with corresponding colonization of the roots of wheat seedlings at soil depths of 3 - 9 cm. In addition ..... of bare patch disease), common fungal root pathogens of ...
Applkd
soil Ecology ELSEVIER
Applied Soil Ecology 1 ( 1994) 3-10
Interactions between earthworms, beneficial soil microorganisms and root pathogens Bernard M. Doubea**, Peter M. Stephens”, Christopher
W. Davorena, Maarten H. Ryderb
“CSIRO Divrsronof Sorls, Waite Campus, Adelaide, SA 5064, Austraha bCooperatlveResearch Centrefor Soil and Land Management, Waite Campus, Adelaide, SA 5064, Australra (Accepted 5 November 1993)
Abstract The capacity of earthworms and soil microorganisms to influence plant growth is well known. However, the interactions between them have been little studied. Recent work in southern Australia has examined these associations with a view to managing both earthworms and beneficial microorganisms to promote cereal and pasture productivity. In this paper we examine the role of earthworms as vectors of beneficial soil bacteria and their capacity to influence the population dynamics and impact of microorganisms on soil and plants. The earthworms Aporrectodea trapezoides and Aporrectodea rosea are the most common species in agricultural ecosystems in southern Australia. They have been shown to spread Pseudomonas corrugata 2 14OR, a biocontrol agent for Take-all (a fungal disease of wheat roots caused by Gaeumannomyces graminrs var. tritd) through soil with corresponding colonization of the roots of wheat seedlings at soil depths of 3 - 9 cm. In addition, A. trapezoides can spread Rhlzobium through the soil and cause increased root colonization and nodulation of legume roots. Tl\p”..:*p =“,UcYUbG -.-An-n- lLUQL l.nc y”yU’4r1”II~ ..~....l.-L~..” “1 ,.rnfi:, -:,..r,..,,,:,.-.I_^ illlc;CLc;U ^XY?..-r-A L.. ---rLLIIWUIIII __.^- acrlvlry --r:-LA-. a1 -I a- small ---11 spar1a1 ---*Z-1 uruylr= 3”ll IIIIU”“IgilC;illll~lU~ IIC uy car scale (e.g. in casts or tunnel walls), there is little evidence to demonstrate that earthworms affect populations at larger spatial scales. Take-all and Rhizoctonia bare patch are the most serious fungal root diseases of wheat in southern Australia. Laboratory trials have shown that, in the presence of the worms A. trapezoides and A. rosea, symptoms of these diseases in wheat seedlings are substantially reduced. In a further trial, the presence of A. trapezoides was associated with a loo-fold decrease in population size of Rhizobium recoverable from soil after 40 days. Key words:Aporrectodea;Biocontrol; Earthworms; Psueudomonas; Rhizobium, Root disease
1. Introduction
The separate capacities of earthworms and soil microorganisms to influence plant growth are *Corresponding author at: CSIRO Division of Soils, PM Bag No. 2 Glen Osmond, SA 5064, Australia. Tel: 08 303 8475, Fax: 08 303 8550.
well known (Edwards and Lofty, 1977; Lee, 1985; Homby, 1990) but the interactions between them have been little studied (Edwards and Fletcher, 1988; Gammack et al., 1992). We are currently examining these associations with a view to managing earthworms, microorganisms and their interactions to promote cereal and pasture productivity in southern Australia.
