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Reduced severity of rhizoctonia solani disease on wheat seedlings associated with the presence of the earthworm aporrectodea trapezoides (lumbricidae)
Soil Biol.Btochem.Vol. 25, No. 11, pp. 1477-1484,1993 Printed in Great Britain. All rights reserved
0038-0717/93 $6.04 + 0.00 Copyright 0 1993 Pergamon Press Ltd
ACCELERATED PAPER
REDUCED SEVERITY OF RHIZOCTONIA SOLANI DISEASE ON WHEAT SEEDLINGS ASSOCIATED WITH THE PRESENCE OF THE EARTHWORM APORRECTODEA TRAPEZOIDES (LUMBRICIDAE) P. M. STEPHENS,‘* C. W. DAVOREN,’ B. M. DouBE,‘*~ M. H. RYDER,‘** A. M. BENGER~ and S. M. NEATE”~ ‘Cooperative Research Centre for Soil and Land Management and %SIRO, Division of Soils, Private Bag No. 2, Glen Osmond, SA 5064, Australia (Accepted 26 March 1993) Sununary-The influence of the earthworm Aporrectodea trapezoides on wheat plants grown in a calcareous sand loam and a red-brown earth soil, artificially infested with Rhizoctonia solani, was examined. Presence of the earthworm A. trapezoides, at a number equivalent to 471 me2 was associated with a significant increase in shoot weight and a reduction in the root disease rating of wheat in both soil types artificially infested with R. solani on wheat chaff. In both soil types, the shoot weight of wheat grown in the presence of A. trapezoides and R. solani was equivalent to that of wheat grown in the absence of R. solani. One further treatment was applied to each soil type In the calcareous sand inoculated with R. so/ant on wheat chaff, A. trapezoides was as effective as mechanical soil disturbance in reducing the root disease rating on wheat. In the red brown earth, in which R. sofani was introduced via naturally infected roots, the presence of A. trapezoides was associated with a significant increase in shoot weight and a reduction in the root disease rating of wheat. These results demonstrate the potential of the earthworm A. trapezoides to reduce the effect of Rhizoctonia root rot on wheat. INTRODUCTION
Soil degradation resulting from over cultivation is a major problem in southern Australia (Rovira et al., 1987). Conservation tillage practices, including direct drilling, have significant advantages over conventional farming practices in reducing soil degradation (Rovira, 1986). Conservation tillage practices may also result in reduced soil preparation costs and increase the availability of annual pastures for grazing (Rovira, 1986). However, the adoption of conservation tillage practices for growing wheat in many parts of southern Australia has been limited due to Rhizoctonia solani Kuhn, the causative agent of the cereal root disease ‘Rhizoctonia bare patch’ (Banyer, 1966). R. sohzni cannot be economically controlled by crop rotation or chemicals and its severity is increased with conservation tillage (MacNish, 1985; Neate, 1989). To exploit the benefits derived from conservation tillage, factors which influence yield loss due to R. solani need to be identified, so that they might be manipulated to control this root disease. Major differences have been observed in the natural capacity of some soils to suppress R. sol& (Rovira et al., 1990). Previous efforts to isolate the causes of this suppression and use them to control R. solani, have focused primarily on isolating biological antagonists of R. sohtni from soil. Microbial
*Author for correspondence.
antagonists that have been shown to reduce the incidence of root disease due to R. solani, include Trichoderma spp (Beagle-Ristaino and Papavizas, 1985; Lewis and Papavizas, 1991) Ghocladium virens Miller (Lumsden and Locke, 1989; Lewis and Papavizas, 1991), Corticum sp. (Odvody et al., 1980klater described as Laetisariu arvalis Burdsall (Burdsall et al., 1980), Streptomyces hygroscopicus var. geldanus (Rothrock and Gottlieb, 1984), Pseudomonasfluorescens (Dahiya et al., 1988) and nonpathogenic isolates of R. soluni (Ichielevich-Auster et al., 1985; Cardoso and Echandi, 1987; Harris et al., 1992). Control of R. soluni has also been reported using large populations of the nematode Aphefenchus avenue (Barnes et al., 1981) and the collembolans Proisotoma minutu Tullberg (Isotomidae) and Onychiurus encarpatus Denis (Porudidae) (Curl et al.,
1985). The severity of disease due to R. solani is reduced by mechanical soil disturbance (Rovira, 1986). The observation that earthworms disturb the soil profile would suggest that earthworms may also influence R. solani. Earthworms have been shown to influence plant fungal pathogens, by acting as vectors of the causative agent of potato wart disease (Synchytrium endobioticum Schilb. Pert.) (Hampson and Coombes, 1989) verticillium wilt of mint (Verticiflium dahliue Kleb.) (Melouk and Horner, 1976) and dwarf bunt (Tilletiu controversa Kuhn) (Hoffman and Purdy, 1964). In this context, we have assessed whether the
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%‘EPHENS et
earthworm Aporrecta~u trapezoides Duges, which is widespread under wheat in southern Australia (Mele, 1991; Baker el af., 1993; Buckerfield, 1993) can influence the disease severity of R. sofani in soil. This study was conducted in two different soil types, a red-brown earth and a calcareous sand loam, representing two of the major soil groups in which wheat is grown in southern Australia. MATERIALS
AND METHODS
