Soil is the most important component for maintaining ecosystem balance on the earth. The relation of soil fertility and microorganisms for expression of better crop health, production and quality is well known. In rhizosphere, there are different microorganisms which directly or indirectly influence the plant health and growth development
(Kamal et al., 2015). Several diseases of field crops are soil borne and are considered a major limitation to better crop production. Soilborne plant pathogens such as
Rhizoctonia spp.,
Fusarium spp.,
Verticillium spp.,
Sclerotinia spp.,
Pythium spp. and
Phytophthora spp. can cause 50 to75 per cent yield loss for many crops such as wheat, cotton, maize, vegetables, fruits and ornamentals (
Baysal-Gurel and Kabir, 2018). The use of fungicides against soilborne plant pathogens can help to manage some diseases, in contrast, frequent and indiscriminate use can increase environmental and health concerns, and lead to development of fungicide resistance
(Christopher et al., 2010). Some environment-friendly approaches have been developed to manage the soil borne diseases while maintaining the soil health. In soil, microorganisms reside in rhizosphere and constitute a complex organization of endophytes, saprophytes and actinomycetes, both harmful and beneficial ones
(Baysal-Gurel et al., 2019).
Various strains of
Bacillus species are known as potential elicitors of ISR and exhibit significant reduction in the incidence or severity of various diseases on diverse hosts
(Choudhary et al., 2008). It is believed that plants have the ability to acquire enhanced level of resistance against pathogens after getting exposed to biotic stimulation provided by many plant growth-promoting rhizobacteria (PGPR’s) and this is known as rhizobacteria mediated induced systemic resistance (ISR)
(Choudhary et al., 2007). Arbuscular mycorrhizal fungi (AMF) can suppress plant pests and diseases through induction of systemic resistance
(Jung et al., 2012 and
Pineda et al., 2010). Mycorrhiza-induced resistance (MIR) involves an ISR component elicited by bacteria in the mycorrhizosphere, as AMF, rhizobacteria possess MAMPs, which can trigger MAMP-induced immune responses
(Berendsen et al., 2012). Green manuring can play an important role in this regard as it showed versatile impacts like improvement of soil physio-chemical and biological properties, fertility, nutrient supply to succeeding crops, checking erosion and plant protection
(Saini et al., 2019). This review paper seeks to summarize the different methods used for management of soil borne diseases by improving soil health including mycorrhiza, cover crops, bioagents, mulching, good compost, good aeration, suppressive soils, nutrients, intercropping and crop rotation.
Soil health
Doran et al., (1994) defined soil quality (health) as “The capacity of a soil to function, within ecosystem and land use boundaries, to sustain biological productivity, maintain environmental quality and promote plant and animal health.” It is worth noting here that “soil health” and “soil quality” are essentially synonymous terms. Soil moisture, soil temperature, soil pH, organic matter, nutrients, beneficial microorganisms, organic amendments, crop rotation, cover crops and green manures are the various factors which influence soil health.
Soil moisture
Water contained in soil is called soil moisture. Optimum soil moisture is important for plant growth. There is a relation of moisture with diseases. Late blight of potato, apple scab, downy mildew of grapes and fire blight are favoured by high rainfall and high relative humidity. Most severe symptoms of
Phytophthora, Rhizoctonia, Sclerotinia and
Sclerotium can be observed on plants when the soil is wet but not flooded. In case of bacterial (
e.g., Erwinia and
Pseudomonas) and nematode infection, more severe symptoms are produced on plants when the soil is wet.
Streptomyces scabies which causes the common scab of potato becomes more severe in soils drying out after wetting.
Fusarium roseum, the cause of seedling blights,
Macrophomina phaseolina, the cause of charcoal rot of sorghum and root rot of cotton and
Fusarium solani, the cause of dry root rot of beans occur in dry environments. Verticillium and canker diseases of forest trees are more severe in water stress conditions (
Agrios, 2005). Soil moisture and temperature have direct relation with seedling blight of wheat and corn (
Dickson, 1923). We can manage all these diseases by maintaining the soil moisture level below or above optimal soil moisture required by the pathogen.
