The ring nematode is a plant-parasitic nematode that is associated with grapevine, with it being regarded as being the most difficult nematode to control.
The ring nematode, Criconemoides xenoplax, is described as a migratory ectoparasitic nematode that feeds on the roots of plants, mainly those of woody perennials, such as grapevine. They feed by means of a long stylet that they insert into the roots of grapevine plants. The name ‘ring nematode’ has its origin in the unique ring-like ornamentation that covers the body, which is extremely rare in plant-parasitic nematodes (Figure 1). The name ‘ring nematode’ applies to a collection of nematodes in the same family, which show great diversity, but which are not necessarily of agricultural importance. Ring nematode occurs throughout the world where grapevine is grown. The ring nematode is regarded, worldwide, as a significant pest in vineyards, with it having become a common soil pest in South African production areas.
FIGURE 1. Close-up view of ring nematodes.
During a recent survey to determine the occurrence of ring nematode in grapevine and deciduous fruit, ring nematodes were found in 100% of the samples analysed (i.e. 49 samples). The mean number of ring nematodes observed on the grapevine (2 627 ± 303) was found to be much higher than that for plum, peach and apricot (160 ± 43.8). To determine whether all the specimens obtained were of the same species, the nematodes were identified morphologically and morphometrically, as well as by using molecular techniques. The results indicate that the species in all the grapevine production areas surveyed were the same.
Unique problems faced concerning ring nematode
The ring nematode is unique among plant-parasitic nematodes, requiring that a highly specific sugar flotation technique be used for their extraction from the soil. The rings are believed to prevent the nematodes from crawling through a filter and being trapped in water. Unfortunately, the use of the sugar flotation technique compromises the viability of the nematodes concerned, due to the amount of osmotic pressure involved, which prevents their use by researchers as an inoculum for any kind of laboratory or glasshouse experiment. The ring nematodes for use in experiments under glasshouse conditions require inoculation as infected soil. An annual host for the cultivation of the nematodes in high numbers is currently not available.
With the minimum agricultural disturbance of the soil involved, all other nematodes in the ring nematode complex that can occur in the vineyard environment will disappear, and only C. xenoplax will be left. However, with the application of chemicals, the problem with ring nematodes is only temporarily alleviated in the upper layers of the soil. They can then easily move from the deeper levels of the soil back to the upper treated soil, where, in the absence of competition and natural enemies, they can reproduce more rapidly.
Nemlab routine sample results
The different private diagnostic laboratories, during the past decade, have encountered very high numbers of ring nematodes in the routine samples taken from grapevine that have been analysed for the presence of plant-parasitic nematodes. However, such was not previously proved to be the case. Soil samples (3 250) from Nemlab (Klapmuts, Paarl, Western Cape province), which were taken from grapevine-growing areas over a period of three years, were selected for analysis. The severity of ring nematode infection was rated, with the different commercial rootstocks being separately analysed.
Rating the number of nematodes according to an artificial scale of zero, low-mild and high to severe, indicated that approximately 18% of 3 426 routine samples were found to be without ring nematodes, 36% were low to mildly infested, and 46% were high to severely infested (Figure 2). The severely infested samples were regarded as infested with >1 000 ring nematode/250 g of soil. The highest number of nematodes found over the three-year period was >6 160, which is extremely high for ring nematodes. The results might, however, have been biased, as the samples might have originated from vineyards with health problems.
FIGURE 2. Soil samples (3 426) from Nemlab, over a period of three years, rated according to the number of nematodes detected in the samples.
In South Africa, there was uncertainty regarding which of the commercial grapevine rootstocks had resistance against ring nematodes. The analysis of soil samples from the Nemlab routine samples indicated that most of the samples received were from the rootstock Ramsey (60%), followed by those from the rootstock Richter 110 (15%). However, in terms of the mean number of ring nematodes per rootstock, the highest was found for the US 8-7 (24%), followed by Richter 110 (21%), Paulsen 1103 (14%), Richter 99 (14%), Ramsey (15%), and 101-14 (12%), with the latter having the least number of ring nematodes (Figure 3).
FIGURE 3. A: Ratio of the routine Nemlab samples analysed per rootstock. B: Mean number of ring nematodes isolated from the different rootstocks.
Previous experiments indicated variable results under glasshouse conditions. However, Richter 110 was shown to be the most susceptible, with Richter 99 and Ramsey being the least susceptible.
- The only nematode in the ring nematode group found during a recent survey was the species C. xenoplax, proven by a recent survey and by the use of molecular techniques.
- Ring nematode is widespread throughout all the different grape-growing areas, with the presence of relatively high numbers, especially towards the relatively warm northern areas.
- The attempts made at conducting glasshouse experiments did not produce reliable results that gave a true picture, depending, as they did, on the availability of perfect conditions, in terms of humidity, temperature and irrigation, for a period of at least six months.
- An annual host is not currently available for the culture of large numbers of nematodes for experimental purposes.
- Inference is possible from the Nemlab routine samples that all South Africa’s commercial rootstocks are susceptible to ring nematode, with only US 8-7 being notably more susceptible than the others.
- Chemical control of ring nematode should prove not to be conducive to future soil health practices in vineyards.
- In the development of different rootstocks selections, ring nematode should be included as an important criterion for selection.
Future prospects for research
Significantly, a holistic approach should be followed in the control of ring nematode. Previous studies, conducted on the use of cover crops with biofumigation capability, showed a gradual decrease in the ring nematode numbers over the number of years. The Nemlab routine samples showed that approximately 18% of the grapevine samples were free of ring nematodes. These vineyards might indicate specific management practices that are not conducive to the increase of ring nematode numbers, so that they should be compared with the management practices of those vineyards that are severely infested with ring nematode.
– For more information, contact Antoinette Malan at firstname.lastname@example.org.