Surface soil (O-10 cm) was taken from two sites: (1) Kapunda-containing volunteer pasture, pH 5.0, red-brown earth; classified as a thermic Cal& Natrixeralf (Soil Survey Staff, 1990) and (2) Avon-previously cropped to wheat, pH 8.2, calcareous sand loam, classified as a Petrocalcic Palexerolf (Soil Survey Staff, 1990); these sites being respectivefy 75 and 100 km north of Adelaide, South Australia. Both soils were air dried, sieved (4 mm), adjusted to 75% of field capacity and the equivalent of 400 g air dried soil was added to SOOml plastic pots. Each pot contained either (a) no inoculum, (b) R. soluni, (c) Aporrectodea trapezoides or (d) R. solani + Aporrectodea trapezoides, with six replications of each treatment for each soil type. Inoculum of R. solani Kuhn AGX (McDonald and Rovira, 1985) was produced on autoclaved wheat chaff according to the method of Benger (1990). Six propagules of this inoculum were added to the appropriate pots at soil depths of 3 and 6 cm. In addition, three adult earthworms of Aporrectodea trapezoides, each weighing 0.75-l 5 g, were added to the appropriate pots. Aluminium fly wire (mesh size 2 mm) was attached around the circumference of each pot to a height of 15 cm, in order that Aporrectodea trapezoides did not escape. Pots were placed in random order in a plant growth cabinet, maintained at 15°C with a 12 h day and a photosynthetic photon flux density of 260gE s-’ mm2 at plant height. After 14 days, 13 seeds of wheat (Triticum aestivum cv. Spear) were planted at a depth of 1 cm into each pot. One extra treatment was applied to each soil type. In the calcareous sand, the influence of mechanical soil disturbance (in the absence of Aporrectodea trapezoides) on R. solani was evaluated. Soil in pots, previously inoculated with R. solani for 14 days, was transected vertically four times with a sterile blade, followed by four vertical cuts at 90” angle to the initial cuts, prior to being sown to wheat. In the red-brown earth, an evaluation was made of the influence of Aporrectodea trapezoides on R. sofani that was added to soil via naturally-infected wheat roots. Roots infected with R. solani were collected from wheat plants, previously grown for 16 days in the same red-brown earth soil (inoculated with R. solani in the manner described previously). Roots were air dried, cut into 2 cm sections and 25 mg added at 3 cm soil depth to the appropriate pots. Eighteen days after sowing, plants were removed
al.
from pots and the roots were rated for disease severity on a scale of O-5, where 0 = no disease and 5 = maximum disease with 100% of primary roots severely truncated (McDonald and Rovira, 1985). Weights of roots and shoots were measured after drying at 60°C for 48 h. The causal agents of disease were identified by placing four roots (exhibiting signs of root necrosis) from each treatment, on to water agar amended with 1OOpg tetracycline ml-’ and 100 pg ml-’ streptomycin ml-‘. Fungi were identified according to morphological characteristics. Statistical analysis was conducted using ANOVA and significant F-tests (P < 0.05) were further analysed by LSD for comparison of treatment effects.
RESULTS
The ability of the earthworm Aporrectodea trapezoides to influence the disease severity of R. solani on wheat was assessed in two different soil types. All earthworms added to pots were recovered at the termination of the experiment. Fungi, identified as R. solani, were isolated from roots derived from all treatments in which R. sofani was introduced. Seuerity of ~~zocto~~a root damage. The addition of the inoculum of R. solani grown on wheat chaff caused a significant (P < 0.05) increase in the root disease rating of wheat in both soils, with the greatest increase in the red-brown earth soil (Figs 1 and 2). Aporrectodea trapezoides caused a significant reduction in the root disease rating in both soil types, where R. solani was introduced on wheat chaff (Figs 1 and 2). However, in both soil types, the root disease rating of wheat grown in the presence of Aporrectodea trapezoides and R. solani was significantly (P < 0.05) greater than that of wheat grown in the absence of inoculum containing R. solani. In the calcareous sand inoculated with R. soiani in wheat chaff, Aporrectodea trapezoides was as effective as soil disturbance in reducing the root disease rating (Fig. 1). In the red-brown earth, the presence of R. soiuni in naturally-infected roots caused a significant (P < 0.05) increase in the root disease rating. Under these conditions, Aporrectodea trapezoides caused a significant reduction in the root disease rating, to a level not signi~cantly different (P > 0.05) to that of wheat plants grown in the absence of inoculum containing R. soluni (Fig. 2). Plant growth. Aporrectodea trapezoides had no significant effect upon either shoot or root weight of wheat in the absence of inoculum containing R. solani in both soil types (Figs 3-6). In the presence of Aporrectodea trapezoides there was a significant (P < 0.05) increase in the shoot weight of wheat in the calcareous sand and red-brown earth soil, where R. soiani was introduced in wheat chaff (Figs 3 and 4). In both soil types, the shoot weight of wheat grown in the presence of Aporrectodea trapezo~des and R. solani was similar to that
Reduced severity of R. solani caused by A. trapezoides
1479
Fig. 1. Influence of the earthworm Aporrectodea trapezoides and R. solani on the root disease rating (O-5 scale) of wheat grown in a calcareous sand soil. Columns containing the same letter were not significant at the P < 0.05 level.