Soil temperature
Warm, moist soils with high levels of carbon to nitrogen ratio will have higher level of microbial activity and a relatively higher level of suppression. Soil temperature can greatly affect the activity of locomotion, infection and reproduction of nematodes. Some species of the fungi
Typhula and
Fusarium, which causes snow mold of cereals and turf grasses, thrive only in cool season or cold regions. Also, the late blight pathogen
Phytophthora infestans is more serious in the Northern latitudes whereas in the subtropics it is serious only during the winter. Many diseases, such as brown rot of stone fruits caused by
Monilinia fructicola, are favored by relatively high temperatures and are limited in range to areas and seasons in which such temperatures are prevalent. Several diseases, such as the Fusarial wilts, anthracnoses caused by
Colletotrichum spp. and the bacterial wilts of solanaceous plants caused by
Ralstonia solanacearum, are favored by high temperatures and are limited to hot areas, being particularly severe in the subtropics and tropics. At temperature much below or above the optimum for the pathogen, or near the optimum for the host, disease development is slower
(Jones et al., 1926).
Soil pH and soil structure
Soil pH is an estimate of the activity of hydrogen ions in the soil solution. It is also an indicator of plant available nutrients. High activity is not desirable and the soil may require liming with base cations Ca or Mg in order to bring the solution back to neutral. Soil pH increases by adding lime while it decreases by adding sulphur and ammonium. The pH of the soil is important in the occurrence and severity of plant diseases caused by certain soil borne pathogens. For example, the club root of crucifers caused by
Plasmodiophora brassicae is most prevalent and severe at about pH 5.7, whereas its development drops sharply between pH 5.7 and 6.2 and is completely checked at pH 7.8. Common scab of potato caused by
Streptomyces scabies can be severe from pH 5.2 to 8.0 or above, but its development drops sharply below pH 5.2. Soil texture and structure could have effects on plant diseases because they affect water holding capacity, nutrient status and gas exchange as well as root growth. Poor soil aeration caused by poor soil structure, soil type or water logging is associated with the development of cavity spot (
Pythium spp.) disease in carrot
. The pea root rot complex
(Fusarium spp.) is known to be affected by compaction
, temperature and moisture of the soils (
Chupp, 1928). All these diseases can be managed by maintaining good aeration or soil pH and avoiding waterlogged conditions.
Soil microorganisms
Microflora (bacteria, fungi, actinomycetes,
etc.) constitute up to 75 to 90 per cent of the soil-living biomass and are the primary decomposers of organic matter. Two major components of microbial biota in soil are: a). Disease-inducing microbes: Fungi and bacteria can cause diseases to plants or degrade the soil quality by interfering with beneficial microorganism(s) thereby affecting plant health. Fungal pathogens like
Plasmodiophora brassicae (club root of crucifers),
Spongospora subterranea (powdery scab of potato),
Fusarium, Rhizoctonia, Verticillium, Phytophthora, Pythium and
Sclerotinia and bacterial pathogens like
Ralstonia solanacearum (wilt),
Erwinia sp. (soft rot) and
Streptomyces scabies (potato scab) causes a great extent of damage to many crops. Soil-borne viruses also cause diseases such as
Lettuce big vein virus (LBVV) and
Lettuce stunt necrotic virus (LSNV). These soil-borne pathogens can survive in soil for many years
(Saha et al., 2016). b). Biocontrol agents inhabiting in soil (Resident biocontrol agents): Soil residing microorganisms (bacteria/fungi) are used successfully for controlling diseases. Disease control and better crop health can be achieved through various activities performed by these soil microbes like hydrocyanic acid (HCN) production, siderophore production, nitrogen assimilation, antibiotic production, hydrolytic enzyme (lipase, chitinase,
etc.) production, induced systemic resistance and systemic acquired resistance.
