In this section:
Lesion Nematodes
Seasonal Variation in Abundance of Lesion Nematodes in Grapevines
Greg Walker and Barry Morey
SARDI Plant Research Centre, GPO BOX 397 Adelaide South Australia 5001.
This article was published in Grapegrower and Winemaker No. 445, pp. 15–18 (2001).
Many species of (Root) Lesion Nematodes (genus Pratylenchus) have been recorded on grapevines, and several of these have been shown to damage grapevine roots by feeding on root cells and by predisposing them to rotting by secondary micro-organisms. However, the economic importance of Lesion Nematodes as agents of vine decline in Australia is not known. These nematodes occur inside roots as well as in the soil, and because of this have often been spread in planting material in the past. Damage caused to grapevines depends on the levels of those nematode species present, and once roots are destroyed nematode levels may decline so it is important to sample at a time when levels are at their highest. Also the seasonal abundance of Lesion Nematodes in roots and soil is important when considering application of chemical control agents, some of which are active only in soil, whilst others also possess systemic activity and can kill nematodes inside roots. A study was initiated to determine the optimum time to sample for these nematodes in both roots and soil.
Soil and roots from 6 different rootstocks and 3 different grapevine varieties on own roots growing at Loxton Centre, in the warm, semi-arid Riverland climate of South Australia, were sampled mid-monthly at 2-month intervals between August, 1999 and August, 2000. Motile stages of nematodes were extracted separately from feeder roots and from associated soil; soil levels were corrected for extraction efficiency, except in preliminary experiments. Soil moisture levels were determined gravimetrically, and mean monthly soil temperatures recorded at depth 20 cm at 9 a.m. at Loxton Centre.
Soil depth and nematode levels
A preliminary experiment was undertaken to evaluate changes in nematode levels by soil depth. Soil samples from 5 replicate vines of 3 different rootstocks were taken at depths 0–30 cm and 30–60 cm. The topsoil here, a sandy loam, was quite deep with a lime layer at depth 65 cm. The results indicated that levels of Lesion Nematodes were higher at the 30–60 cm depth than from the shallower layer, but it was decided to take the repeated bimonthly samples from a depth of about 30 cm to accommodate areas of shallower sandy clay loam soil elsewhere in the vineyard. Levels of other important nematodes including Root knot Nematode (Meloidogyne spp.) and Citrus Nematode (Tylenchulus semipenetrans) were also higher at depth 30-60 cm than at 0-30 cm, however, this did not apply to all nematodes, for example, Ring Nematode (Criconemella xenoplax).
These results suggest that growers wishing to have soil and roots from vines on rootstocks tested for nematodes should first check their own soil structure and root distribution; where similar conditions exist, samples could be taken from depth 30–60 cm. But where topsoil and root distribution are restricted, samples may need to be taken from shallower depths. Also, own-rooted vines, or different rootstocks, could vary in this regard.
Seasonal cycle in levels of Lesion Nematodes
Soil levels of Lesion Nematodes remained low during most of the year but rose rapidly in spring, reaching maximum levels in October. Following this spring increase in soil, root levels of Lesion Nematodes began to rise and kept rising until reaching maximum levels in December. The median dates for budburst and flowering in Loxton are 15 September and 10 November respectively (Coombe, 1988). The main flush of root growth in grapevines (and hence the highest potential for root damage from Lesion Nematodes) occurs in spring; most studies have placed this flush at about 3 weeks after budburst and prior to flowering, that is, in October. It can be seen that soil levels of Lesion Nematodes reach their maximum during this root flush, so that most active stages are present in soil precisely when most new root tips present themselves for infection. Not coincidentally, soil temperatures at this time rise to levels which permit nematode activity; mean temperatures in June – August fall at or below 12° C which are too cold for many nematode species. The other important soil variable which can influence nematode activity, soil moisture, also rises to favourable levels at this time due to the commencement of irrigation at the start of October at this site. Soil moisture levels are kept higher by regular irrigation during the growing season than during winter when irrigation is suspended (despite rainfall, of which only 35 and 61 mm fell during the non-irrigated periods of August – September, 1999, and May – August, 2000, respectively at Loxton Centre). The rise in levels of motile stages of Lesion Nematodes in soil in spring is probably due mainly to hatching of over-wintering eggs. This hatching is probably stimulated chiefly by rising soil temperatures, but increased soil moisture levels and exudates from new root growth could also play a part. Other possible factors in the spring increase could be a re-activation of nematodes from a quiescent state (some nematodes respond to drying by entering a quiescent state, but it is questionable whether autumn/winter soil moisture levels are low enough to induce this, and the water-based extraction method used here should overcome this anyway), and a movement of nematodes from deeper soil layers, but egg hatching is likely to be most important.