”
Conlrol
A.trapazott#e8
R.sola&
A.inpM?ki~* + R.soknf
rod ittfd by Rsohi
A.irrpemidm f toots infd
by R.solani Fig. 2. Influence of the earthworm Aporrectodea traperoides and R. solani on the root disease rating (O-5 scale) of wheat grown in a red-brown earth soil. Columns containing the same letter were not significant at the P < 0.05 level.
1480
P.
M.
STEPHENS et al.
”
Control
A.trapwoides
R.Solatll
A.trapozoldes
R.solani
+ R.soli&
+ dhturbance
Fig. 3. Influence of Aporrectodea trapezoides and R. solani on the average shoot weight of wheat grown in a calcareous sand soil. Columns containing the same letter were not significant at the P < 0.05 level.
A.trapezoides + z?.soimi
roots lnfd by Rmtani
A. tr~pezokies + rob Inf’d by R.sol#~
Fig. 4. Influence of Aporrectodea trapezoides and R. solani on the average shoot weight of wheat grown in a red-brown earth soil, Cohm-ms containing the same letter were not significant at the P < 0.05 level.
Reduced severity of R. solani caused by A. trapezoides
1481
12
10
8
6
4
2
0
Control
A. trapwaldes
Rsokni
R.solani + dirturbutcre
Fig. 5. Influence of Aporrectodea trapezoides and R. solani on the average root weight of wheat grown in a calcareous sand soil. Columns containing the same letter were not significant at the P K 0.05 level.
”
COntrOl
AJraparoldes
R.8dmi
A.trepemidea + Rsmw
mow inf’d by U.~Q/##
A.trrpomidos + root8 infd by A.ro&ni
Fig. 6. Infiuence of Aporrecrodea ~ra~zoi~s and R. solani on the average shoot weight of wheat grown in a red-brown earth soil. Columns containing the same Ietter were not significant at the P < 0.05 level.
P. M. STEPHENS et al.
1482
of wheat grown in the absence of inoculum containing R. solani. In the calcareous sand soil inoculated with R. solani in wheat chaff, soil disturbance did not significantly (P > 0.05) influence wheat shoot weight (Fig. 3). In the red-brown earth, there was a significant (P < 0.05) decrease in wheat shoot weight when R. solani was added to the soil via naturally-infected roots. Under these conditions, the presence of Aporrectodea trapezoides was associated with a significant (P < 0.05) increase in wheat shoot weight; to a level equivalent to that of wheat grown in the absence of inoculum containing R. solani (Fig. 4). In the calcareous sand, neither the introduction of R. solani in wheat chaff or the presence of Aporrectodea trapezoides, significantly (P c 0.05) influenced root weight (Fig. 5). In the red-brown earth inoculated with R. solani in wheat chaff, the presence of Aporrectodea trapezoides was associated with a significant (P < 0.05) increase in root weight; but to a level significantly (P < 0.05) lower than that of wheat roots grown in the absence of inoculum containing R. solani (Fig. 6). DISCUSSION
The presence of Aporrectodea trapezoides did not significantly influence the root or shoot weight of wheat after 18 days growth in the absence of R. solani, in either the calcareous sand or red-brown earth soil. However, recent studies in the same soils have shown that in glasshouse trials, the presence of Aporrectodea trapezoides (at a density similar to the level used here; 460m-2) was associated with a significant (P < 0.05) increase in wheat top biomass by 8.2% in the calcareous sand and 38.7% in the red-brown earth, when plants were harvested at maturity (G. H. Baker and P. M. L. Williams; pers. commun.). In both the calcareous sand and red-brown earth soil, where R. solani was introduced in wheat chaff, the level of root disease due to R. solani was substantially reduced in the presence of Aporrectodea trapezoides. There was also a significant reduction in root disease and a significant increase in shoot weight when Aporrectodea trapezoides was present and R. solani was introduced into the red-brown earth soil via naturally-infected wheat roots. In these experiments, Aporrectodea trapezoides was added to pots at a number equivalent to 471 mm2 Peak earthworm numbers ranging from 85 to 800mm2 have been recorded at two sites containing red-brown earth soils and direct drilled wheat in southern Australia, with the peak number of Aporrectodea trapezoides ranging from 19 to 140 me2 (Buckerfield, 1993). The peak number of Aporrectodea trapezoides in a red-brown earth soil in southern Australia, containing direct drilled canola (after 4 yr pasture), was 382 mm2 (Doube, Buckerfield and Kirkegaard; pets. commun.). There is no pub-