0929-1393/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0929-1393(93)00007-G
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B M Doube et al. /Applied Sod Ecology 1 (1994) 3- 10
In this paper we examine recent data on the ways in which earthworm activity can, ( 1) influence both the distribution of microorganisms in soil and root colonization, (2) directly affect the expression of fmgal root diseases of cereal crops and (3) influence the abundance of microorganisms in soil. Microorganisms in soil affect plant growth in a variety of ways. Many beneficial microorganisms promote plant growth by mutualistic or symbiotic relationships (e.g. nitrogen-fixing bacteria, mycorrhizal fungi and plant growthpromoting rhizobacteria), by controlling root diseases (biocontrol agents ) , by fixing nitrogen (e.g. free-living nitrogen-fixing bacteria), by mineralization of plant nutrients bound in organic matter or soil minerals (e.g. phosphate from rock phosphate) and by recycling organic matter. Other soil microorganisms have a deleterious effect on plant growth (e.g. plant root pathogens). All these processes can be influenced by earthworm activity. In this paper we are primarily concerned with the control of fungal root diseases of cereal crops using earthworms and biocontrol agents, and with the influence of earthworms on Rhizobium populations. 2. Earthworms as vectors of beneficial microorganisms One of the major constraints on effective root colonization by beneficial soil bacteria is their minimal capacity for unaided dispersal through soil (Homby, 1990; Gammack et al., 1992 ). Except when carried by water movement (Liddell and Parke, 1989; Huysman and Verstraete, 1993 ), animal vectors (e.g. earthworms) (Doube et al., 1993a; Stephens et al., 1993b) or on roots growing from inoculated seed (Bull et al., 199 1) , these bacteria are essentially sedentary and root colonization occurs largely when roots grow into soil colonized by them. A variety of methods have been used to inoculate beneficial microorganisms into soil (e.g. seed inoculation, soil inoculation) but these procedures when used in the field commonly inoculate only a small portion of the soil volume which is available to plant roots.
The activity of earthworms has been shown to promote the dispersal through soil of a variety of types of beneficial soil microorganisms including pseudomonads, rhizobia and mycorrhizal fungi. Madsen and Alexander ( 1982) reported that the earthworm Lumbricus rub&s enhanced the vertical transport of Pseudomonas putida and Bradyrhizobwm japonicum in soil in the presence of percolating water, although less than 5% and l%, respectively of the recovered viable bacteria were present below a soil depth of 2.7 cm. Henschke et al. ( 1989) and Thorpe et al. ( 1993 ) reported that genetically altered Pseudomonas survived passage through the intestine of Lumbricus terrestris and were present in castings indicating the potential of L. terrestris to disperse bacteria through soil. Buckalew et al. ( 1982) reported that an earthworm (unspecilied) was capable of acting as a vector of Rhzzobium trifolii. Rouelle ( 1983) indicated that the presence of L. terrestris was associated with improved distribution of nodules on the root system of soybean and suggested that this resulted from worm-dispersal of Bradyrhizobium japonicum. Reddell and Spain ( 199 1a,b) reported the spread of infective Frankia spp. (an actinomycete) and spores and hyphal fragments (in undigested root fragments) of mycorrhizal fungi by the pantropical earthworm Pontoscolex corethrurus. 2. I. Dispersal of a biocon trol agent for Take-all
Take-all disease is possibly the most serious root pathogen of wheat (Asher and Shipton, I98 I ) and is caused by the fungus Gaeumanne myces graminis var. tritici (Ggt). A number of bacterial and fungal biocontrol agents for this pathogen have been identified (Weller, 1988; Harmer and Lumsden, 199 1; Ryder and Rovira, 1993 ) but the effectiveness of such species is limited, in part, by their very slow rate of unaided dispersal (Weller, 1988). Our results (Doube et al., 1993a) have shown that the earthworms Aporrectodea trapezoides and Aporrectodea rosea, two of the most common species in cereal soils in southern Australia (Baker et al., 1993 ), dispersed Pseudomonas corrugata strain
B.M Doube et al. /Applied Sod Ecology I(1 994) 3-10
”
CON Wl
5
WC
CON
WT WC
CON W-T WC
DidnOce distributEI (cm)
5
CON WT WC
20
