Suppressive Soils

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When soils are characterized by a very low level of disease development even though a virulent pathogen and susceptible host are present, they are known as suppressive soils. Biotic and abiotic elements of the soil environment contribute to suppressiveness, however most defined systems have identified biological elements as primary factors in disease suppression. Many soils possess similarities with regard to microorganisms involved in disease suppression, while other attributes are unique to specific pathogen-suppressive soil systems. The organisms' operative in pathogen suppression does so via diverse mechanisms including competition for nutrients, antibiosis and induction of host resistance (Mazzola, 2002). Non-pathogenic Fusarium spp. and fluorescent Pseudomonas spp. play a critical role in naturally occurring soils that are suppressive to Fusarium wilt. Suppression of take-all of wheat, caused by Gaeumannomyces graminis var. tritici, is induced in soil after continuous wheat monoculture and is attributed, in part, to selection of fluorescent pseudomonads with capacity to produce the antibiotic 2,4-diacetylphloroglucinol. Cultivation of orchard soils with specific wheat varieties induces suppressiveness to Rhizoctonia root rot of apple caused by Rhizoctonia solani AG 5. Wheat cultivars that stimulate disease suppression enhance populations of specific fluorescent pseudomonad genotypes with antagonistic activity toward this pathogen. Methods that transform resident microbial communities in a manner which induces natural soil suppressiveness have potential as components of environmentally sustainable systems for management of soilborne plant pathogens (Mazzola, 2002).

Actually, agricultural soils suppressive to soilborne plant pathogens occur worldwide, and for several of these soils the biological basis of suppressiveness has been described. Two classical types of suppressiveness are known. General suppression owes its activity to the total microbial biomass in soil and is not transferable between soils. Specific suppression owes its activity to the effects of individual or select groups of microorganisms and is transferable. The microbial basis of specific suppression to four diseases, Fusarium wilts, potato scab, apple replant disease, and take-all, is discussed by Weller et al. (2002). One of the best-described examples occurs in takeall decline soils. In Washington State, take-all decline results from the buildup of fluorescent Pseudomonas spp. that produce the antifungal metabolite 2,4-diacetyl-phloroglucinol. Producers of this metabolite may have a broader role in disease-suppressive soils worldwide. By coupling molecular technologies with traditional approaches used in plant pathology and microbiology, it is possible to dissect the microbial composition and complex interactions in suppressive soils.

In three of 12 soils obtained from agricultural fields in California, population density development of Meloidogyne incognita under susceptible tomato was significantly suppressed when compared to identical but methyl iodide (MI)-fumigated, M. incognita re-infested soils. When the 12 soils were infested with second-stage juveniles (J2) of M. incognita and the juveniles were extracted after 3 days, significantly fewer J2 were recovered from 9 of the 12 non-treated soils than from the MI-fumigated equivalents. In one of the 12 soils, infestation 3 weeks before planting resulted in lower nematode population densities than infestation at planting in both MI-fumigated and non-treated soil. The combination of infestation 3 weeks before planting with infestation at planting did not alter the occurrence or degree of root-knot nematode suppressiveness (Pyrowolakis et al., 2002).

Yin et al. (2004) established that for suppressive soils that have a biological nature, one of the first steps in understanding them is to identify the organisms contributing to this phenomenon. They presented a new approach for identifying microorganisms involved in soil suppressiveness. This strategy identifies microorganisms that fill a niche similar to that of the pathogen by utilizing substrate use assays in soil. To demonstrate this approach, they examined an avocado grove where a Phytophthora cinnamomi epidemic created soils in which the pathogen could not be detected with baiting techniques, a characteristic common to many soils with suppressiveness against P. cinnamomi. Substrate utilization assays were used to identify rRNA genes (rDNA) from bacteria that rapidly grew in response to amino acids known to attract P. cinnamomi zoospores. Six bacterial rDNA intergenic sequences were prevalent in the epidemic soils but uncommon in the non-epidemic soils. These sequences belonged to bacteria related to Bacillus mycoides, Renibacterium salmoninarum, and Streptococcus pneumoniae. We hypothesize that bacteria such as these, which respond to the same environmental cues that trigger root infection by the pathogen, will occupy a niche similar to that of the pathogen and contribute to suppressiveness through mechanisms such as nutrient competition and antibiosis.