lished data on earthworm numbers per species under direct drilled wheat in calcareous sand soils in southern Australia. However, in a brown calcareous earth in southern Australia, previously sown to barley using conventional tillage, the peak number of earthworms recorded was ca 300me2 with Aporrectodea rosea Savigny representing over 90% of the earthworms found (Baker et al., 1993). The density of Aporrectodea trapezoides used in the experiments outlined in this paper is therefore probably higher than what may be found in a calcareous sand and red-brown earth, but comparable to the total number of earthworms that may be found in a red-brown earth under direct drilled wheat in southern Australia. Further studies are required to determine if the ability to reduce damage caused by R. solani varies between earthworm species. No mechanism is known by which Aporrectodea trapezoides might influence the severity of disease due to R. solani on wheat roots. Results from our study (in the calcareous sand), together with those of Rovira and Venn (1985) and Rovira (1986), have shown that soil disturbance reduces the disease severity of R. solani on cereal roots. The observation that movement of Aporrectodea trapezoides disrupts the soil profile would suggest that soil disturbance may be one mechanism by which Aporrectodea trapezoides reduces the severity of R. solani. MacNish (1985) reported that certain forms of nitrogen fertilizers may also reduce rhizoctonia bare patch of cereals. Reports that earthworms increase the amount of plant available N in soil (Svensson et al., 1986; Anderson et al., 1991), would suggest that Aporrectodea trapezoides may also have controlled R. solani by increasing the concentration of plant available nitrogen in soil. Alternatively, Aporrectodea trapezoides may have reduced the infection level of R. sofani in soil, by ingesting hyphae and killing the fungus in its intestine or through producing unfavourable conditions for R. solani in its cast material or tunnel lining. Kamal and Weinhold (1967) reported that R. solani required a suitable nutrient supply in order to be pathogenic. Earthworms may limit the nutrient supply to R. solani and therefore also its pathogenicity, by accelerating the rate of decomposition of plant residues (Lee, 1985; Bouche et al., 1987), which act as important substrates for propagules of R. solani in soil (Weinhold, 1977). A similar trend has been shown for Gaeumannomyces graminis var. tritici, where fragmentation of plant residues due to mechanical cultivation reduces take-all on wheat (Moore and Cook, 1984). The results from these pot trials demonstrate the potential of Aporrectodea trapezoides to control R. solani on wheat. These observations need to be tested in the field, in order to ascertain whether Aporrectodea trapezoides can reduce the effects of rhizoctonia root rot where conservation tillage practices have been introduced. The observation that conservation tillage practices favour the build up of
Reduced severity of R. solani caused by A. trapezoides
earthworms
(Rovira et nl., 1987) and can favour trupezoides in particular (Buckerfield, 1993), will facilitate the development of management strategies involving the use of earthworms to control
Aporrectodea
R. solani.
Harris A. R., Schisler D. A. and M. H. Ryder (1992) Binucleate Rhizoctonia isolates control Pythium and Rhizoctonia solani and promote seedling growth. Phytopathology 82, 1109. Hoffman J. A. and Purdy L. H. (1964) Germination of dwarf bunt teliospores after ingestion by earthworms. Phytopathology
was supported in part by the Grains Research and Development Corp., Australia. Thanks are due to Dr Geoff Baker, John Buckerfleld and Dr Ken Lee for critical reading of this manuscript. Acknowledgements-This
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Kamal M. and Weinhold A. R. (1967) Virulence of Rhizoctonia solani as influenced by age’of inocuhnn in soil. REFERENCES Anderson J., Knight D. and Elliot P. (1991) Effects of invertebrates on soil properties and processes. In Advances in Management and Conservation of Soil Fauna (G. K. Veerish. D. Raianooal and C. A. Viraktamath,
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Cardoso J. E. and Echandi E. (1987) Nature of protection of bean seedlings from Rhizoctonia root rot by a binucleate Rhizoctonia-like fungus. Phytopathology 77, 1548-1551.