Fig. 1. The capacity of two species of earthworms to disperse the biocontrol bactena, P. corrugata, through the laboratory. Earthworms and a mixture of sheep dung and P. cowugata were applied at one end of a tube worm casts (WC), worm tunnels ( WT ) and soil unaffected by worms (CON) were sampled for bacteria at from the point of introduction of the bacteria (adapted from Doube et al., 1993a). Data represent the mean
2 140R, a biocontrol agent for Take-all (Ryder and Rovira, 1993 ) , 10 - 20 cm through tubes of pasteurised soil in the laboratory after 8 days. The biocontrol agent was introduced to the tubes in a mixture with sheep dung on which both species of earthworms fed. The larger earthworm, A. trapezoides, moved the bacteria further and more quickly than did A. rosea (Fig. 1). Similar densities of P. corrugata were present in casts of both earthworm species and in the soil associated with the walls of earthworm tunnels ( lo4 - lo6 bacteria g- ’ soil), and the numbers of bacteria recovered from casts were not affected by the clay content of the soil in which worms were living (Doube et al., 1993a). Further experiments by Stephens et al. ( 1993b) showed that when P. corrugata 2140R was mixed with worm food ( EzimulchR, a mixture of milled pea and cereal straw), and applied to the surface of soil in pots, the worm A. trapezoides fed on the EzimulchR, dispersed the bacteria through the soil and resulted in bacterial colonization of the roots of wheat seedlings. After 18 days there were ca. lo3 P. corrugata per cm of root at a soil depth of 3 - 9 cm in the presence of the earthworm A. trapezoides while none was recovered at this depth in the absence of the earthworm. Seed inoculation, even in the presence of
soil microcosms m of soil. After 8 days 5, lo,15 and 20 cm of three replicates.
earthworms which dispersed the bacteria away from the seed, was a poor method of achieving root colonization by 2 140R. At a soil depth of 3 - 9 cm, root colonization was ca. 100 times greater when the bacteria were applied in worm food than when they were applied on the seed. From these results it is clear that at least two species of earthworms can disperse biocontrol bacteria from inoculated earthworm food to the surrounding soil, which promotes colonization of wheat roots by the biocontrol agent. Whether such levels of root colonization are sufficient to control Take-all needs to be determined. Further, a carrier material which maximizes the inoculum delivered to the soil by earthworms needs to be developed and tested to assess whether, under field conditions with natural worm populations, adequate control of Take-all can be achieved by earthworm-dispersed P. corrugata. 2.2. Dispersal of Rhizobium by earthworms
The most frequently used procedure for introducing effective rhizobia into soil is seed inoculation (Danso and Bowen, 1989). However, the inoculation of highly effective rhizobia onto legume seed does not always result in the formation of effective nodules and a corresponding high
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B h4. Doube et al / Applted Sod Ecology I (1994) 3- 10
level of nitrogen fixation (Hardarson et al., 1989). Seed inoculation tends to result in localization of high cell densities of bacteria in the vicinity of the seed and emerging roots, with considerably lower numbers further away, especially in the absence of percolating water (Lowther and Patrick, 1993 ). Danso and Bowen ( 1989) have reported that soil and seed inoculation of B. japonicum produced a l.6-fold increase in nitrogen fixation when compared with seed inoculation. In another study, soil inoculation gave a four-fold increase in nitrogen fixation over seed inoculation alone (Hardarson et al., 1989). These results suggest that the use of techniques which increase the dispersal of inoculant rhizobia through soil may increase the nodulation by an introduced Rhizobium strain and increase the level of nitrogen fixed by a legume. Our work has concerned the capacity of earthworms to disperse Rhizobium through soil, to increase levels of colonization of legume roots by Rhizobium, and to increase levels of root nodulation. One set of experiments by Stephens et al. ( 1994) showed that the earthworms A. trapezoides and Microscolex dub&s dispersed Rhizobium meliloti strain L5-30R through pots of soil. The bacterium was inoculated into a variety of worm foods (including sheep dung and EzimulchR) and placed on the soil surface. Eighteen days later more than lo4 cells per g of soil were recovered at a soil depth of 9 cm. A second experiment, using EzimulchR as the carrier, showed that in the presence of A. trapezoides about 1O4R. meliloti cm- 1 of alfalfa root was detected at a depth of 3 - 9 cm while none were detected at that root depth in the absence of A. trapezoides. A further experiment (B. Doube, C. Davoren, P. Fraser and P. Stephens, unpublished data) examined the capacity of seven species of earthworms (A. trapezoides, A. rosea, Aporrectodea caliginosa, Aporrectodea Zonga,Octalasion cyaneum, Gemmascolex walkeri and Eisenia fetida) to disperse R. meliloti inoculated into EzimulchR and placed as a pellet either on the soil surface or buried at a depth of 7 cm. Only A. trapezoides, A. Zongaand E. fetida consumed significant amounts of the EzimulchR pellets and more buried than surface-applied material was