Bacillus spp. and
Pseudomonas spp. serveserves as excellent examples of biocontrol agents having prominent PGPR
(plant growth-promoting rhizobacteria
) activities and disease reduction ability
(Saha et al., 2016).
On the basis of microbial activity of soil, it is a). Disease-inducing soils: are disease causing soil which means that pathogenic microbes comprise of 5 to 20 per cent of total soil microflora. When fresh organic matter is applied in this type of soil, incomplete oxidized products are released, which are hazardous to plants and in turn, are easily attacked by pathogens or insects. Such soils can be amended into disease suppressive soils by addition of inoculum of effective microorganisms
(Parr et al., 1994). b). Plant growth promoting microorganisms (PGPM): exert direct effect on plant growth promotion
e.g. Rhizobium and
Glomus spp. Bacteria which possess the tendency to colonize roots actively are called as PGPR (
Schroth and Hancock, 1982). For soil-borne pathogens and disease management, rhizospheric microbes emerged as a biological weapon that triggers the mechanism of disease reduction through Systemic Acquired Resistance (SAR) and Induced Systemic Resistance (ISR). Soil microflora facilitate uptake of nutrients from soil which results in enhanced yield as well as disease reduction or suppression.
Bacillus subtilis has the potential in disease reduction and more than twenty antibiotics are produced by them. Efficacy of
Bacillus spp. has been reported in different crop plants like tomato, chilli, brinjal
etc. to control different pathogens like
Colletotrichum acutatum, C. capsici, C. gloeosporioides, Pythium aphanidermatum and
R. solani (
Abdul et al., 2007).
Antibiotic production
Rhizobacteria contributes in disease suppression with antibiotic production. There are six classes of antibiotic compounds (for which their modes of action are partly understood) that are related to the biocontrol of root diseases
viz. phenazines, phloroglucinols, pyrrolnitrin, pyoluteorin, cyclic lipopeptides (all of which are diffusible) and HCN (
Haas and Defago, 2005).
Pseudomonas fluorescens-5 (Pf-5), source of pyoluteorin and pyrrolnitrin, was found to be effective in protecting damping off in cotton (
Pythium ultimium /Rhizoctonia solani) (
Howell and Stipanovic, 1980).
Maksimov et al., (2011) reported majority of
Bacillus spp. for the production of antibiotics such as polymyxin, circulin and colistin, which were found active against Gram-positive and Gram-negative bacteria and many pathogenic fungi. An antibiotic, Geldanamycin produced by
Streptomyces hygroscopicus var.
Geldanus, reduced the
Rhizoctonia root rot of pea (
Rothrock and Gottlieb, 1984). Similarly, HCN is a volatile compound and is useful in the biocontrol of diseases
(Defago et al., 1990).
Root colonization
The main aspect for biocontrol is the colonization of rhizosphere soil or external / internal root zone by microbes, especially bacteria (
Bahme and Schroth, 1987). In biocontrol method, root colonization is completed in two phases: Firstly, bacteria get attached to rhizosphere and are then transported on the elongating root tip. Secondly, bacteria spread locally, proliferate to the boundaries of niche by competing with other indigenous microorganisms and survive. Root colonization involves recognition of pathogens by potential antagonists and it is important for base line of effective biocontrol strategies (
Barak and Chet, 1990).
Induced systemic resistance
Induced systemic resistance (ISR) was explained and reported in carnation plant in which
Pseudomonas strain (Pf WCS 417r) was found effective against
F. oxysporum f. sp.
dianthi (Van
Peer et al., 1991). Induced resistance is the ability of plants to develop an enhanced defensive ability when appropriately stimulated (Van
Loon et al., 1998). Some pathogenesis-related proteins (PRs) like 1, 3-glucanases and chitinases are capable of hydrolyzing fungal cell walls and insects
(Singh et al., 2015). Pseudomonas and
Bacillus spp. are the most popular rhizobacteria encompassing ISR (Van
Wees et al., 2008).
