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Rice root-knot nematode
Meloidogyne graminicola (Golden & Birchfield)
Female adult and egg mass of root knot-nematode
The roots of the host plants can be examined for hooked-like galling. They can be stained to determine the presence and populations of M. graminicola.
The juveniles of M. graminicola can be extracted from the roots of the host plants.
Other nematodes cause similar damage symptoms.
M. graminicola is a damaging parasite on upland, lowland and deepwater rice. It is well adapted to flooded conditions and can survive in waterlogged soil as eggs in eggmasses or as juveniles for long periods. Numbers of M. graminicola decline rapidly after 4 months but some egg masses can remain viable for at least 14 months in waterlogged soil. M. graminicola can also survive in soil flooded to a depth of 1 m for at least 5 months. It cannot invade rice in flooded conditions but quickly invades when infested soils are drained. It can survive in roots of infected plants. It prefers soil moisture of 32%. It develops best in moisture of 20% to 30% and soil dryness at rice tillering and panicle initiation. Its population increases with the growth of susceptible rice plants.
The presence of relatively broad host range and many of the alternative vegetable crops that are grown during dry season are favorable for this nematode.
Adult females appear to be pear-shaped to spheroid with elongated neck, which is usually embedded in root tissue. Their body does not transform into a cystlike structure. Females have six large unicellular rectal glands in the posterior part of the body, which excrete a gelatinous matrix to form an egg sac, in which many eggs are deposited. The stylet is mostly 9-18 µm long with tree small, prominent, dorsally curved basal knobs. The esophageal glands overlap the anterior end of the intestine. The females have two ovaries that fill most of the swollen body cavity. The vulva is typically terminal with the anus, flush with or slightly raised from the body contour and surrounded by cuticular striae, which form a pattern of fine lines resembling human fingerprints called the perennial pattern.
Infective second stage juveniles are short (0.3-0.5 mm) and have a weak cephalic framework. The esophageal gland lobe overlaps the intestine ventrally. The tail tip tapers to a long, fine point with a long hyaline region.
Aside from the rice plant, it also prefers Alopecurus sp., Avena sativa L., Beta vulgaris L., Brachiaria mutica (Forsk.) Stapf, Brassica juncea (L.) Czem. & Coss, B. oleraceae L., Colocasia esculenta (D.) Schott, Cyperus procerus Rottb., C. pulcherrimus Willd. ex Kunth, C. rotundus L., Echinochloa colona (L.) Link, Eleusine indica (L.) Gaertn., Fimbristylis miliacea (L.) Vahl, Fuirena sp., Glycine max (L.) Merr., Lactuca sativa L., Lycopersicon esculentum Mill., Monochoria vaginalis (Burm. f.) Presl, Panicum miliaceum L., P. repens L., Paspalum scrobiculatum L., Pennisetum typhoides (Burm. f.) Stapf & Hubbard, Phaseolus vulgaris L., Poa annua L., Ranunculus sp., Saccharum officinarum L., Sorghum bicolor (L.) Moench, Sphaeranthus sp., Sphenoclea zeylanica Gaertn., Spinacia oleracea L., Triticum aestivum L., and Vicia faba L.
1. The larvae of the second stage (a) are attracted to the roots. They usually penetrate the roots closely behind the root tip. The larvae then migrate first towards the root tip, where the absence of differentiated endodermis allows them to enter the vascular cylinder. This migration happens intercellularly by mechanical and possibly enzymatic softening of the middle lamella. The parasites finally start feeding on three to ten cells, which are rapidly turned into multinucleated giant cells, by endomitosis and cell hypertrophy.
2. At the same time as the giant cells are formed, the cells of the neighboring pericycle start to divide, giving rise to a typical gall or root-knot. Inside the gall, a female (b) and a male (c) of the J3 larval stage are shown.
3. The gall continues to swell, while females (d) and males (e) are in their J4 stage.
4. During the last moult, the male (h) dramatically changes its shape, then leaves the root, and fertilizes the female (f) in the case of amphimictic species. However, parthenogenesis is often encountered in root-knot nematodes. The female lays its eggs in a gelatinous matrix (g) outside the root. From there, the larvae of the second stage (a) hatch and are attracted to roots. Depending on environmental conditions, this cycle is completed in one to two months.
Infective second stage juvenile of M. graminicola penetrates through the root tips and takes about a minimum of 41 hours. Females develop within the root and eggs are laid in the cortex. Galls are formed in 72 hours. The juveniles or immatures remain in the maternal gall or migrate intercellularly through the aerenchymatous tissues of the cortex to new feeding sites within the same root.
The rice root-knot nematode attacks the rice plant in all growth stages.
The rice root-knot nematode is considered one of the limiting factors in rice production in all rice ecosystems. In upland rice, there is an estimated reduction of 2.6% in grain yield for every 1000 nematodes present around young seedlings. In irrigated rice, damage is caused in nurseries before transplanting or before flooding in the case of direct seeding. Experiments have shown that 4000 juveniles per plant of M. graminicola can cause destruction of up to 72% of deepwater rice plants by drowning out.
There are cultural, biological, physical, mechanical, use of resistant varieties and chemical control that are available for the rice root-knot nematode. For example, cultural control includes continuous flooding, raising the rice seedlings in flooded soils, and crop rotation. These practices will help prevent root invasion by the nematodes. Soil solarization, bare fallow period and planting cover crops such as sesame and cowpea has been reported to decrease nematodes. Rotation crop like marigold (Tagetes sp.) is also effective in lowering root knot nematode populations because of its nematicidal properties.
There is some IR cultivars, which are resistant against the nematode. Likewise, some related rice species such as some accessions of Oryza longistaminata Chev. et Roehr. and O. glaberrima Steud are also resistant.
Several nematicidal compounds can be used as chemical control. They are volatile (fumigants) and nonvolatile nematicides applied as soil drenches and seedling root dips or seed soaks to reduce nematode populations. Seeds can be treated with EPN and carbofuran. The roots can be dipped in systemic chemicals such as oxamyl or fensulfothion, phorate, carbofuran, and DBCP. Telone (1,3- dichloropropene) can be injected into the soil before the crop is planted.
JLA Catindig, LC Fernandez, and KL Heong