consumed. Only A. trapezoldes and A. Zongadispersed R. meliloti through the vertical soil column: E. fetida remained in the vicinity of the food pellet. It is possible that the other species would have dispersed R. meliloti if different carriers of suitable food had been used. A further experiment examined the effect of A. trapezoides on root nodulation by R. trifolii in subclover ( Trifolium subterraneum) seedlings (Doube et al., 1993b). When R. trifolii was inoculated into sheep dung and placed on the soil surface, there were three times as many root nodules in the presence of A. trapezoides than in its absence. Furthermore, there was a highly significant increase in the number of root nodules on the main root at a soil depth of 2 - 8 cm compared with plants from pots lacking earthworms (Fig. 2). The presence of earthworms was associated with increased shoot and root biomass but the additional nodulation did not affect plant growth. From these results it appears that at least three species of common earthworms have the capacity to disperse Rhizobium through soil from an inoculated food source and that better dispersal
”
O-2 2-4 4-6 6-8 Depth below soil surface (cm)
Fig. 2. The mfluence of the earthworm A. trapezoldes on the distribution of nodules of R. trrfolil along the primary root of subterranean clover seedlings m pots m the laboratory. Sheep dung with or without added R. trrfolrr was applied to the so11 surface of pots containing earthworms and young subclover plants and the roots were examined after 6 weeks at 15°C (from Doube et al., 1993b) Data represent the mean of five replicates.
B. M. Doube et al. / Applred Sod Ecology 1 (I 994) 3-10
is achieved with buried than with surface-applied carrier material. Dispersal can be relatively rapid, and increased nodulation is associated with earthworm activity. The prospect that earthworms or R&o&urn-inoculated earthworm food can be used to improve the dispersal, and corresponding root nodulation/nitrogen fixation by nitrogen-fixing bacteria, through field soil needs to be examined, especially in acidic pasture soils where root nodulation by rhizobia is often inhibited by low soil pH. 3. Earthwormeffects on populations of microorganisms in soil Microorganisms provide a primary source of food for earthworms. If earthworms are present in soil in moderate numbers, their feeding, casting and tunnelling behaviour will substantially alter the physical, chemical and biological environment for microorganisms and thereby favour some species and suppress others. In a review considering the diet of earthworms, Edwards and Fletcher (1988) concluded that protozoa and fungi were the major sources of nutrients for earthworms, and that bacteria were of minor nutritional importance and soil algae were of moderate importance. Other authors have considered the feeding and digestive processes of earthworms (e.g. Flack and Hartenstein, 1984; Dash et al., 1986; Lavelle, 1988; reviewed by Brown, 1993) and concluded that fungi are a primary source of food for earthworms and that some types are preferentially ingested while others (e.g. basidiomycetes) are rejected. It is also generally considered that earthworms increase bacterial populations in soil patches by providing them with high-quality substrates for growth, e.g. in the earthworm gut, tunnels and casts (Lee, 1985; Lavelle et al., 1992). However, it must be emphasised that there are also cases in which bacteria are digested in the earthworm gut and examples of fungi which survive passage through the earthworm gut (Brown, 1993 ) . Despite the evidence that earthworms cause local changes in populations of soil microorganisms (e.g. in the drilosphere ) , evidence demonstrating that
7