Corresponding rises in levels of Lesion Nematodes in roots during spring lag slightly behind the rise in soil levels but continue rising until a peak is reached in December. This probably reflects multiplication of the nematode occurring within the newly formed roots of the spring flush. Once these roots start to die and decay, which begins quite quickly, within 2 months, conditions for Lesion Nematode multiplication become unfavourable and root levels decline. A corresponding increase in soil levels after December was not observed, which might be expected if nematodes moved out of decaying roots into the soil. This suggests that the nematodes remain within decaying roots and probably over-winter there, and/or that they die off as roots also die. However, a small rise in soil level was observed in June so some nematodes probably moved out of the roots into the soil in the late autumn/early winter. A smaller rise in root levels of Lesion Nematodes also was observed in April. This corresponds to a second, smaller root flush which is known to occur just after harvest. So nematode reproduction also occurs in flushing roots in autumn but to a lesser extent than in spring.
Optimum sampling times for Lesion Nematodes
These results have practical significance to growers in determining when is the best time to sample vineyards to diagnose or predict growth problems caused by Lesion Nematodes. If soil samples are taken the optimum time for sampling based on this study is during October; if root samples are to be taken for analysis then December would be a better time to sample.
Species of Lesion Nematode
Two main species of Lesion Nematode were identified in samples from this site; Pratylenchus neglectus and P. zeae. These 2 species are commonly associated with field crops, especially cereals, grasses and related crops. Earlier research (Walker and Morey, 2000) did not find any adverse affects from inoculation with a cereal population of Pratylenchus neglectus on grapevines, nor did the nematode increase in numbers on vines, suggesting that vines were a poor host for this population. The present findings suggest that different populations of P. neglectus and P. zeae occur with different host ranges; some are adapted to grapevines, while others are adapted to cereals and related hosts. These populations are currently not distinguishable by conventional identification methods, and even the newer, rapid, DNA-based diagnostic tests are likely to have difficulties separating them. So predictions based on pre-planting soil tests, conventional or otherwise, will need to take these findings into account. Cropping history of the land may be a good indicator of what populations are likely to be present.
Are these Nematodes Damaging Grapevines?
Lesion Nematodes were present in grapevine roots at this site so it highly likely that they were causing some damage, especially to new root flushes, but we do not know whether this damage was economically significant. Population levels certainly exceeded recently published damage thresholds (summarised in Nicol et al. 1999) especially in spring (and reached maxima exceeding 800 nematodes per kg soil or per g roots, dry weight), and higher levels were present deeper in the profile. Other nematodes also occurred at this site on some vines, but there were no gross signs of ill-health. Yield data are not available from rootstock source blocks.
Conclusions
Lesion Nematodes are closely tied to the cycle of grapevine growth and environmental conditions, particularly to the flushes of root growth in spring, and to a lesser extent in autumn. Peak levels of these nematodes occur in soil during October at the time of the main root flush, but peak levels in roots do not occur until later in the season in December after nematodes have undergone multiplication within flushing roots. Soil levels do not increase immediately after this suggesting that the nematodes largely remain within decaying roots at this time. However, nematode levels within roots rise again to a lesser extent in April with the second, smaller root flush in autumn. In late autumn/early winter soil levels rise slightly suggesting some exodus of nematodes from decaying roots into the soil.
The best months for growers wishing to diagnose or predict damage by Lesion Nematodes by taking samples for analysis are October for soil samples and December for root samples. At the vineyard studied here, with rootstocks growing in deep topsoils, samples were best taken from depth 30-60 cm, but samples may need to be taken from shallower depths where root distribution is more restricted.
Populations of Pratylenchus neglectus and P. zeae occur which are adapted to grapevines but are morphologically indistinct from populations on cereal and grassy hosts. These are likely to be difficult to distinguish even using DNA-based tests, making pre-planting predictive testing difficult and requiring detailed consideration of cropping history.
These nematodes occurred at this site at levels above recently published damage thresholds but the economic significance of this damage is not yet known.
References
Coombe, B. G. (1988). Grape Phenology. In Viticulture Vol. 1 Resources in Australia (B.G. Coombe & P.R. Dry eds.), Australian Industrial Publishers, Adelaide.
Nicol, J.M., Stirling, G.R., Rose, B.J., May, P. and Van Heeswijck, R. (1999). Impact of nematodes on grapevine growth and productivity: current knowledge and future directions, with special reference to Australian viticulture. Australian Journal of Grape and Wine Research 5: 109-27.
Walker, G. and Morey, B. (2000). Effects of Lesion Nematodes associated with cereals on grapevine growth. The Australian Grapegrower and Winemaker 438a: 130-2.
Acknowledgements
Funding support from Grape and Wine Research and Development Corporation is gratefully acknowledged; I would like to thank Dr Jackie Nobbs for identifying Pratylenchus species, and the dedicated PIRSA Loxton Centre weatherpersons.
