Radopholus citri

Notes on Taxonomy and Nomenclature

Top of pageR. citri Machon and Bridge, 1996 is accepted as a valid species.

Description

Top of pageDescription (after Machon and Bridge, 1996)

Female: Body vermiform, assuming an almost straight to ventrally arcuate form when heat relaxed. Cuticle annulated, ventral annules 1.3-1.6 µm apart at mid body. Four lateral incisures reducing to three in region of phasmids, outer incisures crenate. Outer bands of lateral fields marked by occasional transverse striae, particularly towards posterior extremity. Head slightly offset, low, rounded and somewhat flattened apically with four or five annules. En face view similar to that of type species. Stylet moderately strong. Basal knobs about 5 µm across; distal surface of dorsal knob extending anteriorly; much smaller and indistinct anterior projection usually visible on each subventral knob. Dorsal oesophageal gland opening 4-5 µm behind stylet knobs. Procorpus cylindroid; median bulb round to oval in form and slightly offset from rest of oesophagus. Oesophageal gland well developed, overlapping intestine mostly on dorsal side with glands in tandem. Nerve ring immediately posterior to median bulb. Excretory pore just posterior to hemizonid and located about two to three bulb lengths posterior to bulb. Vulva postmedian with markedly protuberant lips. Genital branches amphididelphic, outstretched, with oval to rod-shaped sperm in axial spermathecae. Uterine egg 20.1 x 58.3 µm. Tail conoid, tapering to rounded terminus which is regularly annulated. Phasmids located at 16 (12-17) annules posterior to anus, about midway along tail.

Male: Body vermiform, showing marked sexual dimorphism in anterior region. Head high, offset, knob-like with four or five annules. Stylet conus highly refringent, relatively strongly developed for males of this genus; shaft and knobs less well developed, but distinct. Oesophagus degenerate and apparently non-functional. Spicules paired, ventrally arcuate. Gubernaculum rod-like, with a dorsally directed process, observed in SEM studies, on dorsal surface; protrusible. Bursa crenate, extending almost to tail tip. Phasmids located about 16 annules posterior to cloaca, close to midway along tail. Tail conoid, tapering to a finely pointed, partially offset, terminus.

Measurements
(after Machon and Bridge, 1996)

Females (n=20): L = 0.69±0.05 (0.62-0.81) mm; a = 28.8±2.5 (25.1-32.5); b = 7.62±0.4 (6.9-8.3); b' = 4.4±0.3 (3.8-4.9); c = 14.3±1.2 (12.3-16.7); c' = 3.0±0.4 (2.5-3.6); V = 59.5±2.0 (54.1-62.3); stylet = 18.6±0.6 (17.4-19.4) µm; tail = 48.7±3.4 (43.6-57.0) µm; h* = 7.3±0.9 (5.4-8.7) µm.

Males (n=20): L = 0.53±0.03 (0.47-0.58) mm; a = 34.5±3.1 (28.4-41.7); b = 6.6±0.7 (5.4-8.0); c = 14.2±1.1 (12.4-15.9); c' = 3.2±0.5 (2.6-3.9); stylet = 13.5±0.7 (12.7-14.7); tail = 37.8±4.0 (31.5-43.6) µm; h* = 6.7±0.8 (5.4-8.0) µm; spicules = 15.5±1.0 (14.1-17.4) µm; gubernaculum = 8.8±0.9 (7.4-10.7) µm.

Holotype (female): L = 0.72 mm; a = 32.5; b = 7.7; b' = 4.5; c = 14.7; c'= 2.8; V = 60.7; stylet = 19.4 µm; tail = 48.9 µm; h* = 7.4 µm.

Risk of Introduction

Top of pageR. citri can be spread on infested vegetative planting material such as rootstocks and seedlings. It is currently localized, but there is a high probability that it could infest citrus in other subtropical and tropical areas, particularly those with sandy soils similar to the type locality.