Management of diseases by improving soil health
Different methods used to improve soil health and manage diseases are use of organic amendment, cover crops and green manures, beneficial organisms (bioagents), crop rotation
, nutrient management
, VAM fungi and suppressive soil.
Organic matter amendments
The addition of organic matter such as cover crop green manure (single and mixed species), seed meals, dried plant material, good quality compost, organic waste and peats can aid in reducing diseases caused by soil borne pathogens
. Organic biochemicals are environment friendly, easily biodegradable and safe for humans. Organic matter thus is capable of restoring ecological balance
e.g. Phytophthora root rot is suppressed by using composted tree bark. Verticillium wilt of tomato is decreased by using blood meal and fish meal. Organic manure supply nutrients which are suitable for antagonists, which favor them in competition. Soil amendments with plant materials help in management of root rot of apple. Fusarium crown rot of Chinese yam, cucumber Fusarium wilt are suppressed by application of pig manure and rape seed bio-organic fertilizer to field plots
(Zhang et al., 2008).
Green manuring crops
Cover crops are defined as any living ground cover that is planted into or after agriculture crops mainly cash crops and turned into the soil before planting next crop plants.
Hartwig and Ammon (2002) stated that first benefit of cover crops is reduction of runoff and soil erosion which subsequently results in improvement of soil health and sustainable soil productivity. Cover crops also keep the soil moist, which is favourable for microbial activity. Cover crops also affects soil microbial biomass C and N of long term field. This control can come through direct suppression of the insects by the plants or by attraction of beneficial insects that prey or parasitize the pests.
Barley (
Hordeum vulgare) suppresses leafhopper, aphids, armyworms, root-knot nematodes; buckwheat (
Fagopyrum esculentum) attracts hover flies, wasps, minute pirate bugs, insidious flower bugs, tachinid flies, lady beetles that prey or parasitize aphids, mites, other pests; cowpeas (
Vigna unguiculata) attracts wasps, honeybees, lady beetles, ants that prey on crop pests; rapeseed (
Brassica nupus and
Brassica rapa) suppresses plant parasitic nematodes; red clover (
Trifolium pretense) attracts beneficial insects; rye (
Secale cereal) reduces insect pest problems in rotations, attracts beneficial insects; sorgum-sudangrass hybrids (
Sorhum bicolor x S. bicolor var
. Sudanese) disrupts life cycles of diseases, nematodes; subterranean clovers (
Trifolium subterraneum, T. yanninicum, T. brachycalcycinum) reduces pest egg laying and larval populations in cabbage, thrips in leeks, tarnished plant bug; sweet clovers (
Melilotus officinalis, M. alba) attracts honeybees, tachinid flies, large predatory wasps; woollypod vetch (
Vicia villosa ssp
. dasycarpa) attracts beneficial insects (
Brevik, 2009).
Intercropping and crop rotation
Intercropping and crop rotations promotes microbial multiplication and activities. Intercropping or polyculture is defined as the cultivation of two or more crops on the same piece of land to the extent that crops interact biologically. Such biological nitrogen fixation improves the fertility of the soil and both the soil health and quality as the availability of nitrogen positively affects the C: N ratios by increasing the N value. This in turn improves the general microbial ecology of the soil by allowing proliferation of diverse and increased number of soil biota. Growing crops in a planned sequence on the same field is known as crop rotation. The rotation of host crop with non-host crop plays an important role in managing diseases. Crop rotation is a key factor in residue management for disease control as it breaks disease cycle by reducing inoculum level
i.e. some pathogens that cause disease, survives in soil for many years in the form of sclerotia, spores, hyphae. Monoculture encourages many soil borne diseases because the pathogen specific to a crop can multiply out of all proportions when that crops is grown in the same place year after year. The club root in brassicas requires a 7-year rotation. The diseases are easily managed by crop rotation as some pathogens survive on roots and leaves of taxonomically diverse weeds
e.g. bacterial spot of pepper and tomato (
Xanthomonas campestris pv.
vesicatoria), bacterial speck of tomato (
Pseudomonas syringae pv.
tomato), root-knot nematode (
Meloidogyne hapla),
etc. There are certain pathogens that survives in soil and cannot be easily managed
e.g.