Curl E. A., Gudauskas R. T., Harper J. D. and Peterson C. M. (1985). Effects of soil insects on populations and germination of fimgal propagules. In Ecology and Management of Soilborne Plant Pathogens (C. A. Parker, A. D. Rovira, K. J. Moore, P. T. W. Wong and J. F. Kollmorgen, Eds), pp. 20-23. American Phytopathology Society, St Paul, Minn. Dahiya J. S., Woods D. L. and Tewari J. P. (1988) Control of Rhizoctonia solani, causal agent of brown girdling root rot of rapeseed, by Pseudomonas fluorescens. Botanical Bulletin Academia-Sinica
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McDonald H. J. and Rovira A. D. (1985) Development of an inoculation technique for Rhizoctonia solani and its application to screening cereal cultivars for resistance. In Ecology and Management of Soilborne Plant Pathogens (C. A. Parker, A. D. Rovira, K. J. Moore, P. T. W.
Wong and J. F. Kollmorgen, Eds), pp. 174-176. American Phytopathology Society, St Paul, Minn. Mele P. (1991) What species and how many on local farms. Earthworms, improving soil for agriculture. Proceedings of a conference held at TAFE College Wangaratta, Victoria, Australia. AIAS occasional publication No. 62. Melouk H. A. and Homer C. E. (1976) Recovery of Verticillium dahliae pathogenic to mints from castings of earthworms. American Phytopathology Society Proceedings 3, 265.
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0038-0717/93 $6.04 + 0.00 Copyright 0 1993 Pergamon Press Ltd
ACCELERATED PAPER
REDUCED SEVERITY OF RHIZOCTONIA SOLANI DISEASE ON WHEAT SEEDLINGS ASSOCIATED WITH THE PRESENCE OF THE EARTHWORM APORRECTODEA TRAPEZOIDES (LUMBRICIDAE) P. M. STEPHENS,‘* C. W. DAVOREN,’ B. M. DouBE,‘*~ M. H. RYDER,‘** A. M. BENGER~ and S. M. NEATE”~ ‘Cooperative Research Centre for Soil and Land Management and %SIRO, Division of Soils, Private Bag No. 2, Glen Osmond, SA 5064, Australia (Accepted 26 March 1993) Sununary-The influence of the earthworm Aporrectodea trapezoides on wheat plants grown in a calcareous sand loam and a red-brown earth soil, artificially infested with Rhizoctonia solani, was examined. Presence of the earthworm A. trapezoides, at a number equivalent to 471 me2 was associated with a significant increase in shoot weight and a reduction in the root disease rating of wheat in both soil types artificially infested with R. solani on wheat chaff. In both soil types, the shoot weight of wheat grown in the presence of A. trapezoides and R. solani was equivalent to that of wheat grown in the absence of R. solani. One further treatment was applied to each soil type In the calcareous sand inoculated with R. so/ant on wheat chaff, A. trapezoides was as effective as mechanical soil disturbance in reducing the root disease rating on wheat. In the red brown earth, in which R. sofani was introduced via naturally infected roots, the presence of A. trapezoides was associated with a significant increase in shoot weight and a reduction in the root disease rating of wheat. These results demonstrate the potential of the earthworm A. trapezoides to reduce the effect of Rhizoctonia root rot on wheat. INTRODUCTION
Soil degradation resulting from over cultivation is a major problem in southern Australia (Rovira et al., 1987). Conservation tillage practices, including direct drilling, have significant advantages over conventional farming practices in reducing soil degradation (Rovira, 1986). Conservation tillage practices may also result in reduced soil preparation costs and increase the availability of annual pastures for grazing (Rovira, 1986). However, the adoption of conservation tillage practices for growing wheat in many parts of southern Australia has been limited due to Rhizoctonia solani Kuhn, the causative agent of the cereal root disease ‘Rhizoctonia bare patch’ (Banyer, 1966). R. sohzni cannot be economically controlled by crop rotation or chemicals and its severity is increased with conservation tillage (MacNish, 1985; Neate, 1989). To exploit the benefits derived from conservation tillage, factors which influence yield loss due to R. solani need to be identified, so that they might be manipulated to control this root disease. Major differences have been observed in the natural capacity of some soils to suppress R. sol& (Rovira et al., 1990). Previous efforts to isolate the causes of this suppression and use them to control R. solani, have focused primarily on isolating biological antagonists of R. sohtni from soil. Microbial
*Author for correspondence.