earthworms influence the overall size and impact of populations of microorganisms in soil at a larger spatial scale is scarce, although such effects are implicit in the literature (e.g. Brown, 1993). 3.1. Earthworms suppress root disease in wheat plants Ggt and Rhizoctonia solani (the causal agent of bare patch disease), common fungal root pathogens of wheat in southern Australia, are saprophytic species which also colonize living roots. Because earthworms consume microorganisms, decomposing organic matter and possibly living roots (Lee, 1985; Edwards and Fletcher, 1988; Cortez and Bouche, 1992 ), they have obvious potential to interact directly (as predators) and indirectly (as competitors or facilitators) with soil-dwelling saprophytic fungi. Further, the effects of some fungal root pathogens (e.g. Rhizoctonia) are substantially reduced by mechanical disruption of soil (Lee and Pankhurst, 1992), and it is possible that earthworm activity may perform a similar function. Recent laboratory data have shown that the activity of the earthworm A. trapezoides can substantially decrease the symptoms caused by Rhizoctonia in wheat seedlings (Stephens et al., 1993a). R. solani was introduced into soil on wheat chaff and the presence of A. trapezoides caused a significant reduction in the severity of the symptoms of the disease at a density equivalent 470 earthworms m-* in both a calcareous sand loam and a red-brown earth. Similar results were obtained using a red-brown earth in an experiment which examined the effect of A. trapezoides on the expression of Take-all disease on roots of wheat seedlings (M. Ryder, B. Doube, C. Davoren, unpublished data). The mechanisms are still not well understood, but a number of possible processes have been discussed by Stephens et al. (1993~).
3.2. Earthworms reduce populations of bacteria in soil
Recently Stephens et al. ( 1993~) measured changes in population size of Rhizobium main-
B M Doube et al. / Applied Sod Ecology 1 (1994) 3- 10
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_ Rhizobium meliloti
0 1 2 3 Number of worms per pot Fig. 3. The mfluence of the earthworm, A. trapezordes, on the abundance of R. melilotl maintained for 40 days at 15°C m soil m the laboratory (from Stephens et al., 1993~). Data represent the mean of six replicates.
tamed in soil held in pots at 15°C in the presence and absence of the earthworm A. trapezoides. After 20 days no change in population size was detected but between Days 20 and 40 the density of bacteria declined to less than 1% of that in the pots without earthworms (Fig. 3 ) . The mechanism by which this occurred is not clear but is unlikely to be related to digestion of the bacteria by the earthworms because only a small proportion of the soil in the pot would have been consumed by the worms during the second 20-day period. 4. Concluding remarks Both beneficial and harmful microorganisms can be dispersed by earthworms. Such harmful microorganisms dispersed by earthworms include the spores of Pythium and Fusarium (Edwards and Fletcher, 1988). Here we have been concerned with recent results from beneficial microorganisms used in Australian soils. Reviews of the literature (Brown, 1993; Edwards and Fletcher, 1988; Gammack et al., 1992) sug-
gest that the beneficial effects of interactions between earthworms and microorganisms may well outweigh the harmful effects, especially if management practices which favour beneficial microorganisms can be devised and implemented. The importance of earthworms in influencing the composition and activity of microbial communities has only recently been recognised as an important aspect of the way in which earthworms affect the productivity of agricultural ecosystems. It may be that by manipulating the composition and activity of earthworm communities (e.g. by introducing species or altering agricultural management practices to favour beneficial species) earthworms may be used to manipulate populations of soil microorganisms to reduce the severity of plant diseases and to increase plant productivity in the field. However, before recommendations can be devised, much more needs to be known about the population dynamics of soil microorganisms and earthworms and the growth of plant roots, especially their temporal patterns of activity at different spatial scales. These results also open the possibility that a mixture of earthworm food and beneficial microorganisms may provide the basis of a new technology for introducing and dispersing biocontrol bacteria and rhizobia into soil. There are also other beneficial species of microorganisms and types of microbial activity (e.g. fungi such as Trichoderma for control of root diseases and Metarrhizium for control of soil-dwelling insect pests of crops) which may also be dispersed through soil via earthworms, using this procedure. Because soil microbial communities are a key element in controlling soil structure and fertility, their interactions with plant roots and the soil macrofauna are a promising area for further research.