Fusarium wilts of crucifers, cucurbits, pea, spinach, tomato and
Rhizoctonia solani. If legume crops are used in the rotation, nutrient cycling is improved through biological nitrogen fixation (BNF) by symbiotic rhizobacteria (
Rhizobium spp
.,
Sinorhizobium spp
., Bradyrhizobium spp
.). Similarly, free-living nitrogen-fixing bacteria (
Beijerinckia fluminensis, Azotobacter paspali, Azospirillum spp.) associated with non-legume plants like sugar cane, maize, or rice contribute to BNF, explaining the productivity of these crops under low input management (
Baldani and Baldani, 2005). AMF populations are favored when a cover crop like hairy vetch (
Vicia villosa) or sorghum-sudangrass (
Sorghum × drummondi), Arbuscular Mycorrhiza Fungi (AMF)-root-symbiosis increases phosphorus (P) uptake (
Douds and Millner, 1999).
Nutrient management
Plants suffering a nutrient stress will be more susceptible to diseases, while adequate crop nutrition makes plants more tolerant or resistant to disease. Mineral nutrition can affect two primary resistance mechanisms: a) formation of mechanical barrier (
e.g. thickness of cell walls) and b) synthesis of natural defense compounds (
e.g. phytoalexins). There is a relation between disease and nutrients.
Fusarium spp
., Plasmodiophora brassicae, Sclerotium rolfsii, Pyrenochaeta lycopersici and the diseases they cause (root rots and wilts, clubroot of crucifers, damping off and stem rots and corky root rot, respectively) increase in severity when an ammonium fertilizer is applied.
Phymatotrichopsis omnivora, Gaeumannomyces graminis and
S. scabies and the diseases they cause (cotton root rot, take-all of wheat and scab of potato, respectively) are favoured by nitrate nitrogen. Phosphorus has been shown to reduce the severity of take-all disease of barley (caused by
G. graminis) and potato scab (caused by
S. scabies). Potassium reduced the severity of numerous diseases, including stem rust of wheat, early blight of tomato and gray leaf spot and stalk rot of corn. Potassium in high amount increased the severity of rice blast (caused by
Magnaporthe grisea), corn gray leaf spot (caused by
Cercospora zeae-maydis) and root knot (caused by the nematode
Meloidogyne incognita). Magnesium increased the severity of corn leaf blight caused by
Cochliobolus heterostrofus. Molybdenum reduced the severity of late blight of potato and Ascochyta blight of beans and peas (Agrios, 2005). Septoria glume blotch of wheat is reduced with the application of potassium and phosphorus
(Gunifer et al., 1980).
Biocontrol agents
Trichoderma harzianum, T. viride, T. hamatum, Gliocladium spp.,
Flavobacterium spp.,
Streptomycetes spp., VAM fungi (Vesicular Arbuscular Mycorrhizae),
Bacillus subtilis, Pseudomonas fluorescens helps in suppressing the effect of pathogen and reduces the disease incidence.
Trichoderma strains are potential biocontrol against several fungal phytopathogens. Several soil-borne antagonists are reported to control fungal wilt of tomato caused by
F. oxysporum f. sp.
lycopersici (
Singh et al., 2013).