A similar experimental approach was developed by Borneman et al. (2004) for identifying microorganisms involved in specified functions such as pathogen suppressiveness. in this approach, it was postulated that the microorganisms involved in pathogen suppressiveness could be discovered by identifying those organisms whose populations positively correlate with high levels of suppressiveness. The approach has three phases. The first phase is to identify bacterial and fungal rRNA genes (rDNA) from soils possessing various levels of suppressiveness. Ribosomal DNA sequences that are more abundant in the highly suppressive soils than in the less suppressive soils are considered candidate sequences. A method termed oligonucleotide fingerprinting of rRNA genes (OFRG) is used to obtain extensive analysis of microbial community composition. The second phase of this experimental approach is to verify the results obtained from phase one using quantitative PCR. Here, selective PCR primers for each of the candidate rDNA sequences are designed. These primers are then used to determine the relative amounts of the candidate sequences in soils possessing various levels of suppressiveness produced by several different methods such as mixing various quantities of suppressive and fumigation-induced non-suppressive soil, biocidal treatments and temperature treatments. In phase three, the organisms that consistently correlate with suppressiveness are isolated and amended to non-suppressive soils to assess their abilities to produce suppressiveness. The utility of this experimental approach was demonstrated by using it to identify microorganisms involved in suppressiveness against the plant-parasitic nematode, Heterodera schachtii. This general experimental approach should also be useful for the identifying microorganisms involved in functions other then pathogen suppressiveness.

Kloepper et al. (1999) discussed concepts and examples of how naturally occurring bacteria (plant-associated bacteria residing in the rhizosphere, phyllosphere, and inside tissues of healthy plants -endophytic), and introduced bacteria may contribute to management of soilborne and foliar diseases. some introduced rhizobacteria have been found to enhance plant defences, leading to systemic protection against foliar pathogens upon seed or root-treatments with the rhizobacteria. In these cases, introduction of the rhizobacteria results in reduced damage to multiple pathogens, including viruses, fungi and bacteria. An alternative strategy to the introduction of specific antagonists is the augmentation of existing antagonists in the root environment. This augmentation may result from the use of specific organic, amendments, such as chitin, which stimulate populations of antagonists, thereby inducing suppressiveness. Intercropping or crop rotation with some tropical legumes, including velvetbean (Mucuna deeringiana), lead to management of phytoparasitic nematodes, partly through stimulation of antagonistic microorganisms. some biorational nematicides, such as specific botanical aromatic compounds, also appear to induce suppressiveness through alterations in the soil microbial community.

single isolates of bacterial endophytes, obtained from the nematode antagonistic plant species African (Tagetes erecta) and French (T patula) marigold, were introduced into potatoes (Solanum tuberosum). Several bacterial species possessed activity against root-lesion nematodes (Pratylenchus penetrans) in soils around the root zone of potatoes, namely: Microbacterium esteraromaticum, Tsukamurella paurometabolum, isolate TP6, Pseudomonas chlororaphis, Kocuria varians and K. kristinae. Of these, M. esteraromaticum and K. varians depressed the population densities of root-lesion nematodes without incurring any yield penalty (tuber wet weight). No significant differences were found in the total numbers of P. penetrans nematodes, rhabditid nematodes or 'other' parasitic nematode species within the root tissues of bacterized potato plants compared to the unbacterized check. Overall, tuber fresh weights and tuber number were equal to or significantly lower (P < 0.05) in bacterized plants than their unbacterized counterpart (Sturz and Kimpinski, 2004). The authors of this study conclude that endoroot bacteria from Tagetes spp. can play a role in nematode suppression through the attenuation of nematode proliferation, and proposed that these nematode control properties are capable of transfer to other crops in a rotation as a beneficial 'residual' microflora - a form of beneficial microbial allelopathy.