5. References Asher, M.J.C. and Shrpton, P.J.. 1981. Btology and control of Take-all. Academrc Press, London, 538 pp. Baker, G.H., Barrett, V.J., Carter, P J., Wlliams, P.M.L. and Buckerfield, J.C., 1993 Seasonal changes in the abun-
B.M. Doube et al /AppliedSodEcology dance of earthworms ( Annelids:Lumbricidae and Acanthodrilidae) in soils used for cereal and luceme production in South Australia. Aust. J. Agric. Res., 44: 12911301. Brown, G.G., 1993. How do earthworms affect microfloral and faunal community diversity? Plant and Soil, in press. Buckalew, D.W., Riley, R.K, Yoder, W.A. and Vail, W.J., 1982. Invertebrates as vectors of endomycorrhizal fungi and Rhrzobtum upon surface mine soils. West Virginia Acad. Sci. Proc., 54: 1. Bull, C.T., Weller, D.M. and Thomashow, L.S., 199 1. Relationship between root colonization and suppression of Gaeumannomyces gramrms var. tritwzby Pseudomonas jluorescens Strain 2-79. Phytopathology, 8 1: 945-95 9. Cortez, J. and Bouche, M.B., 1992. Do earthworms eat livmg roots? Soil Biol. Biochem., 9: 913-915. Danso, K&A. and Bowen, G.D., 1989. Methods of moculation and how they influence nodulation pattern and mtrogen fixation using two contrasting strains of BradyrhizobiumJaponuum. Soil Biol. B&hem., 21: 1053-1058. Dash, H.K., Beura, N.B. and Dash, M.C., 1986. Gut load, transit time, gut microflora and turnover of soil, plant and fungal material m some tropical earthworms. Pedobiologia, 29: 13-20. Do&e, B.M., Ryder, M.H., Davoren, C.W. and Meyer, T., 1993a. Earthworms: A down-under delivery service for biocontrol agents of root disease. Acta Zool. Fenn., in press. Doube, B.M., Ryder, M.H., Davoren, C.W. and Stephens, P.M., 1993b. Enhanced root nodulatron of subterranean clover (Trtfolium subterraneum) by Rhrzobtum trtfohi in the presence of the earthworm Aporrectodea trapezotdes. Biol. Fertil. Soils, in press. Edwards, C.A. and Fletcher, K.E., 1988. Interactions between earthworms and microorgamsms in orgamc-matter breakdown. Agric. Ecosystems Environ., 24: 235-247. Edwards, C.A. and Lofty, J.R., 1977. Biology of Earthworms. 2nd Edn., Chapman and Hall, London, 333 pp. Flack, EM. and Hartenstein, R., 1984. Growth of the earthworm Etsenzafetida on microorganisms and cellulose. Soil Biol. B&hem., 16: 491-495. Gammack, S.M., Paterson, E., Kemp, J.S., Cresser, M.S. and Killham, K., 1992. Factors affectmg movement of microorganisms in soils. In: G. Stotzky and L.M. Bolla (Editors), Soil Biochemistry, 7. Marcel Dekker, New York, pp. 263-305. Hardarson, G., Golbs, M. and Danso, K.S.A., 1989. Nitrogen fixation in soyabean (Glycme ma.x L. Merrill) as affected by nodulatron patterns. Soil Btol. Blochem., 21: 782-787 Harmer, G.E. and Lumsden, R.D., 199 1. Btological disease control. In: J.M. Lynch (Editor), The Rhizosphere. Wiley, New York, pp. 259-277. Hensche, R.B., Nucke, E. and Schmidt, F.R.J., 1989. Fate and dispersal of recombinant bacteria m a soil microcosm containing the earthworm Lumbncus terrestrts.Biol. Fertil. Soils, 20: 887-890. Homby, D. (Editor), 1990. Biological control of sot1 borne
