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 1914.

Thomas at the Citrus Experiment Station (Riverside, California) reported association with poor-growing trees.

Recently retired Dean of the College of Agriculture at UC Riverside, Seymour Van Gundy, was hired in 1954 to work on this nematode.

Morphology and Anatomy:

Distribution:

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

Economic Importance:

Feeding:

Hosts:

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

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 masses contain about 100 eggs.

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.

Damage:

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.

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.

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:

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

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).

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.