Feeding		References
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Classification:

     
       Tylenchina
        Criconematoidea
         Tylenchulidae
          Tylenchulinae
           Tylenchulus semipenetrans
  

• First reported by J. R. Hodges in 1912 - Horticultural inspector for Los Angeles County, California.  The nematode was described by N.A. Cobb in 1913.
• Thomas at the Citrus Experiment Station (Riverside, California) reported association with poor-growing trees.
• Baines at UC Riverside worked on the use of resistant rootstocks and nematicides for 30 years.  
• Retired Dean of the College of Agriculture at UC Riverside, Seymour Van Gundy, was hired in 1954 to work on this nematode.

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Morphology and Anatomy:

	Small nematodes, 0.25-0.35mm long.  Procorpus and metacorpus not amalgamated, distinct isthmus, postcorpus in bulb, butts intestine - distinct in juveniles (note that these features are characteristic of Criconematoidea). Females: swollen, have single ovary, vulva subterminal, eggs deposited in matrix. Excretory pore posterior (just in front of vulva). Pore surrounded by small, irregularly shaped lobes; excretory duct directed forward.  Rectum and anus atrophied or absent; non-functional.       Below:  Tylenchulus semipenetrans female dissected from a citrus root.  Photo by Ulrich Zunke.
Male:  reduced esophagus and stylet. Has no bursa. Do not feed from J2 stage onward.

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Distribution:

Worldwide with that of citrus.  

Occurs in 95% of citrus in California and is common in eastside San Joaquin Valley grapes, particularly in former citrus-growing areas.  

Originally spread with planting stock and further with irrigation water.

Commonly found in grape vineyards planted after citrus orchards.  Common in grape vineyards in Australia.
    

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Economic Importance:

C-rated pests in California.

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Feeding:

Ectoparasite as juvenile and young adult female, feeding on epidermal and outer cortical cells.	Sedentary semiendoparasite as mature female.  Move deeper into root as young female so  that head is near pericycle.  Establish feeding site of 8-10 nurse cells with thick walls, large nucleus and nucleolus.  The area may become invaded by other micro-organisms.   Female body outside root swells, and eggs are produced in a gelatinous matrix .
Juvenile Citrus Nematode

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Hosts:

Citrus (29 species and 21 hybrids), olive, grape, lilac, and persimmon.

Currently, there are at least four biotypes.  Biotypes are distinguished by their ability to parasitize citrus rootstocks (host range test).

For an extensive list of host plant species and their susceptibility, copy the name

Tylenchulus semipenetrans

select Nemabase and paste the name in the Genus and species box

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Life Cycle:

Egg development occurs in 14 days.  After first molt in egg, J2 hatches.  Male and female J2s can be distinguished - male has a shorter esophagus and greater body diameter, with a clear area in the tail where spicules will develop.   The male passes through 3 molts without feeding, and the stylet becomes progressively less distinct; males reach maturity in one week.
Female J2s are longer and more slender.   They feed during development, initially on epidermal and outer cortical cells.  In about 21 days they molt to young females which move deeper into the root so  that the head is near the pericycle. They establish a feeding site of 8-10 nurse cells - thick walls, large nucleus and nucleolus.  The area may become invaded by micro-organisms.
Female body outside root swells, and eggs are produced in a gelatinous matrix (the soil adheres to matrix, causing a  "dirty root" symptom).
Video of Citrus Nematode Root Symptoms  >>>>
Egg mass of each female contains about 100 eggs.  When females are aggregated on a root their egg masses coalesce and cover the group. Reproduction occurs by parthenogenesis, males are not required; both male and female J2s are produced by unfertilized females.
The J2 female is the persistent stage, and has been recovered from stored soil after 2.5 years and from field soil 4 years after pulling lemon trees; however, J2 females appear susceptible to dry soil conditions, although capable of "shallow" anhydrobiosis (Tsai and Van Gundy).	In studies in California vineyards, T. semipenetrans has been found as deep as 12 feet below the soil surface.
Leachates from citrus cultivars repel T. semipenetrans, and leachates from resistant cultivars are  more repellant than those from susceptible cultivars (Duncan and Graham, 1992).   Citrus and root-knot nematodes respond differently to components of citrus root exudates (citrus is a non-host to the root-knot nematode, Meloidogyne javanica): Attractiveness and repellancy, relative to water, of components of citrus root exudates to Tylenchulus semipenetrans and Meloidogyne javanica .   Na- and K-acetate Na- and K-formate NaCl, KCl and CaCl2 Na- and K-citrate NaCO3 CO2 Citrus Roots T. semipenetrans ++++ ++ ++ -- --   +++ M. javanica -- -- --   ++ ++ -- Greatest aggregation of T. semipenetrans was at 0.08-0.10 M Na-acetate. Above 0.2 M recovery was very low. Preference for Na-acetate by T. semipenetrans was pH-dependent. [Data from Duncan et al (1993) and Abou-Setta and Duncan (1998)] Duncan and Abou-Setta (1995) used the attractiveness of acetates to attract citrus nematode into soil zones with high concentrations of  nematicides or bacterial antagonists.
Reproduction of citrus nematode in the upper soil was greater when only lower soil levels were irrigated than when the soil was uniformly moist or uniformly dry. Why?  Some possibilities: hydraulic lift of water from the lower soil to the nematode-infested regions of the root accessible to the sedentary endoparasite females. availability of O2 to the reproducing females. unfavorable soil conditions for biological antagonists. combinations of all these factors. Research by Duncan and El-Morshedy (1996)   Duncan (personal communication) considers that soil moisture may be the most important factor regulating populations of this nematode.  He notes that population densities and crop losses associated with citrus nematode are higher in dryland or Mediterranean citrus production than in the humid tropics and subtropics.  Also, he notes that the host range of the citrus nematode is limited to a few deep- rooted perennials so there may not have been selection pressure for anhydrobiosis.