antagonists that have been shown to reduce the incidence of root disease due to R. solani, include Trichoderma spp (Beagle-Ristaino and Papavizas, 1985; Lewis and Papavizas, 1991) Ghocladium virens Miller (Lumsden and Locke, 1989; Lewis and Papavizas, 1991), Corticum sp. (Odvody et al., 1980klater described as Laetisariu arvalis Burdsall (Burdsall et al., 1980), Streptomyces hygroscopicus var. geldanus (Rothrock and Gottlieb, 1984), Pseudomonasfluorescens (Dahiya et al., 1988) and nonpathogenic isolates of R. soluni (Ichielevich-Auster et al., 1985; Cardoso and Echandi, 1987; Harris et al., 1992). Control of R. soluni has also been reported using large populations of the nematode Aphefenchus avenue (Barnes et al., 1981) and the collembolans Proisotoma minutu Tullberg (Isotomidae) and Onychiurus encarpatus Denis (Porudidae) (Curl et al.,
1985). The severity of disease due to R. solani is reduced by mechanical soil disturbance (Rovira, 1986). The observation that earthworms disturb the soil profile would suggest that earthworms may also influence R. solani. Earthworms have been shown to influence plant fungal pathogens, by acting as vectors of the causative agent of potato wart disease (Synchytrium endobioticum Schilb. Pert.) (Hampson and Coombes, 1989) verticillium wilt of mint (Verticiflium dahliue Kleb.) (Melouk and Horner, 1976) and dwarf bunt (Tilletiu controversa Kuhn) (Hoffman and Purdy, 1964). In this context, we have assessed whether the
1477
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1478
%‘EPHENS et
earthworm Aporrecta~u trapezoides Duges, which is widespread under wheat in southern Australia (Mele, 1991; Baker el af., 1993; Buckerfield, 1993) can influence the disease severity of R. sofani in soil. This study was conducted in two different soil types, a red-brown earth and a calcareous sand loam, representing two of the major soil groups in which wheat is grown in southern Australia. MATERIALS
AND METHODS
Surface soil (O-10 cm) was taken from two sites: (1) Kapunda-containing volunteer pasture, pH 5.0, red-brown earth; classified as a thermic Cal& Natrixeralf (Soil Survey Staff, 1990) and (2) Avon-previously cropped to wheat, pH 8.2, calcareous sand loam, classified as a Petrocalcic Palexerolf (Soil Survey Staff, 1990); these sites being respectivefy 75 and 100 km north of Adelaide, South Australia. Both soils were air dried, sieved (4 mm), adjusted to 75% of field capacity and the equivalent of 400 g air dried soil was added to SOOml plastic pots. Each pot contained either (a) no inoculum, (b) R. soluni, (c) Aporrectodea trapezoides or (d) R. solani + Aporrectodea trapezoides, with six replications of each treatment for each soil type. Inoculum of R. solani Kuhn AGX (McDonald and Rovira, 1985) was produced on autoclaved wheat chaff according to the method of Benger (1990). Six propagules of this inoculum were added to the appropriate pots at soil depths of 3 and 6 cm. In addition, three adult earthworms of Aporrectodea trapezoides, each weighing 0.75-l 5 g, were added to the appropriate pots. Aluminium fly wire (mesh size 2 mm) was attached around the circumference of each pot to a height of 15 cm, in order that Aporrectodea trapezoides did not escape. Pots were placed in random order in a plant growth cabinet, maintained at 15°C with a 12 h day and a photosynthetic photon flux density of 260gE s-’ mm2 at plant height. After 14 days, 13 seeds of wheat (Triticum aestivum cv. Spear) were planted at a depth of 1 cm into each pot. One extra treatment was applied to each soil type. In the calcareous sand, the influence of mechanical soil disturbance (in the absence of Aporrectodea trapezoides) on R. solani was evaluated. Soil in pots, previously inoculated with R. solani for 14 days, was transected vertically four times with a sterile blade, followed by four vertical cuts at 90” angle to the initial cuts, prior to being sown to wheat. In the red-brown earth, an evaluation was made of the influence of Aporrectodea trapezoides on R. sofani that was added to soil via naturally-infected wheat roots. Roots infected with R. solani were collected from wheat plants, previously grown for 16 days in the same red-brown earth soil (inoculated with R. solani in the manner described previously). Roots were air dried, cut into 2 cm sections and 25 mg added at 3 cm soil depth to the appropriate pots. Eighteen days after sowing, plants were removed
al.
from pots and the roots were rated for disease severity on a scale of O-5, where 0 = no disease and 5 = maximum disease with 100% of primary roots severely truncated (McDonald and Rovira, 1985). Weights of roots and shoots were measured after drying at 60°C for 48 h. The causal agents of disease were identified by placing four roots (exhibiting signs of root necrosis) from each treatment, on to water agar amended with 1OOpg tetracycline ml-’ and 100 pg ml-’ streptomycin ml-‘. Fungi were identified according to morphological characteristics. Statistical analysis was conducted using ANOVA and significant F-tests (P < 0.05) were further analysed by LSD for comparison of treatment effects.
