Session 13 – Constraints on Forage and Grassland Production 13-13ABSTRACT [623252]

Session 13 – Constraints on Forage and Grassland Production 13-13ABSTRACT
A simple pot test was used to indicate the combined effects of several
pathogens common in pasture soils (plant parasitic nematodes and
pathogenic fungi e.g. Pythium spp.) by comparing the dry weight
yields of clover seedlings grown in untreated soil with those from
soil treated in a microwave oven. Response to microwave treatment,
expressed as a "Soil Pathogenicity Index", was greater with soil from
old pasture or from areas in 2 year old pasture plots showing poor
regrowth after grazing of white clover ( Trifolium repens L) or
Caucasian clover ( T. ambiguum Bieb.), than from soil from new
pasture or areas in the 2 year old plots with vigorous clover regrowth.
KEYWORDS
Soilborne pathogens, nematodes, seedling vigour, white clover,
Caucasian clover
INTRODUCTION
Soilborne pathogenic nematodes, fungi and bacteria can be major
biological constraints to productivity of pasture and forage crop plants
(Skipp and Watson, 1996; Mercer and Watson, 1996) through their
adverse effects on seedling establishment, persistence, drought
tolerance, regrowth after grazing, N-fixation, and/or efficiency of
fertilizer use. However, it has been difficult to define and quantify
losses because damage frequently results from complex interactions
among several soilborne pathogens (Skipp and Watson, 1996).
Current attempts to develop bioindicators of 'soil quality' to monitor
changes in the sustainability of farming systems (Pankhurst, 1994)
have largely ignored the impact of plant pathogens. Cook (1994)
has advocated measurement of increases in plant growth resulting
from disinfestation of soil (e.g. by heat sterilization or chemical
fumigation) for field-scale detection of effects of soilborne pathogens.
This paper describes the use of simple pot tests which indicate
'pathogen potential' of soils by comparison of seedling growth in
untreated soil and in soil treated with microwave (MW) radiation.
METHODS
Core samples of a light volcanic ash soil (Paengaroa Shallow Sand,
a typic Hapludand) were obtained from a dairy farm near Te Puke,
North Island, New Zealand which had shown declining milk yields
associated with poor performance of white clover ( Trifolium repens
L) (Watson et al. 1994). Pot experiments were conducted using soil
from pastures containing perennial ryegrass ( Lolium perenne L) and
white clover or Caucasian clover ( T. ambiguum Bieb.). Samples were
taken from areas showing differences in plant vigour as follows: (a)
‘New pasture’ (first year pasture with cv. Kopu white clover); (b)
‘Old pasture’ (>10 years, local white clover ecotype); (c) ‘High vigour
white clover’ (from areas in a second year pasture after maize
cropping within patches showing good regrowth of cv Kopu white
clover after grazing); (d)‘Low vigour white clover’ (as ‘c’ showing
poor regrowth); (e) ‘High vigour Caucasian clover’ (as ‘c’ from
pasture containing cv Endura Caucasian clover); (f) ‘Low vigour
Caucasian clover’. Soil was air dried, crushed and sieved (9 mm).
Bulk soil from each category was placed in 1 kg quantities in 2
polyethylene bags and moistened to 40% water holding capacity
(WHC). One bag of each category was untreated and the other treated
in a 650 watt microwave oven for 3 min at full power (Nan et al.
1991). Cool soil (150g) was placed in 65mm diam. unperforated
plastic cups, sown with 20 seeds of white clover (cv. Huia) or
Caucasian clover (cv. KZ1) and covered with 20g of the same soil.ID NO. 1146
INDICATORS OF PATHOGEN POTENTIAL OF PASTURE SOILS
R.A. Skipp1 , R.N. Watson2, and G.C.M Latch1
1 AgResearch, Grasslands Research Centre, PB11008, Palmerston North, New Zealand
2 AgResearch, Ruakura Agricultural Centre, PB3123, Hamilton, New Zealand
Five replicate pots of treated and untreated soil were prepared per
soil category. Pots were maintained at 80% WHC and 20°C in
temperature-controlled water baths in the glasshouse for 2 months.
Shoot dry weight (DW) data were analysed by 2 way ANOVA and
the influence of soilborne pathogens expressed as "Soil Pathogenicity
Index" (SPI) = 1 – (DW shoots from untreated soil/DW shoots from
MW treated soil).
RESUL TS AND DISCUSSION
More (P<0.01) white clover seedlings established in pots of MW
treated soil (76%) than in untreated soil (65%), whereas establishment
of Caucasian clover was not affected by MW treatment. The mean
DW of shoots of seedlings grown in untreated soil from new pasture
was greater than that of seedlings in untreated old pasture soil (Table
1), while the shoot DW of white clover and Caucasian clover
seedlings from their respective ‘high’ and ‘low’ vigour soils did not
differ significantly.
