2013 R&D Results Compendium
The number of root lesion nematodes (Pratylenchus
neglectus) declines after strand medic pasture
Ross Ballard, Jake Howie, David Peck, Nigel Charman, Jeff Hill and Alan Mckay
South Australian Research & Development Institute (SARDI)
Background
Annual medics are widely grown in neutral and alkaline soil regions where they are
valued for livestock production, contributions of fixed N to the farming system and
for their ability to reduce the impacts of soil borne disease on the cereal and oilseed crops that follow.
There have been conflicting reports in the literature and popular press regarding the impact of medics
on the population densities of Pratylenchus neglectus (root lesion nematode, hereafter referred to as
Pn). For example, Collins et al. (2013) referred to a greenhouse study as the basis for listing cultivars
of burr and barrel medic as susceptible or very susceptible, i.e. likely to result in significant
multiplication of the nematode. This is at odds with previous field studies in SA (Ballard et al. 2006)
that had shown medics maintained or reduced Pn number.
This field study quantifies Pn numbers following the growth of strand medic (Medicago littoralis), the
best adapted and most extensively grown medic in Mallee agricultural regions. It differs from previous
field studies in that it considers if changes to Pn multiplication are influenced by the initial (before
medic) nematode population density.
About the trials
The work reported here is part of a larger effort to understand the impacts of Pn on the production of
medic pastures and cereal crops. Trials were sown at Arthurton (Yorke Peninsula) and Pinery (Lower
Mid-North) in South Australia, where population densities of Pn had been manipulated in the previous
year. This was achieved by sowing 20 cereal varieties that varied in their Pn susceptibility/resistance,
in a randomised block design with four replications.
A cone seeder was used to sow strand medic into the cereal stubbles persisting from the previous
year. Medic cultivar Herald and breeder’s line RH-1 (selected for putative tolerance to Pn: Oldach et
al. 2014) were sown to establish each as a 2m × 1.2 m sub-plot (comprising 8 rows at 15 cm spacing)
within the original 5m cereal (main) plots. Medic was sown at 20 kg/ha (2 June) at Arthurton and at
15 kg/ha (18 May) at Pinery.
Medic establishment was erratic at Arthurton, but by September, plots had achieved a good level of
growth. At Pinery, medic establishment and growth was excellent. Weeds were controlled at both
sites through a combination of herbicide application and hand weeding to minimise the potential for
Pn susceptible hosts to affect the results (Fig. 1).
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Figure 1 Examples of the growth and purity of strand medic plots at Arthurton (left) and Pinery.
A sub-set (3 of the original 20 cereal treatments) was subsequently selected for this study. The
selected plots had previously grown the cereals Abacus, EGA Wylie or Brookton and provided a broad
range of initial Pn number.
Plots were sampled prior, or close to the time of medic establishment, and again in the summer
following the medic phase. On each occasion, twenty cores (15 mm diameter to 100 mm depth) were
taken from each plot to provide a soil sample of approximately 400g.
Population density of Pn was determined by the Root Disease Testing Service (SARDI) using rDNA
probe sequences specific to Pn that were applied to total DNA extracted from the soil.
Results
Pn number after cereal, before medic
The number of Pn after cereal varied widely from less than 10 to more than 50 per g soil, at both sites.
At Arthurton, the selected plots were estimated to contain 2 (cv. Abacus), 16 (EGA Wylie) and 42
(Brookton) Pn per g soil. At Pinery, the corresponding Pn numbers were 6, 22 and 37 per g soil.
Pn number after medic
At both sites the number of Pn declined, following the growth of medic (Fig. 1).
The magnitude of reduction was greatest where initial numbers were highest. Where Pn number was
initially high (37 to 42 after Brookton wheat), the mean decline was 60% after the medic phase. Where
Pn number was initially moderate (16 to 22 after EGA Wylie wheat) the mean decline was 46%. Where
nematode number was initially low (2 to 6 after Abacus triticale) there was no significant effect on
nematode number.
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60
Pratylenchus neglectus (number/g soil)
Initial Pn
50
Final Pn
40
30
20
10
0
Herald
RH-1
Herald
RH-1
Abacus
Abacus
Abacus
Abacus
Pinery
Pinery
Arthurton Arthurton
Herald
RH-1
Herald
RH-1
EGA Wylie EGA Wylie EGA Wylie EGA Wylie
.