RESULTS
The ability of the earthworm Aporrectodea trapezoides to influence the disease severity of R. solani on wheat was assessed in two different soil types. All earthworms added to pots were recovered at the termination of the experiment. Fungi, identified as R. solani, were isolated from roots derived from all treatments in which R. sofani was introduced. Seuerity of ~~zocto~~a root damage. The addition of the inoculum of R. solani grown on wheat chaff caused a significant (P < 0.05) increase in the root disease rating of wheat in both soils, with the greatest increase in the red-brown earth soil (Figs 1 and 2). Aporrectodea trapezoides caused a significant reduction in the root disease rating in both soil types, where R. solani was introduced on wheat chaff (Figs 1 and 2). However, in both soil types, the root disease rating of wheat grown in the presence of Aporrectodea trapezoides and R. solani was significantly (P < 0.05) greater than that of wheat grown in the absence of inoculum containing R. solani. In the calcareous sand inoculated with R. soiani in wheat chaff, Aporrectodea trapezoides was as effective as soil disturbance in reducing the root disease rating (Fig. 1). In the red-brown earth, the presence of R. soiuni in naturally-infected roots caused a significant (P < 0.05) increase in the root disease rating. Under these conditions, Aporrectodea trapezoides caused a significant reduction in the root disease rating, to a level not signi~cantly different (P > 0.05) to that of wheat plants grown in the absence of inoculum containing R. soluni (Fig. 2). Plant growth. Aporrectodea trapezoides had no significant effect upon either shoot or root weight of wheat in the absence of inoculum containing R. solani in both soil types (Figs 3-6). In the presence of Aporrectodea trapezoides there was a significant (P < 0.05) increase in the shoot weight of wheat in the calcareous sand and red-brown earth soil, where R. soiani was introduced in wheat chaff (Figs 3 and 4). In both soil types, the shoot weight of wheat grown in the presence of Aporrectodea trapezo~des and R. solani was similar to that
Reduced severity of R. solani caused by A. trapezoides
1479
Fig. 1. Influence of the earthworm Aporrectodea trapezoides and R. solani on the root disease rating (O-5 scale) of wheat grown in a calcareous sand soil. Columns containing the same letter were not significant at the P < 0.05 level.
”
Conlrol
A.trapazott#e8
R.sola&
A.inpM?ki~* + R.soknf
rod ittfd by Rsohi
A.irrpemidm f toots infd
by R.solani Fig. 2. Influence of the earthworm Aporrectodea traperoides and R. solani on the root disease rating (O-5 scale) of wheat grown in a red-brown earth soil. Columns containing the same letter were not significant at the P < 0.05 level.
1480
P.
M.
STEPHENS et al.
”
Control
A.trapwoides
R.Solatll
A.trapozoldes
R.solani
+ R.soli&
+ dhturbance
Fig. 3. Influence of Aporrectodea trapezoides and R. solani on the average shoot weight of wheat grown in a calcareous sand soil. Columns containing the same letter were not significant at the P < 0.05 level.
A.trapezoides + z?.soimi
roots lnfd by Rmtani
A. tr~pezokies + rob Inf’d by R.sol#~
Fig. 4. Influence of Aporrectodea trapezoides and R. solani on the average shoot weight of wheat grown in a red-brown earth soil, Cohm-ms containing the same letter were not significant at the P < 0.05 level.
Reduced severity of R. solani caused by A. trapezoides
1481
12
10
8
6
4
2
0
Control
A. trapwaldes
Rsokni
R.solani + dirturbutcre
Fig. 5. Influence of Aporrectodea trapezoides and R. solani on the average root weight of wheat grown in a calcareous sand soil. Columns containing the same letter were not significant at the P K 0.05 level.
”
COntrOl
AJraparoldes
R.8dmi
A.trepemidea + Rsmw
mow inf’d by U.~Q/##
A.trrpomidos + root8 infd by A.ro&ni
Fig. 6. Infiuence of Aporrecrodea ~ra~zoi~s and R. solani on the average shoot weight of wheat grown in a red-brown earth soil. Columns containing the same Ietter were not significant at the P < 0.05 level.