In relation with this same type of study, Hallman et al. (1998) performed a greenhouse experiments with cotton and cucumber to determine the effects of inoculation of the parasitic nematode Meloidogyne incognita on population dynamics of indigenous bacterial endophytes and introduced endophytic bacterial strains JM22 (Enterobacter asburiae) and 89B-61 (Pseuedomonas fluorescens) applied as seed treatments. Internal communities of endophytic bacteria in roots were generally largest in the presence of M. incognita. Recovery of JM22 from cucumber roots was positively, but not significantly, associated with soilborne nematode inoculum size, except at 2 weeks after inoculation. The internal populations of 89B-61 applied to seed also increased with nematode applications. The diversity of indigenous bacterial endophytes changed within 7 d after M. incognita inoculation. Species richness and diversity of endophytic bacteria were slightly, but not significantly, greater for nematode-infested plants than for non-infested plants. Alcaligenes piechaudii and Burkholderia pickettii occurred only in nematode-infested plants, whereas Bievundimonas vesicularis was mainly isolated from nematode-free plants. Agrobacterium radiobacter and Pseudomonas spp. were the most common taxa found in both treatments, accounting for a total of 41% and 37% of the community for non-inoculated and inoculated plants, respectively. JM22 colonized cotton roots internally and was also found in high numbers on the root surface around nematode penetration sites and on root galls where the root tissue had been disruptured due to gall enlargement. Single cells of JM22 were attached to the cuticle of M. incognita juveniles. Sturz et al. (2000) assesses that endophytic bacteria and M. incognita form complex associations and an understanding of these associations will aid efforts to develop and manage microbial communities of endophytic bacteria for practical use as biocontrol agents against plant-parasitic nematodes and soil-borne pests and pathogens.

In addition, Postma et al. (2003) found that compost amended soil has also been found to be suppressive against plant diseases in various cropping systems. The level and reproducibility of disease suppressive properties of compost might be increased by the addition of antagonists. In this study, the establishment and suppressive activity of two fungal antagonists of soil-borne diseases was evaluated after their inoculation in potting soil and in compost produced from different types of organic waste and at different maturation stages. The fungal antagonists Verticillium biguttatum, a mycoparasite of Rhizoctonia solani, and a non-pathogenic isolate of Fusarium oxysporum antagonistic to Fusarium wilt, survived at high levels (103-105 CFU g-1) after 3 months incubation at room temperature in green waste compost and in potting soil. Their populations faded-out in the organic household waste compost, especially in the matured product. In bioassays with R. solani on sugar beet and potato, the disease suppressiveness of compost increased or was similar after enrichment with V biguttatum. The largest effects, however, were present in potting soil, which was very conducive for the disease as well as the antagonist. similar results were found in the bioassay with F. oxysporum in carnation where enrichment with the antagonistic F oxysporum had a positive or neutral effect. Postma et al. (2003) foresee great potential for the application of antagonists in agriculture and horticulture through enrichment of compost or potting soil with antagonists or other beneficial micro-organisms.

All soils are suppressive to phytonematodes to some degree. The degree of suppressiveness to them or other soilborne pathogens in a soil can be enhanced not only by infesting soil with selected microorganisms, but by the use of appropriate cropping systems and the application to soil of specific organic amendments or chemical compounds. Conducive cropping systems such as monoculture can reduce soil suppressiveness to the point where the soil is not resistant to plant parasitic nematodes (Wang et al., 2002).

Inorganic fertilizers containing ammoniacal nitrogen or formulations releasing this form of N in the soil are most effective for suppressing nematode populations. Anhydrous ammonia has been shown to reduce soil populations of Tylenchorhynchus claytoni, Helicotylenchus dihystera, and Heterodera glycines. The rates required to obtain significant suppression of nematode populations are generally in excess of 150 kg N/ha. Urea also suppresses several nematode species, including Meloidogyne spp., when applied at rates above 300 kg N/ha. Additional available carbon must be provided with urea to permit soil microorganisms to metabolize excess N and avoid phytotoxic effects. There is a direct relation between the amount of "protein" N in organic amendments and their effectiveness as nematode population suppressants. Most nematicidal amendments are oil cakes, or animal excrements containing 2-7% (w/w) N; these materials are effective at rates of 4-10 t/ha. Organic soil amendments containing mucopolysaccharides (e.g., mycelial wastes, chitinous matter) are also effective nematode suppressants (Rodriguez-Kabana, 1986).

