Meloidogyne graminicola
(rice root knot nematode)

Preferred Common Name

• rice root knot nematode

EPPO code

• MELGGC (Meloidogyne graminicola)

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Nematoda
•             Family: Meloidogynidae
•                 Genus: Meloidogyne
•                     Species: Meloidogyne graminicola

The taxonomy of the genus Meloidogyne is complex and becoming more so - many species can only be reliably identified by an expert.

Measurements (After Golden and Birchfield, 1965)

20 females: L = 0.445-0.765 (0.573) mm; width = 0.275-0.520 (0.419) mm; a = 1.2-1.8 (1.37); stylet = 10.64-11.20 (11.08) µm.

20 males: L = 1.020-1.428 (1.222) mm; a = 72.8-215.0 (117.4); length of oesophagus (anterior end to base of oesophagus) = 196.0-250.0 (222.0) µm; stylet = 16.24-17.36 (16.8) µm.

20 second-stage juveniles: L = 0.415-0.484 (0.441) mm; a = 22.3-27.3 (24.8); b = 2.9-4.0 (3.2); c = 5.5-6.7 (6.2); stylet = 11.20-12.32 (11.38) µm.

20 eggs: L = 96-101 (99) µm; width = 42-47 (44) µm.

Description (after Mulk, 1976)

Female
Pearly white, globular to pear-shaped with small neck; cuticle distinctly annulated but often marked with irregular punctations. Lip region smooth, anteriorly flattened, not distinctly set off from neck, with inconspicuous framework. Stylet slender and delicate; knobs rounded with posteriorly sloping anterior margins. Orifice of dorsal oesophageal gland 3.2 (2.8-3.9) µm behind stylet base. Excretory pore conspicuous, anterior to median oesophageal bulb, more than one stylet length posterior to stylet knobs and 7-16 annules behind lip region. Procorpus elongate cylindrical; median oesophageal bulb large, situated in the hind part of the neck, highly muscular, rounded to hemispheroid, 20-23 µm long and 10-12 µm wide with strongly cuticularized valve in the middle; isthmus short and narrow; three oesophageal glands, each with a prominent nucleus, extend ventrally and ventro-laterally over the intestine. Nerve ring obscure.

Ovaries two, well developed, convoluted, filling body cavity and overlying the intestine; uterus with several eggs. Six large radially arranged, uninucleate rectal glands with prominent nuclei, surround the rectum. Posterior cuticular pattern (= perineal pattern) dorso-ventrally oval, sometimes almost circular; dorsal arch low with smooth striae; tail tip marked with prominent, coarse, fairly well separated and disorganized striae, forming an irregular tail whorl; sometimes a few lines converge at either end of vulva. Lateral fields obscure or absent. A few well-marked, irregular, short, zig-zag striae, distinct from the rest and interrupting the general pattern, distinguish it from other species. Phasmids minute, rather close together; distance between the phasmids about two-thirds the length of the vulva. Distance from anus to vulva about 2.5-3.0 times the distance between anus and level of phasmids.