P. M. STEPHENS et al.
1482
of wheat grown in the absence of inoculum containing R. solani. In the calcareous sand soil inoculated with R. solani in wheat chaff, soil disturbance did not significantly (P > 0.05) influence wheat shoot weight (Fig. 3). In the red-brown earth, there was a significant (P < 0.05) decrease in wheat shoot weight when R. solani was added to the soil via naturally-infected roots. Under these conditions, the presence of Aporrectodea trapezoides was associated with a significant (P < 0.05) increase in wheat shoot weight; to a level equivalent to that of wheat grown in the absence of inoculum containing R. solani (Fig. 4). In the calcareous sand, neither the introduction of R. solani in wheat chaff or the presence of Aporrectodea trapezoides, significantly (P c 0.05) influenced root weight (Fig. 5). In the red-brown earth inoculated with R. solani in wheat chaff, the presence of Aporrectodea trapezoides was associated with a significant (P < 0.05) increase in root weight; but to a level significantly (P < 0.05) lower than that of wheat roots grown in the absence of inoculum containing R. solani (Fig. 6). DISCUSSION
The presence of Aporrectodea trapezoides did not significantly influence the root or shoot weight of wheat after 18 days growth in the absence of R. solani, in either the calcareous sand or red-brown earth soil. However, recent studies in the same soils have shown that in glasshouse trials, the presence of Aporrectodea trapezoides (at a density similar to the level used here; 460m-2) was associated with a significant (P < 0.05) increase in wheat top biomass by 8.2% in the calcareous sand and 38.7% in the red-brown earth, when plants were harvested at maturity (G. H. Baker and P. M. L. Williams; pers. commun.). In both the calcareous sand and red-brown earth soil, where R. solani was introduced in wheat chaff, the level of root disease due to R. solani was substantially reduced in the presence of Aporrectodea trapezoides. There was also a significant reduction in root disease and a significant increase in shoot weight when Aporrectodea trapezoides was present and R. solani was introduced into the red-brown earth soil via naturally-infected wheat roots. In these experiments, Aporrectodea trapezoides was added to pots at a number equivalent to 471 mm2 Peak earthworm numbers ranging from 85 to 800mm2 have been recorded at two sites containing red-brown earth soils and direct drilled wheat in southern Australia, with the peak number of Aporrectodea trapezoides ranging from 19 to 140 me2 (Buckerfield, 1993). The peak number of Aporrectodea trapezoides in a red-brown earth soil in southern Australia, containing direct drilled canola (after 4 yr pasture), was 382 mm2 (Doube, Buckerfield and Kirkegaard; pets. commun.). There is no pub-
lished data on earthworm numbers per species under direct drilled wheat in calcareous sand soils in southern Australia. However, in a brown calcareous earth in southern Australia, previously sown to barley using conventional tillage, the peak number of earthworms recorded was ca 300me2 with Aporrectodea rosea Savigny representing over 90% of the earthworms found (Baker et al., 1993). The density of Aporrectodea trapezoides used in the experiments outlined in this paper is therefore probably higher than what may be found in a calcareous sand and red-brown earth, but comparable to the total number of earthworms that may be found in a red-brown earth under direct drilled wheat in southern Australia. Further studies are required to determine if the ability to reduce damage caused by R. solani varies between earthworm species. No mechanism is known by which Aporrectodea trapezoides might influence the severity of disease due to R. solani on wheat roots. Results from our study (in the calcareous sand), together with those of Rovira and Venn (1985) and Rovira (1986), have shown that soil disturbance reduces the disease severity of R. solani on cereal roots. The observation that movement of Aporrectodea trapezoides disrupts the soil profile would suggest that soil disturbance may be one mechanism by which Aporrectodea trapezoides reduces the severity of R. solani. MacNish (1985) reported that certain forms of nitrogen fertilizers may also reduce rhizoctonia bare patch of cereals. Reports that earthworms increase the amount of plant available N in soil (Svensson et al., 1986; Anderson et al., 1991), would suggest that Aporrectodea trapezoides may also have controlled R. solani by increasing the concentration of plant available nitrogen in soil. Alternatively, Aporrectodea trapezoides may have reduced the infection level of R. sofani in soil, by ingesting hyphae and killing the fungus in its intestine or through producing unfavourable conditions for R. solani in its cast material or tunnel lining. Kamal and Weinhold (1967) reported that R. solani required a suitable nutrient supply in order to be pathogenic. Earthworms may limit the nutrient supply to R. solani and therefore also its pathogenicity, by accelerating the rate of decomposition of plant residues (Lee, 1985; Bouche et al., 1987), which act as important substrates for propagules of R. solani in soil (Weinhold, 1977). A similar trend has been shown for Gaeumannomyces graminis var. tritici, where fragmentation of plant residues due to mechanical cultivation reduces take-all on wheat (Moore and Cook, 1984). The results from these pot trials demonstrate the potential of Aporrectodea trapezoides to control R. solani on wheat. These observations need to be tested in the field, in order to ascertain whether Aporrectodea trapezoides can reduce the effects of rhizoctonia root rot where conservation tillage practices have been introduced. The observation that conservation tillage practices favour the build up of
Reduced severity of R. solani caused by A. trapezoides
earthworms
(Rovira et nl., 1987) and can favour trupezoides in particular (Buckerfield, 1993), will facilitate the development of management strategies involving the use of earthworms to control
Aporrectodea
R. solani.
Harris A. R., Schisler D. A. and M. H. Ryder (1992) Binucleate Rhizoctonia isolates control Pythium and Rhizoctonia solani and promote seedling growth. Phytopathology 82, 1109. Hoffman J. A. and Purdy L. H. (1964) Germination of dwarf bunt teliospores after ingestion by earthworms. Phytopathology
was supported in part by the Grains Research and Development Corp., Australia. Thanks are due to Dr Geoff Baker, John Buckerfleld and Dr Ken Lee for critical reading of this manuscript. Acknowledgements-This
work
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