Vargas-Ayala and Rodriguez-Kabana (2001) established a field microplot trial to evaluate nematode population dynamics in a rotation program utilizing nematode-suppressive and non-suppressive legumes, and nematode-host and nonhost grass species. The rotation treatments consisted of velvetbean (Mucuna deeringiana) or cowpea (Vigna unguiculata) during the first year, followed in winter by oat (Avena sativa), wheat (Triticum aestivum), rye (Secale cereale), rye grass (Lolium sp.), clover (Trifolium sp.), hairy vetch (Vicia villosa), lupine (Lupinus sp.) or fallow. Rotation in the second and third year consisted of soybean (Glycine max). Results showed that velvetbean had a generally suppressive effect on populations of root-knot (Meloidogyne incognita), cyst (Heterodera glycines), and stunt (Tylenchorhynchus claytoni) nematodes in soil and roots. It had little effect on populations of Helicotylenchus dihystera. Velvetbean rotations with winter grass species were also effective in reducing nematode population densities in soil. Soybean yields were positively correlated with velvetbean in rotations with winter grass species. High populations of M. incognita were negatively correlated with soybean yields. The use of velvetbean as a rotation crop assures reduction of important plant-parasitic nematodes in soil and an improvement in soybean yield.

Wang et al. (2002) made an extensive review on the use of Crotalaria spp. (Fabaceae) as a suppressor of agricultural pests, particularly nematodes. These authors summarized the knowledge of the efficacy of Crotalaria spp. for plant-parasitic nematode management, described the mechanisms of nematode suppression, and outline prospects for using this crop effectively. They mentioned that Crotalaria is a poor host to many plant-parasitic nematodes including Meloidogyne spp., Rotylenchulus reniformis, Radopholus similis, Belonolaimus longicaudatus, and Heterodera glycines. It is also a poor or non-host to a large group of other pests and pathogens. Besides, Crotalaria is competitive with weeds without becoming a weed, grows vigorously to provide good ground coverage for soil erosion control, fixes nitrogen, and is a green manure. However, most Crotalaria species are susceptible to Pratylenchus spp., Helicotylenchus sp., Scutellonema sp. and Criconemella spp.

Crotalaria species are used as preplant cover crops, intercrops, or soil amendments. When used as cover crops, Crotalaria spp. reduces plant-parasitic nematode populations by: i) acting as a nonhost or a poor host, ii) producing allelochemicals that are toxic or inhibitory, iii) providing a niche for antagonistic flora and fauna, and iv) trapping the nematode.

A non-host to a nematode species is a plant in which the nematode fails to reproduce. A plant is considered as resistant to nematodes when these fail to live inside the host or early dead in the host; they decreased the production of eggs; or their growth or development are inhibited by the plant (Wang et al., 2002).

Allelopathic effects of the plant against nematodes were described as a mechanism of suppression of nematodes. Soler-Serratosa et al. (1996) evaluated the nematicidal activity of thymol, a phenolic monoterpene present in the essential oils of several plant families. Thymol was added to soil at rates 25-250 ppm. Initial and final population densities of Meloidogyne arenaria, Heterodera glycines, Paratrichodorus minor, and Dorylaimoid nematodes, as well as disease incidence, declined sharply with increased dosages of thymol. Thymol was also applied at 0, 50, 100, and 150 ppm to soil in combination with 0, 50, and 100 ppm benzaldehyde, an aromatic aldehyde present in nature as a moiety of plant cyanogenic glycosides. Combinations in which benzaldehyde was applied at 100 ppm showed synergistic effects in suppressing initial and final soil populations of M. arenaria and H. glycines. Significant reductions in root galling and cyst formation in soybean were attributable to thymol at > 50 ppm.