Habitat

Top of pageThe rhizosphere of Citrus seedlings cv. Japanese Citron in Tulungagung, East Java, Indonesia.

Hosts/Species Affected

Top of pageAt present there is no record of other hosts, apart from citrus, in the field. However, R. citri can be cultured on excised maize roots and carrot discs (Machon and Bridge, 1996) and the former could be a host.

Growth Stages

Top of pageFlowering stage, Fruiting stage, Seedling stage, Vegetative growing stage

Symptoms

Top of pageR. citri causes very severe cortical necrosis and root destruction in citrus. Nematodes observed in stained roots were confined to the cortex and often associated with complete destruction of the cortical tissues. In pot trials, all levels of the nematode caused highly significant reductions in fresh and dry weights of tops, number of leaf nodes and growth as measured by length of stems and branches. Fresh root weights were also highly significantly reduced in all nematode treatments (Machon and Bridge, 1996).

Biology and Ecology

Top of pageR. citri is a migratory endoparasite which can be found in both roots and soil. It completes its life cycle within the root cortex. Females and juveniles are infective but males, although having a relatively strongly developed stylet, have an apparently non-functional, degenerate oesophagus. Eggs are laid singly or in groups in the root tissues and hatch after about 10 days at 25°C (Machon, 1996, CABI Bioscience UK Centre, St Albans, UK, unpublished data). The juveniles and females feed on the cortical cells causing cell death and necrosis of the root.

Impact

Top of pageThe very severe root destruction and growth reduction observed in controlled pathogenicity experiments (Machon and Bridge, 1996) have demonstrated that the nematode is potentially a destructive pest of citrus.

Diagnosis

Top of pageThe cause of stunting and poor growth of citrus plants is not confined to parasitism by the nematode and can be confused with other causal organisms.

Detection and Inspection

Top of pageFemales, males and juveniles can be found within the root cortex associated with lesions and areas of necrosis and also in the soil surrounding the roots up to a depth of at least 60 cm. Nematodes can be extracted from soil using a tray modification of the Baermann funnel method (Hooper, 1990) left for 24 to 48 hours and also by cutting the roots into small pieces, macerating in a blender for 20 s, pouring on to a 85 µm aperture sieve in a Petri dish of water and leaving for 48 hours. Observations of nematodes within root tissues can be made by staining roots in lactoglycerol plus 0.05% acid fuchsin (Bridge et al., 1982).

Similarities to Other Species/Conditions

Top of pageR. citri has a superficial resemblance to the genus Pratylenchus, but can be distinguished by having a median vulva with two genital tracts in the female as opposed to a posterior vulva with one tract. It can be most easily distinguished from other species of Radopholus in the male having a stronger stylet but particularly from the female of R. similis and R. s. citrophilus (the citrus race of R. similis) in having a shorter tail with a more rounded terminus.

Prevention and Control

Top of pageControl methods include the production of nematode-free planting material. Growing seedlings in field soil free of nematodes or use of sterilized soil for pots or containers is advised.

References

Top of page

Bridge J, Page SLJ, Jordan SM, 1982. An improved method for staining nematodes in roots. Rothamsted Report for 1981, Part 1. Harpenden, UK: Rothamsted Experimental Station.

Hahn ML, Burrows PR, Wright DG, 1996. Genomic diversity between Radopholus similis populations from around the world detected by RAPD-PCR analysis. Nematologica, 42(5):537-545; 15 ref.

Hooper DJ, 1990. Extraction and processing of plant and soil nematodes. In: Luc M, Sikora RA, Bridge J, eds. Plant Parasitic Nematodes in Subtropical and Tropical Agriculture. Wallingford, UK: CAB International, 45-68.

Machon JE, Bridge J, 1996. Radopholus citri n. sp. (Tylenchida : Pratylenchidae) and its pathogenicity on citrus. Fundamental and Applied Nematology, 19(2):127-133; [16 reef.].

Distribution Maps

Top of page