Male
Body cylindrical, vermiform, tapering more towards anterior than posterior extremity. Cuticle prominently annulated. Annules about 2.1-2.5 µm apart near mid-body. Lip region continuous with body or slightly offset by a constriction, nearly flat anteriorly, 3.5-4.0 µm high and 8.5-9.0 µm wide, consisting of a prominent labial annule followed by 1 or sometimes 2 wide post-labials. Cephalic framework conspicuously sclerotized. Stylet fairly strong with rounded posteriorly sloping knobs, 3.5-4.0 µm across; anterior conical part of stylet about 50% of the whole length. Orifice of the dorsal oesophageal gland 3.5-4.8 µm (2.80-3.92 µm according to Golden and Birchfield, 1965) from base of stylet. Anterior and posterior cephalids at about 2nd and 7th annules behind lip region. Excretory pore distinct, 51-64 annules behind lip region (about 0-7 annules posterior to nerve ring). Hemizonid 1-2 annules wide, 1-3 annules anterior to the excretory pore. Hemizonion a few annules behind excretory pore but inconspicuous. Procorpus elongate, cylindrical, wider than isthmus. Median oesophageal bulb hemispheroid to fusiform with strongly cuticularized valve in the middle. Isthmus, a narrow tube, encircled by nerve ring near middle; three oesophageal glands forming a compact lobe overlie intestine ventrally and ventro-laterally. Lateral fields 7.7 (6.2-9.5) µm wide or about one quarter of body-width, marked with 4 incisures in young and 8 in large and old specimens, near mid-body. Outer incisures crenate and outer bands areolated at extremities. Testis single, outstretched, sometimes reflexed anteriorly. Spicules arcuate or slightly bent ventrally near middle, 28.1 (27.4-29.1) µm long medially. Gubernaculum rod-shaped 6.1 (5.6-6.7) µm long. Tail 11.1 (6.2-15.1) µm wide with smooth terminus. Phasmids small, postanal, located near middle of tail.

Second-stage juveniles
Body cylindrical, vermiform, tapering towards posterior extremity. Cuticle finely marked with distinct transverse striae, about 1 µm apart near mid-body. Lip region continuous with body, weakly sclerotized, marked with 3 faint post-labial annules. Stylet delicate with posteriorly sloping rounded knobs. Orifice of dorsal oesophageal gland 2.8 (2.8-3.4) µm from base of stylet. Excretory pore at level of nerve ring or slightly behind. Hemizonid just anterior to excretory pore. Median oesophageal bulb rounded, almost spherical, with prominent refractive valve. Lateral fields with 3 incisures, occupying one-quarter to one-third of body width near middle. Outer incisures finely crenate. Tail 70.9 (67.0-76.0) µm long, including the irregularly annulated posterior hyaline portion, which is 17.9 (14.0-21.2) µm long and 4-5 times as long as the anal body width. Tail terminus rounded, often slightly clavate.

Seedling stage, Vegetative growing stage

M. graminicola can be detected when plants are uprooted as it causes swellings and galls throughout the root system. Infected root tips become swollen and hooked, a symptom which is especially characteristic of this nematode.

In upland conditions and shallow intermittently flooded land, it can cause severe growth reduction, unfilled spikelets, reduced tillering, chlorosis, wilting and poor yield. Symptoms often appear as patches in a field.

M. graminicola is known to cause serious damage to deepwater rice. Prior to flooding, symptoms are the typical stunting and chlorosis of young plants. When flooding occurs, submerged plants with serious root galling are unable to elongate rapidly, and do not emerge above the water level (Bridge and Page, 1982). This causes death or drowning out of the plants leaving patches of open water in the flooded fields.

Plant parts not known to carry the pest in trade/transport
Bark
Flowers/Inflorescences/Cones/Calyx
Fruits (inc. pods)
Leaves
Stems (above ground)/Shoots/Trunks/Branches
True seeds (inc. grain)
Wood

M. graminicola can cause economic yield loss in upland, lowland and deepwater rice. In upland rice, there is an estimated reduction of 2.6% in grain yield for every 1000 nematodes present around young seedlings. The population levels which cause 10% loss in yield of upland rice are 120, 250 and 600 eggs/plant at 10, 30 and 60 days age of plants in direct seeded crops (Rao et al., 1986). In flooded rice, damage by M. graminicola is caused in nurseries before transplanting - the tolerance limit of seedlings is <1 J2 per cubic centimetre of soil (Plowright and Bridge, 1990). Damage also occurs prior to flooding where rice is sown directly in well-drained soils. Experiments have shown that 4000 juveniles/plant of M. graminicola can cause destruction of up to 72% of deepwater rice plants by drowning out. Losses as high as this in the field are unlikely as natural root populations vary considerably (Bridge and Page, 1982).