Microwave-treated soil yielded greater seedling DW than untreated
soil for each of the six soils tested (Table 1). However, the percent
increase in response to MW treatment (and the SPI) was greater for
the old pasture and ‘low vigour’ soil than for the new pasture and
‘high vigour’ soil. Roots of seedlings washed from MW treated soil
were white and lacked symptoms of pathogen damage while roots
from untreated soil bore root galls caused by Meloidogyne spp.
nematodes, cysts of Heterodera trifolii Goffart and dark fungal root
lesions. Pathogenic fungi isolated from lesioned roots included
Codinaea fertilis Hughes & Kendrick, Pythium spp., Fusarium spp.
and dark sterile forms.
The SPI provides an indication of the potential effects of the combined
natural inoculum of pathogens in a particular soil in terms of growth
depression relative to an arbitrary optimum value defined by growth
in soil rendered ‘pathogen-free’ by microwave radiation. In the
absence of any substantial inter-plant competition, the index will
reflect the additive effects of pathogens which reduce plant numbers
by killing seedlings (e.g. Pythium spp.) and those which reduce
seedling vigour (e.g. nematodes). The effects of these two types of
pathogens can be differentiated using separate indices for effects on
seedling number and of dry weight per plant. Use of soil treatments
more selective in their action than MW treatment (e.g. soil freezing,
nematicides and fungicides), coupled with the use of root examination
and fungal isolation techniques, can help further differentiate the
influence of different pathogens (Skipp and Watson 1996). Since
nematode and fungal pathogens which invade seedling roots affect
plant growth long after establishment, particularly during periods of
rapid root initiation and growth, the use of such indicators are
applicable to mature permanent pastures.
REFERENCES
Cook, R.J. 1994. Introduction of soil organisms to control root
diseases. Pages 13-22 in C.E. Pankhurst, B.M. Doube, V.V.S.R. Gupta
and P.R. Grace eds Soil biota: Management in sustainable farming
systems. CSIRO Press, Melbourne, Australia.
Mercer, C.F. and R.N. Watson .1996. Nematode pathogens of New
Zealand pastures. Pages 241-256 in S. Chakraborty, K.T. Leath, R.A.
Skipp, G.A. Pederson, R.A. Bray, G.C.M. Latch and F.W. Nutter,
eds. Pasture and forage crop pathology. American Society of

Session 13 – Constraints on Forage and Grassland Production 13-14Agronomy, Crop Science Society of America, Soil Science Society
of America, Madison, Wisconsin.
Nan, Z.B., R.A. Skipp and P.G. Long. 1991. Use of fungicides to
assess the effects of root disease: effects of prochloraz on red clover
and microbial populations in soil and roots. Soil Biol. Biochem. 23:
743-750.
Pankhurst, C.E. 1994. Biological indicators of soil health and
sustainable productivity. Pages 331-351 in D.J. Greenland and I.
Szabolcs, eds Soil resilience and sustainable land use. CAB
International, Wallingford, UK.Skipp, R.A. and R.N. Watson. 1996. Disease complexes in New
Zealand pastures. Pages 429-451 in S. Chakraborty, K.T. Leath,
R.A. Skipp, G.A. Pederson, R.A. Bray, G.C.M. Latch and F.W.
Nutter, eds. Pasture and forage crop pathology. American Society of
Agronomy, Crop Science Society of America, Soil Science Society
of America, Madison, Wisconsin.
Watson, R.N., N.L. Bell, F.J. Neville and S.L. Harris. 1994.
Improving pasture sustainability by reducing the impact of clover
nematodes. Pages 83-85 in C.E. Pankhurst ed. Soil biota:
Management in sustainable farming systems. CSIRO Press,
Melbourne, Australia.
Table 1
Mean shoot dry weight per pot, % increase in shoot DW resulting from MW treatment, and soil pathogen index (SPI) of seedlings grown in
untreated and treated soil from new (first year) and old pasture (>10 years), and high and low vigour patches in second year pasture containing
white or Caucasian clover.
Source of Soil Untreated MW-treated % Increase SPI
White clover seedlings
New pasture 0.477 0.583 22.2 0.18
Old pasture 0.263 0.631 139.9 0.58
High vigour white clover 0.421 0.745 76.7 0.43
Low vigour white clover 0.317 0.696 119.6 0.54
LSD (P<0.05) 0.106
(P<0.01) 0.143
Caucasian clover seedlings
High vigour Caucasian clover 0.356 0.505 41.9 0.30
Low vigour Caucasian clover 0.304 0.615 102.3 0.51
LSD (P<0.05) 0.106
(P<0.01) 0.142