Hallmann and Sikora (1996) confirmed that endophytic fungi isolated from the cortical tissue of surface sterilized tomato roots collected from field plots produced secondary metabolites in nutrition broth that were highly toxic to Meloidogyne incognita. Especially strains of Fusarium oxysporum were highly active with 13 of 15 strains producing culture filtrates toxic to nematodes. They investigated also the mechanism of action of the toxic metabolites produced by the non-pathogenic F oxysporum strain 162 with proven biological control of M. incognita in pot experiments. These metabolites reduced M. incognita mobility within 10 min of exposure. After 60 min, 98% of juveniles were inactivated. Fifty percent of juveniles with exposure of 5 h were dead, and 24 h exposure resulted in 100% mortality. In a bioassay with lettuce seedlings metabolite concentrations >100 mg/l reduced the number of M. incognita juveniles on the roots comparing to the water control. The F. oxysporum toxins were highly effective towards sedentary parasites and less effective towards migratory endoparasites. Non-parasitic nematodes were not influenced at all. Metabolites of strain 162 also reduced significantly the growth of Phytophthora cactorum, Pythium ultimum and Rhizoctonia solani in vitro.

Three out of 15 bacterial strains preselected for antagonistic activity in different pathosystems showed biocontrol activity towards Meloidogyne incognita on lettuce and tomato as described by Hoffmann-Hergarten et al. (1998). They found that seed treatment with the rhizobacteria Pseudomonas sp. W34 or Bacillus cereus S18 resulted in significant reductions in root galling and enhanced seedling biomass. The yield response of M. incognita-infested tomato was tested in a long-term pot experiment using three antagonistic bacteria, i.e., Pseudomonas sp. W34, Bacillus cereus S18 and Bacillus subtilis VM1-32. Significant reduction in M. incognita gall index was observed within 18 weeks after inoculation with all three bacterial strains. B. cereus S18 caused a 9 % yield increase when compared with the nematode control and thereby compensated for the yield loss due to nematode infection. Early maturity of fruits on M. incognita--infested tomato plants after inoculation of B. cereus S18 was observed when compared with both the nematode and the untreated control.

Vargas-Ayala et al. (2000) hypothesized that the induction of soil suppressiveness to plant parasitic nematodes that occurs following planting of velvetbean (Mucuna deeringiana) is associated with the development of an antagonistic microflora in soils and rhizospheres. They performed a crop rotation study in microplots, consisting of three crop cycles. Cycle 1 involved planting of either velvetbean or cowpea (Vigna unguiculata) in the first spring. Cycle 2 during the next fall and winter was fallow or cover-cropped with wheat (Triticum aestivum) or crimson clover (Trifolium incarnatum). Cycle 3 the next spring was soybean (Glycine max). Rhizosphere fungal populations were significantly smaller on velvetbean than on cowpea at the end of cycle 1. The use of velvetbean in cycle 1 significantly decreased rhizosphere bacterial populations on crops in cycle 2, compared to treatments which had cowpea in cycle 1. Velvetbean also influenced bacterial diversity, generally increasing frequency of bacilli, Arthrobacter spp. and Burkholderia cepacia, while reducing fluorescent pseudomonads. Some of these effects persisted through cycle 3. Fungal diversity was influenced in cycle 1 by velvetbean; however, effects generally did not persist through cycles 2 and 3. The results indicate that the use of velvetbean in a cropping system alters the microbial communities of the rhizosphere and soil, and they are consistent with the hypothesis that the resulting control of nematodes results from induction of soil suppressiveness.

6.1. Resistence through natural compounds

Sometimes, natural compounds that confer resistance to a plant against nematode infestation could be of foreign origin (Reitz et al., 2000). Recent studies have shown that living and heat-killed cells of the rhizobacterium Rhizobium etli strain G12 induce in potato roots systemic resistance to infection by the potato cyst nematode Globodera pallida. To better understand the mechanisms of induced resistance, Reitz et al. (2000) focused on identifying the inducing agent. Since heat-stable bacterial surface carbohydrates such as exopolysaccharides (EPS) and lipopolysaccharides (LPS) are essential for recognition in the symbiotic interaction between Rhizobium and legumes, their role in the R. etli-potato interaction was studied. EPS and LPS were extracted from bacterial cultures, applied to potato roots, and tested for activity as an inducer of plant resistance to the plant-parasitic nematode. Whereas EPS did not affect G pallida infection, LPS reduced nematode infection significantly in concentrations as low as 1 and 0.1 mg ml(-1). Split-root experiments, guaranteeing a spatial separation of inducing agent and challenging pathogen, showed that soil treatments of one half of the root system with LPS resulted in a highly significant (up to 37%) systemic induced reduction of G. pallida infection of potato roots in the other half. The results clearly showed that LPS of R. etli G12 act as the inducing agent of systemic resistance in potato roots.