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plant pathogens. CAB International, Wallingford, UK, 479 PP. Huysman, F. and Verstraete, W., 1993. Water-facilitated transport of bacteria in unsaturated soil columns: Influence of cell surface hydrophobicity and soil properties. Soil Biol. B&hem., 25: 83-90. Lavelle, P., 1988. Earthworms and the soil system. Biol. Fertil. Sons, 6: 237-251. Lee, K.E., 1985. Earthworms. Their Ecology and Relationships wrth Land Use. Academic Press, New York, 441 pp. Lee, K.E. and Pankhurst, C.E., 1992. Sot1 organisms and sustainable agnculture. Aust. J. Soil Res., 30: 855-892. Liddell, C.M. and Parke, J.L., 1989. Enhanced colonization of pea taproots by a fluorescent pseudomonad biocontrol agent by water infiltration into sol. Phytopathology, 79: 1327-1332. Lowther, W. and Patrick, H.N., 1993. Spread of Rhtzobium and Bradyrhrzobtum in soil. Soil Biol. Biochem., 25: 607612. Madsen, E.L. and Alexander, M., 1982. Transport of Rhtzobium and Pseudomonas through soil. Soil Sci. Sot. Am. J., 46: 557-560. Rouelle, J., 1983. Introduction of an amoeba and Rhtzobium Japonrcum into the gut of Etsenia fetzda (Sav.) and Lumbruxs terrestrzsL. In: J.E. Satchel1 (Editor), Earthworm Ecology: From Darwin to Vermiculture. Chapman and Hall, New York, pp. 375-38 1. Reddell, P. and Spam, A.V., 1991a. Earthworms as vectors of viable propagules of mycorrhtzal fungi. Soil Biol. Biochem., 23: 767-774. Reddell, P. and Spain, A.V., 199 lb. Transmission of infective Frankta (Actinomycetales) propagules m casts of the endogenic earthworm Pontoscofex corethrurus (Oligochaeta: Glossoscolecidae). Soil Biol. Biochem., 23: 775778. Ryder, M.H. and Rovlra, A.D., 1993. Biological control of Take-all in glasshouse-grown wheat using strains of Pseudomonas corrugata isolated from wheat field soil. Soil Biol. Biochem., 25: 31 l-320. Stephens, P.M., Davoren, C.W., Doube B.M., Ryder, M.H., Benger A. and Neate, S., 1993a. Reduced severity of Rhrzoctontasolant disease on wheat seedlings associated with the presence of the earthworm Aporrectodea trapezotdes (Lumbricidae) . Soil Btol. Btochem., 11: 1477- 1484. Stephens, P.M., Davoren, C.W., Ryder, M.H. and Doube, B.M., 1993b. Influence of the earthworm Aporrectodea trapezordes (Lumbricidae) on the colomsation of wheat ( Trtttcum aestwum cv. Spear) roots by Pseudomonascorrugata strain 2140R and survival of 2140R m soil. Soil Biol. Btochem., in press. Stephens, P.M., Davoren, C.W., Ryder, M.H. and Doube, B.M., 1994. Influence of the earthworm Aporrectodea trapezozdes (Lumbricrdae) on the colonization of alfalfa (Medtcago sat& L.) roots by Rhtzobrum melilott strain LS-30R and the survival of L5-30R m soil. Biol. Fertil. Soils, in press.
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Thorpe, I.S., Ktllham, K., Presser, J.I. and Glover, L.A., 1993 Novel method for the study of the populatron dynamics of a genetically modrfied mxroorganism m the gut of the earthworm Lumbncus terrestns. Biol. Ferttl. Sods, 15: 5559. Weller, D.M.. 1984. Datnbutron of Take-all suppressive
strain of Pseudomonas jluorescens on semmal roots of wmter wheat. Appl. Env. Mtcrobtol., 48: 897-899. Weller, D.M., 1988. Btologrcal control of soilbome plant pathogens m the rhizosphere with bactena. Ann. Rev Phytopathol., 26. 379-407
