Ring nematode (Criconemoides xenoplax), distribution, characterization and culture methods
Stellenbosch University. Department of Conservation, Ecology and Entomology
Addison, M F
Malan, A P
Storey, S G
The ring nematode, Criconemoides xenoplax (Raski, 1952) Loof & De Grisse, is described as a migratory ectoparasitic nematode that feeds entirely on the roots of plants, mainly those of woody perennials. The ring nematode is regarded, worldwide, as a significant pest in stone fruit orchards and vineyards, and it has become a common soil pest in South African production areas. To determine the distribution of the ring nematode, a survey was conducted in grapevine and stone fruit production areas, in both the Western and Northern Cape provinces. Nematode specimens collected during the study were characterised morphometrically and molecularly to determine the ring nematode species present in the areas. Soil samples were obtained from randomly selected farms, with additional data acquired from the diagnostic laboratory, Nemlab (Klapmuts, Paarl, Western Cape province), being investigated in relation to ring nematode occurrence in soil samples from the stone fruit and grapevine areas.
After DNA characterisation of the Internal Transcribed Spacer (ITS) region and the study of the morphology of the ring nematodes collected, the most abundant ring nematode species present in the stone fruit and grapevine production areas was concluded to be C. xenoplax. Another unknown species of ring nematode was recovered at a single site during the course of the study. Criconemoides xenoplax occurred in 100% of the soil samples obtained during the survey, with nematode numbers ranging from 20 to > 2000 individuals per 250 ml of soil. Criconemoides xenoplax infestation in South Africa shows similar trends, as have been observed globally, demonstrating its importance as an economically significant pest. It is, therefore, essential to develop alternative management options to minimise the amount of damage incurred, and to manage C. xenoplax in the areas concerned, to sustain production.
Control of the ring nematode is difficult and is highly dependent on the use of chemicals. Alternative, more sustainable and eco-friendly management options, such as the use of resistant rootstocks in the grapevine and stone fruit industries, have become common practice. Six commercially available grapevine rootstocks were assessed for their susceptibility to C. xenoplax in a glasshouse pot trial. Additional data, from routine grapevine soil samples analysed by Nemlab, were used in conjunction with the glasshouse trial data. Grapevine rootstock susceptibility in the glasshouse was evaluated by taking into account the reproductive factor (RF), and the nematode numbers recovered in the soil of the different vine rootstocks. The results of the glasshouse trial indicated significant differences in grapevine rootstock susceptibility to C. xenoplax. Ring nematode numbers were recorded to be highest on the rootstock 110 Richter for both trials conducted in the glasshouse, whereas rootstocks 1103 Paulsen and 140 Ruggeri performance remained similar in both trials. Contrasting results were recorded between the 99 Richter and Ramsey rootstocks in the two trials, with Ramsey showing higher tolerance than 99 Richter in the first trial, yet a lower tolerance than 99 Richter in the second trial. The data analysed from Nemlab samples showed that the mean number of C. xenoplax were generally higher from the field samples evaluated during the study. Further studies are required examine different rootstock tolerance or resistance to C. xenoplax. To do the above, however, a reliable source of C. xenoplax cultures needs to be available for future studies as well a glasshouse facilities.
In addition to its preferred hosts, being grapevine and stone fruit, C. xenoplax has also been recorded on a variety of other plant species. Five annuals, specifically tomato (Lycopersicon esculentum), lettuce (Lactuca sativa), mint (Mentha), carnation (Dianthus caryophyllus), white clover (Trifolium repens), and sweetcorn (Zea mays var. saccharata), were investigated in a glasshouse trial. The investigation was conducted to determine whether a monoxenic C. xenoplax population could be cultured en masse on these alternative hosts, thus providing a more successful and rapid method of culturing the nematodes for future use. Criconemoides xenoplax was used to inoculate the host plants selected six weeks after replanting the seedlings, where after the plants were left for a duration of seven weeks to allow for nematode reproduction. None of the annual hosts tested during the trial were considered a suitable host for the nematode, as no increases in the population were observed for the duration of the trial. As the RF values calculated were all > 1, using these annuals as an alternative option to culture C. xenoplax is not a viable option for future experiments. The use of grapevine and stone fruit plants should, as a result, remain the primary hosts for the sustained ring nematode populations in culture, with grapevine being the preferred host.
To conclude, C. xenoplax is a significant nematode that requires more research in South Africa. Doing so would enhance the understanding and amount of knowledge pertaining to the biology of such an economically important pest, as well as promoting the understanding of future host damage and plant resistance. The above mentioned will be critical for the employment of efficient control methods to manage nematode populations in the future. The amount of chemical application needed to control this nematode, is to the detriment of soil health expectations for the future. The search for tolerant and resistant rootstocks in both grapevine and stone fruit will be the way forward in the control of ring nematode. Samples from Nemlab indicate that vineyards and orchards with zero ring nematode occurrence do exits as well as those with incredibly high numbers of ring nematode. These extremes should be investigated, in terms of their general environmental conditions and current management practices to find solutions to deal with situations of massively high ring nematode numbers, other than chemical control.