Identifying root-knot nematode infested production fields is generally done through observations of root-knot symptoms on host plants, and/or through the extraction and counting of second-stage juveniles from soil. Root-knot nematodes invade and cause galls near the root tip, thus quantifying the severity of root galling can be difficult unless most of the root system is removed. The latter can be difficult and time consuming, particularly if sampling is done when paddy soils are submerged. A soil bioassay test, either alone or in conjunction with examining plant roots, provides an alternate means for assessing root-knot nematode levels in production fields, through evaluations of root-knot galling severity on test plant roots.

The bioassay test consists of growing a susceptible host (rice, in the case of M. graminicola) in a container filled with a composite soil sample collected from the root zone of the crop plant, from several locations in the selected field. A 1 liter subsample, taken from 3 to 4 liters of soil collected from each sampling area, is sufficient for the test. The pots should be sown soon after the soil is collected, and maintained for 30 to 40 days, after which roots are washed and graded for root-knot galling severity. Dividing a field into several sampling blocks of approximately 1/4 ha in size will aid in accurately assessing nematode density within a field, and its potential damage. The field sampling survey and bioassay method is relatively fast, allowing a large production area to be surveyed, and can be done during periods when crops other than rice (or the host crop of concern) are in the field. In addition, the bioassay test makes it possible to observe symptoms of multiple soil-borne pathogens, and to quantify differences in nematode pressure within a field or among fields.

Volunteer plants that germinate from the shattered grain of the previous year's crop provide another useful, though less quantifiable, indicator of a potential root-knot nematode infestation. In a field study on M. graminicola in Bangladesh, several root-knot infected volunteer rice seedlings were visible in farmer fields just prior to land preparation for monsoon rice, following the harvest of the pre-monsoon crop. In addition to indicating the location of a potential nematode problem, root galling symptoms on volunteer seedlings provide an excellent opportunity to educate farmers and extension agents about root-knot nematode disease.


Modification of the Hussey-Barker sodium hypochlorite method for extracting eggs of M. graminicola

The egg sac of M. graminicola is located inside the root cortex, making it difficult to extract eggs by the Hussey-Barker sodium hypochlorite method (1). A simple modification, involving pulse-blending the roots, for 3 to 4 seconds at 20 to 30 second intervals in a 1% solution of sodium hypochlorite, has been found to be very effective for extracting eggs of M. graminicola from infected roots (Nematology Lab, Cornell University, New York State Agricultural Experiment Station, Geneva NY, USA).

References 1. Hussey RS, Barker KR. 1973. A comparison of methods of collecting inocula of Meloidogyne spp., including a new technique. Plant Disease Reporter 57: 1025-8