Reniform nematodes

Rotylenchulus reniformis (Linford & Olivera), the reniform nematode, is widespread in the tropics and subtropics, where it attacks and multiplies on a wide range of cultivated plants. Infection of soybean roots by the reniform nematode was reported in 1956 on the Gold Coast (now Ghana), West Africa, and in 1967 in South Carolina, USA (Sinclair and Backman, 1989).

Epidemiology and biology

R. reniformis is now distributed in the south-eastern and Gulf Coast areas of the USA and in most tropical regions of the world. It is a semi-endoparasite that partially embeds itself in the root. Because it is small and soil particles adhere to the portion of the body outside the root, the nematode is easily overlooked. The eggs are elongate (72-100 x 33-44 iim) and hatch within 24 h, yielding a sex ratio of about 1:1. Larvae are 330-445 iim long and have a well-developed stylet of 14-18 iim long. They moult three times in the soil with little or no root feeding and become adults (Sinclair and Backman, 1989).

The mature females of R. reniformis are sedentary, semi-endoparasites of roots. They feed on cortical parenchyma, the pericycle or even the phloem, depending on the host species. The male has a poorly developed stylet and median oesophageal bulb and does not feed. Sexual reproduction is the norm, but parthogenesis has also been reported (Sinclair and Backman, 1989). The kidney-shaped females produce a gelatinous matrix (the egg sac) that covers the female body and in which about 50-70 eggs (depending on the host plant) are laid. Soil adhering to this matrix often can hamper detection of the female on the root surface (Whitehead, 1998; Sikora et al., 2005a). All juveniles are passed into the soil in as little as 10 days, with the complete life cycle taking up to about 30 days, depending on the host and soil conditions. R. reniformis survives in the soil in the juvenile and adult male stages. Immature females penetrate the root and establish in the endodermis.

Damage threshold

The reniform nematode can cause stunting and chlorosis on soybean. R. reniformis has been found to be associated with soybean damage in tropical and subtropical countries (Schmitt and Noel, 1984). Significant yield losses may result from such attacks, especially when other root pathogens are also involved.

Management measures

The use of resistant varieties (Birchfield et al., 1971; Lim and Castillo, 1979) and a sound rotation plan with non-host crops for >2 years are good management options. Soybean cultivars that are resistant to H. glycines can also be resistant to R. reniformis (Sikora et al., 2005a).

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