In relation with Crotalaria spp. secondary metabolites, it is well known that these plants produce pyrrolizidine alkaloids and monocrotaline which have high vertebrate toxicity and could potentially be toxic to nematodes; but it is possible also that the low C/N ratio of Crotalaria may also contribute to its allelopathic effect against nematodes. Materials with very low C/N or high content of ammonia will either result in plasmolysis of nematodes, or proliferation of nematophagous fungi due to the release of NH4+-N (Rich and Rahi, 1995).

6.2. Soil Amendments

A study was conducted to determine the effects of combinations of organic amendments and benzaldehyde on plant-parasitic and non-parasitic nematode populations, soil microbial activity, and plant growth (Chavarria-Carvajal et al., 2001). Pine bark, velvetbean and kudzu were applied to soil at rates of 30 g/kg and paper waste at 40 g/kg alone and in combination with benzaldehyde (300 mul/kg), for control of plant-parasitic nematodes. Pre-plant and post-harvest soil and soybean root samples were analyzed, and the number of parasitic and non-parasitic nematodes associated with soil and roots were determined. Soil samples were taken at 0, 2, and 10 weeks after treatment to determine population densities of bacteria and fungi. Treatment effects on microbial composition of the soybean rhizosphere were also determined by identifying microorganisms. Bacteria strains were identified rising fatty acid analysis, and fungus identification was done rising standard morphological measurements and appropriate taxonomic keys. Results showed that most amendments alone or in combination with benzaldehyde reduced damage from plant parasitic nematodes. Benzaldehyde applied alone or in combination with the amendments exerted a selective action on the activity and composition of microbial populations in the soybean rhizosphere. In control soils the bacterial flora was predominantly Gram-negative, while in soils amended with velvetbean or kudzu alone or with benzaldehyde. Grampositive bacteria were dominant. Mycoflora promoted by the different amendments or combinations with benzaldehyde included species of Aspergillus, Myrothecium, Penicillium, and Trichoderma.

Calvet et al. (2001) evaluated the survival of two species of plant parasitic nematodes: the root-lesion nematode Pratylenchus brachyurus, and the root-knot nematode Meloidogyne javanica, in saturated atmospheres of 12 natural chemical compounds. The infectivity of two isolates of arbuscular mycorrhizal fungi: Glomus mosseae and Glomus intraradices, under identical experimental conditions, was also determined. All the compounds tested exerted a highly significant control against M. javanica and among them, benzaldehyde, salicilaldehyde, borneol, p-anisaldehyde and cinnamaldehyde caused a mortality rate above 50% over P. brachyurus. The infectivity of G. intraradices was inhibited by cinnamaldehyde, salicilaldehyde, thymol, carvacrol, p-anisaldehyde, and benzaldehyde, while only cinnamaldehyde and thymol significantly inhibited mycorrhizal colonization by G. mosseae.

When soybean plant responses to Meloidogyne incognita infestation were compared to resistant (Bryan) and susceptible (Brim) cultivars at 0, 1, 3, 10, 20, and 34 days after infestation, Qiu and collaborators (1997) observed that the resistant cultivar had higher chitinase activity than the susceptible cultivar at every sample time beginning at the third day. Results from isoelectric focusing gel electrophoresis analyses indicated that three acidic chitinase isozymes with isoelectric points (pis) of 4.8, 4.4, and 4.2 accumulated to a greater extent in the resistant compared to the susceptible cultivar following challenge. SDS-PAGE analysis of root proteins revealed that two proteins with molecular weights of approximately 31 and 46 kD accumulated more rapidly and to a higher level in the resistant than in the susceptible cultivar. Additionally, three major protein bands (33, 22, and 20 kD) with chitinase activity were detected with a modified SDS-PAGE analysis in which glycolchitin was added into the gel matrix. These results indicate that higher chitinase activity and early induction of specific chitinase isozymes may be associated with resistance to root-knot nematode in soybean.