Arthurton Arthurton
Pinery
Pinery
.
Herald
RH-1
Herald
RH-1
Brookton
Brookton
Brookton
Brookton
Pinery
Pinery
Arthurton Arthurton
Figure 1 Number of root lesion nematodes (Pratylenchus neglectus) in soil, before (closed bars) and
after (open bars) medic (Herald or RH-1) in field trials at Arthurton and Pinery, South Australia. Initial
nematode numbers varied according to the cereal host (Abacus, EGA Wylie or Brookton) grown the
previous year. Bars indicate standard error of treatment means.
Conditions at the sites should have been favourable to nematode multiplication, because Pn
multiplication was previously achieved using susceptible cereals.
Reductions in Pn number were consistent at both sites, which were about 100 km apart. Spring dry
matter yields for Herald and RH-1 were approximately 3.7 and 2.3 t ha respectively at Arthurton; 3.3
and 2.6 t ha at Pinery. These differences in medic production did not obviously affect changes in Pn
number at either site.
Key Messages
The results show that strand medics are unlikely to increase the number of Pn, under field conditions.
In this study, there was no instance where Pn number was increased under the two strand medics
tested. Where moderate population densities of Pn were initially present (>10/g soil) reductions in
nematode number were measured after the growth of strand medic. The results support previous
field studies that have included strand, burr and barrel medics (Ballard et al. 2006, Taylor et al. 2000).
In a practical sense, Pn numbers were sufficiently reduced in this study (by up to 60%) to downgrade
paddock risks with respect to potential yield losses in intolerant crops (McKay et al. 2008). On this
basis, the medic cultivar Herald would be a reasonable rotational choice where Pn is present and
control is sought, so long as other Pn susceptible hosts (broadleaf weeds and grasses) in the pasture
are controlled.
RH-1 (another strand medic) behaved similarly to Herald and provides some evidence of broader Pn
field resistance in the strand medics. The cultivar Angel has similarly been shown to reduce Pn number
in another field study (R.A. Ballard, unpub. data).
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The reason for the disparity in reported resistance ratings for medic most likely stems from whether
the classification is based on greenhouse assays or field tests. Whilst we do not dispute that medics
can host Pn and therefore may increase their number under greenhouse conditions, under less
favourable field conditions where there are fewer or less persistent lateral roots, shortened periods
of optimal growth and interactions with other soil flora, increases in Pn are not the reality. It may
come down to an interpretation of terminology. In a strict pathological interpretation, medics may
well be susceptible and multiply Pn under ideal conditions, but in a practical farming sense they should
be regarded as resistant.
The significance of the effect of initial Pn number on our interpretation of previous studies is that
there appears a slightly improved opportunity to reduce Pn where initial numbers are greater. At very
low numbers of Pn, for example under Abacus, numbers were maintained at the low level after medic.
It also becomes more difficult to measure changes in Pn number at the lower limits of method
sensitivity.
This study shows that strand medics can be grown in rotations to manage levels of Pn.
Acknowledgements
The work was funded by South Australian Grain Industry Trust and GRDC.
The work was conducted on the properties of Andrew Barr (Pinery) and Neville Rowe (Arthurton)
References (further reading)
Ballard RA, Hutton RE, Taylor SP, McKay AC, Howie JH (2006). Field resistance of annual pasture
legumes to the root lesion nematode, Pratylenchus neglectus. Aust. Plant Pathology. 35, 303-308.
Collins S, Wilkinson C, Kelly S, Hunter H, DeBrincat L (2014). ‘Root Lesion Nematode has a Picnic in
2013’. 2014 Crop Updates, Grain Industry Association of Western Australia.
Oldach KH, Peck DM, Nair RM, Sokolova M, Harris J, Bogacki P, Ballard RA (2014). Genetic analysis of
tolerance to the root lesion nematode Pratylenchus neglectus in the legume Medicago littoralis. BMC
Plant Biology. 14, 100.
McKay A, Roget D, Hannam R, Ophel Keller K (2008). ‘Root Disease Risk Management Manual’. South
Australian Research and Development Institute, South Australia. p. 59.
Taylor SP, Hollaway GJ, Hunt CH (2000) Effect of field crops on population densities of Pratylenchus
neglectus and P. thornei in southeastern Australia; Part 1: P. neglectus. Journal of Nematology 32,
591-599.
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