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Damage:

Infected plants exhibit debilitating slow decline, small leaves, twig die- back, reduced foliage, reduced fruit size and number.	Infected feeder roots (left) are stunted and have clinging soil particles; healthy roots on right.		Growth reduction in citrus trees planted in an old orchard. Row on right preplant fumigated with 1,3-D nematicide.
		Mature female Tylenchulus semipenetrans dissected from root.  Distortion of anterior region caused by constriction by host cell walls.

Root destruction causes plant decline over 3-5 years.  

Nematodes occur in very high numbers.  Scotto la Massese in France estimates a 5% yield loss per 1,000 nematodes per g of root.  In Israel, tree performance is reduced at levels of > 40,000 nematodes per 10g of root.

Mashela, Duncan et al, 1992, showed that feeding of T. semipenetrans changed the partitioning of osmoticum ions in citrus: K decreased in  leaves; K⁺, Na⁺, and Cl^- decreased in roots, and Cl^- increased in leaves.

Leaf symptoms in citrus are consistent with the increased Cl^- concentrations.  Starch concentrations increased in infected roots.
   

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Management:

Avoidance - clean planting stock, use of certified material, bare root dips in hot water (45 C for 25 minutes) effective.

Prevent spread - give attention to machinery, planting stock, and irrigation water.

Nematicides

Pretreatment 4-8 months before planting with 1,3-Dichloropropene (1,3-D) at 70-120 gpa placed 12-24" deep at 18" spacing reduces, but does not eradicate nematode populations.  Can also use tarped methyl bromide at 100-200 lb/acre 1  month before planting.  

Prior to 1979, DBCP was used postplant at 34-36 lb/acre, but not for the first 4 years, and then not more than once every 3 years.

Commercial microbial antagonist formulations performed poorly against Tylenchulus semipenetrans and Paratrichodorus lobatus compared to the nematicides Aldicarb, cadusafos and metalaxyl:

• Prosper Nema did not reduce nematode levels even when combined with a sawdust mulch over the soil. 
• Nutri-life 3/20 inhibited citrus growth and caused leaf chlorosis. 
• Potassium silicate reduced levels of T. semipenetrans in soil. 
• Cadusafos and Aldicarb were equally effective in controlling T. semipenetrans on roots but cadusafos was more effective in soil. 
• Relatively low initial combined levels of T. semipenetrans and Paratrichodorus lobatus depressed citrus growth
• Cadusafos (3 applications at 3 g/m2 over 6 months) reduced T. semipenetrans to very low levels, but did not eliminate the nematode; mechanical incorporation of nematicide granules improved the level of nematode control after the first, but not succeeding, applications (Walker and Morey, 1999a)

Green Manures:

Brassica cultivars Ebony and Indian mustards, and Rangi rape residues reduced the soil level of Tylenchulus semipenetrans by up to 76% compared with unamended soil, and in a greenhouse reduced levels on the roots of orange (Citrus sinensis) seedlings. 

Paratrichodorus lobatus reached high levels in pots containing unamended soil but was not detected in pots containing amended soils. 

In field experiments brassica cultivars were grown in orchards (20 kg seed/ha) as green manure crops.  Although soil levels of T. semipenetrans were reduced by 79-91% by incorporation of green manures, brassica cultivars including Ebony, Indian and Yellow mustards, and Humus and Rangi rapes, were no more effective than were self-seeding weeds. Growth of citrus did not differ between soils amended with brassica or weed residues (Walker and Morey, 1999b).

Resistant rootstocks

• Poncirus trifoliata - trifoliate rootstock.
• Troyer citrange - but biotypes pose a threat.

When tested against a Mediterranean biotype, all selections with 'Troyer' citrange (Citrus sinensis X P. trifoliata) in their parentage supported nematode reproduction. (Verdejo et al, 2000).

Citrus hybrid rootstocks were tested against a Mediterranean biotype of Tylenchulus semipenetrans. Seven selections of the cross 'Cleopatra' mandarin (Citrus reshni X Poncirus trifoliata), and one of Citrus volkameriana X P. trifoliata were highly resistant to the citrus nematode and did not support nematode reproduction. The nematode showed very low infectivity and reproductive potential on seven additional selections of 'Cleopatra' mandarin X P. trifoliata, one of 'King' mandarin x P. trifoliata, and two C. volkameriana X P. trifoliata. These selections were considered nematode resistant (Verdejo et al, 2000).

Resistance

For a list of plant species or cultivars (if any) reported to be immune or to have some level of resistance to this nematode species, copy the name

Tylenchulus semipenetrans

select Nemabase Resistance Search and paste the name in the Genus and species box

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References:   

Abou-Setta, M. M.; Duncan, L. W. 1998.  Attraction of Tylenchulus semipenetrans and Meloidogyne javanica to salts in vitro. Nematropica 28: 49-59.

Verdejo-Lucas, S.; Sorribas, F. J.; Forner, J. B.; Alcaide, A. 2000.  Resistance of hybrid citrus rootstocks to a Mediterranean biotype of Tylenchulus semipenetrans Cobb. Hortscience 35: 269-273.

Walker, G. E.; Morey, B. G. 1999a.  Effects of chemicals and microbial antagonists on nematodes and fungal pathogens of citrus roots. Australian Journal of Experimental Agriculture 39: 629-637.

Walker, G. E.; Morey, B. G. 1999b.  Effect of brassica and weed manures on abundance of Tylenchulus semipenetrans and fungi in citrus orchard soil. Australian Journal of Experimental Agriculture 39: 65-72.

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Copyright © 1999 by Howard Ferris.
Revised: November 13, 2012.