Antagonists, most likely favored by selected cover crops, include mainly fungal egg parasites, trapping fungi, endoparasitic fungi, fungal parasites of females, endomycorrhizal fungi, planthealth promoting rhizobacteria, and obligate bacterial parasites. There are several hypotheses on how cover crops can enhance nematode-antagonistic activities. A series of ecological events may be involved. The decomposing organic material is a significant event because the bacteria which proliferate after organic matter incorporation become a food base for microbiovorous nematodes. In turn, these nematodes serve as a food source for nematophagous fungi (Wang et al., 2002). Leguminous crops enhance nematophagous fungi better than other crops. Rootknot symptoms were reduced more by alfalfa amendments in a 4-year microplot test than by chemical fertilization of plots (Mankau, 1968). Microplots amended with alfalfa meal increased nematode-trapping fungal activity of Drechmeria coniospora (Van Den Boogert et al., 1994). Pea enhanced the densities and species diversity of nematode-trapping fungi more than white mustard or barley. In addition, formation of conidial traps of nematode-trapping fungi was more prevalent in the pea rhizosphere than in root-free soil (Persmark and Nordbring-Hertz, 1997; Persmark and Jansson, 1997).

Being a legume, Crotalaria juncea has characteristics that may make the crop useful for nematode antagonism. Plant exudates from Crotalaria spp. were selective for microbial species antagonistic to phytopathogenic fungi and nematodes (Rodriguez-Kabana and Kloepper, 1998). The changes in soil enzymatic activity was investigated by Chavarria and Rodriguez-Kabana (1998) when they incorporated four organic amendments (velvetbean, kudzu, pine bark, and urea-N) to the soil to evaluate their effects on the root-knot nematode (Meloidogyne incognita). The amendments were applied to nematode-infested soil at rates of 0 to 5% and placed in pots planted with 'Davis' soybean (Glycine max). The number of M. incognita juveniles and nonparasitic nematodes associated with the soil and root tissues were determined after 8 weeks. Soil samples were taken at 0, 2, and 10 weeks after amendment application for determination of soil enzyme activities. Most organic amendments were effective in reducing root galling and root-knot nematodes and increasing populations of non-parasitic nematodes. Catalase and esterase were sharply increased by most rates of velvetbean, kudzu, and pine bark. Application of velvetbean, kudzu, and urea to soil stimulated urease activity in proportion to the amendment rates. Results suggest that complex modes of action operating in amended soils are responsible for suppression of M. incognita.

In relation with nematode-trapping fungi, a major group of nematode antagonists, they can be enhanced by incorporation of residues of C. juncea. These fungi have been categorized into two groups: parasitic and saprophytic. The saprophytic group consists of predators characterized by sticky three-dimensional networks and non-spontaneous trap formation. These fungi have a saprophytic and a predatory (trap formation) phase. In the presence of nematodes, or even exudates and homogenates of nematodes, trap formation is induced. The parasitic group consists of nematode-trapping fungi that form constricting rings, adhesive knobs, or adhesive branches. These fungi form traps spontaneously, and thus are more effective trappers (Wang, 2000).

Among these two groups of nematode trapping fungi, the population densities of parasitic fungi are more likely to be enhanced by organic matter due to the rich microbial flora and fauna. The nematode trapping by these fungi are not nematode species- or trophic group specific, therefore the enhancement of nematode-trapping fungi by organic matter incorporation should lead to increased trapping of plant-parasitic nematodes (Wang, 2000).

Soil amended with C. juncea to give a 1:100 (w:w) concentration, enhanced parasitic nematode-trapping fungi, nematode egg parasitic fungi, vermiform stage parasites, and bacterivorous nematode population densities more efficiently than soil amended with chopped pineapple tissues or non-amended soil. Crotalaria juncea amendment enhanced the population densities of nematode-trapping fungi and the percentage of eggs parasit-ized by the fungi. Enhancement of nematode-trapping fungi was most effective in soils that had not been treated with 1,3-dichloropropene for at least 5 months. Suppression of R. reniformis by C. juncea amendment was correlated with parasitic nematode-trapping fungi, fungal egg parasites, and bacterivorous nematodes. Nematode-trapping fungi population densities were higher in C. juncea planted plots than weed fallow plots. However, four months after removal of C. juncea, and replacement with pineapple plants, the population densities of nematode-trapping fungi greatly decreased (Wang, 2000).

Suppressive cropping systems rely on the use of precisely defined sequences of crops to increase populations and activities of naturally occurring antagonistic microorganisms in soil. Some crops such as velvetbean (Mucuna deerengiana) produce compounds which are directly toxic to nematodes and stimulate microbial antagonism to plant parasitic nematodes. These 'active' crops when included in cropping systems can increase suppressiveness of the system against nematodes. There are a number of active crops throughout the world which can be used in a practical manner to enhance naturally occurring biological control of plant parasitic nematodes (Wang, 2000)

Rich and Rahi (1995) conducted two greenhouse trials to determine the influence of ground seed of castor (Ricinus communis), crotalaria (Crotalaria spectabilis), hairy indigo (Indigofera hirsuta), and wheat (Triticum aestivum) on tomato (Lycopersicon esculentum) growth and egg mass production of Meloidogyne javanica (test 1) or M. incognita (test 2). Ground seed from each plant species was individually mixed with an air-dried, fine sandy soil at rates of 0, 0.5, 1.0, and 2.0% (w/w). The mixtures were placed in one-liter plastic pots, and water was added to bring soil to field capacity. After ten days, 0 or 10 000 M. javanica or M. incognita eggs and juveniles were added to each pot. A single 'Homestead' tomato seedling was transplanted into each pot and allowed to grow for 70 days in test 1 and 75 days in test 2. Compared to the non-amended control, egg mass production was significantly reduced by all treatments except the 0.5% levels of wheat and castor and the 1.0% castor treatment. The 2.0% levels of ground seed of Crotalaria and hairy indigo almost completely suppresses egg mass production of both M. javanica or M incognita. With the exception of the 1% Crotalaria treatment in test 2, total plant weight did not differ between treatments and the control.

Morris and Walker (2002) mixed dried ground plant tissues from 20 leguminous species with Meloidogyne incognita-infested soil at 1, 2 or 2.5, and 5% (w/w) and incubated for 1 week at room temperature (21 to 270C). Tomato ('Rutgers') seedlings were transplanted into infested soil to determine nematode viability. Most tissues reduced gall numbers below the non-amended controls. The tissue amendments that were most effective include: Canavalia ensiformis, Crotalaria retusa, Indigofera hirsuta, I. nummularifolia, I. spicata, I. suffruticosa, I. tinctoria, and Tephrosia adunca. Although certain tissues reduced the tomato dry weights, particularly at the higher amendment rates (5%), some tissues resulted in greater dry weights. These non-traditional legumes, known to contain bioactive phytochemicals, may offer considerable promise as soil amendments for control of plant-parasitic nematodes. Not only do these legumes reduce root-knot nematodes but some of them also enhance plant height and dry weight.

Nematode management is rarely successful in the long term with unitactic approaches. It is important to integrate multiple-tactics into a strategy. Crotalaria offers the potential to be one of the tactics. Some Crotalaria species are potential cover crops for managing several important plant-parasitic nematodes including Meloidogyne spp. and R. reniformis. Unfortunately, the residual effects are short term (a few months). Crotalaria, a poor host, generally helps reduce nematode population densities, but the number of nematodes will resurge on subsequent host crops. The damage threshold level, especially on longer-term crops, will often be reached or exceeded. This scenario strongly suggests that integrating the Crotalaria rotation system with other nematode management strategies is necessary. Among the possibilities for integration are crop resistance, enhanced crop tolerance, selection for fast growing crop varieties, soil solarization, and biological control. Chemical nematicides should be avoided in a cropping system if the objective is to enhance nematode-antagonistic microorganisms in the cropping system. Several studies have demonstrated the destructive effect of fumigation treatments to nematode antagonistic microorganisms. Crotalaria juncea amendments failed to enhance nematode-trapping fungi populations in soils that were recently treated with 1,3-dichloropropane. Wang et al. (2002) concluded that the major impediment to using Crotalaria is its short-term effect in agricultural production systems, and suggested that integrating other pest management strategies with Crotalaria could offer promising nematode management approaches.

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