The biocontrol agents produce antibiotics
viz., 2,4-diacetyl-phloroglucinol (
Pseudomonas fluorescens F113) against damping off (
Pythium spp.); agrocin 84 (
Agrobacterium radiobacter) against crown gall (
Agrobacterium tumefaciens); bacillomycin D (
Bacillus subtilis AU195) against aflatoxin contamination (
Aspergillus flavus); bacillomycin, fengycin (
Bacillus amyloliquefaciens FZB42) against wilt (
Fusarium oxysporum); xanthobaccin A (
Lysobacter sp. strain SB-K88) against damping off (
Aphanomyces cochlioides); gliotoxin (
Trichoderma virens) against root rots (
Rhizoctonia solani); herbicolin (
Pantoea agglomerans C9-1) against fire blight (
Erwinia amylovora); iturin A (
B. subtilis QST713) against damping off (
Botrytis cinerea and
R. solani); mycosubtilin (
B. subtilis BBG100) against damping off (
Pythium aphanidermatum); Phenazines (
P. fluorescens 2-79 and 30-84) against take-all (
Gaeumannomyces graminis var.
tritici); pyoluteorin, pyrrolnitrin (
P. fluorescens Pf-5) against damping off (
Pythium ultimum and
R. solani); pyrrolnitrin,pseudane (
Burkholderia cepacia) against damping off and rice blast (
R. solani and
Pyricularia oryzae); Zwittermicin A (
Bacillus cereus UW85) against damping off (
Phytophthora medicaginis and
P. aphanidermatum) (
Pal and Gardener, 2006).
These biocontrol agents show either direct antagonism by mechanisms such as mycoparasitism / predation (Lytic/some nonlytic mycoviruses,
Ampelomyces quisqualis, Lysobacter enzymogenes, Pasteuria penetrans, Trichoderma virens, etc.) or mixed-path antagonism by mechanisms such as antibiotiosis (2,4-diacetylphloroglucinol, phenazines, cyclic lipopeptides,
etc.) / lytic enzymes production (chitinases, glucanases, proteases,
etc.) / unregulated waste products (
e.g. ammonia, carbon dioxide, hydrogen cyanide,
etc.)/ physical or chemical interference (blockage of soil pores, germination signals consumption, molecular cross-talk confused,
etc.) or indirect antagonism by mechanisms such as competition (exudates/leachates consumption, siderophore scavenging, physical niche occupation, etc.)/ promoting plant growth and plant defense mechanisms [induction of host resistance (contact with fungal cell walls, detection of pathogen-associated, molecular patterns, phytohormone-mediated induction,
etc.)
(Kamal et al., 2015). These metabolites can be either combined with appropriate biocontrol strains in order to obtain new formulations for use in more efficient control of plant diseases. Buckeye rot of tomato caused by
Phytophthora nicotianae var. parasitica is a serious threat to the crop production. In investigation, six biological control agents were screened for the efficacy against mycelial growth of
P. nicotianae var. parasitica. Out of the six fungal and bacterial biocontrol agents tested,
Trichoderma virens resulted in maximum mycelial growth inhibition (77.67%) of the test pathogen followed by
T. hamatum (69.40 %),
T. harzianum (68.52%) and
T. viride (67.43%).
Pseudomonas fluorescens and Bacillus sp
. were least effective with 53.67 per cent mycelial growth inhibition
(Sharma et al., 2018).
AM (Arbuscular mycorrhizae)
AM is symbiotic association between a fungus and a root of higher plant. The most important nutritional effect of AM fungi is the improved uptake of immobile nutrients, especially P, Cu and Zn. AM fungi is associated with enhanced chlorophyll levels in leaves and improved plant tolerance to diseases, parasites, water stress, salinity and heavy metal toxicity. Cover plant roots, forming mat protect plant roots from many diseases in several ways by either providing physical barrier, antagonistic chemicals, competing with pathogen, or increasing nutrient uptake. AM mycorrhizal fungi physiologically alter the host competition with pathogen for space or host photosynthate. Nutritional benefits improved by mycorrhizal symbiosis, as well as induced biochemical alterations of the plant, by which VAM mycorrhizal fungi reduced soil borne diseases. The antimicrobial substances produced by the extraradical mycelium of the AM fungi species
Glomus intraradices reduced conidial germination of
F. oxysporum f.sp.
chrysanthemi. Co-inoculation of groundnut with
Glomus fasciculatum, Gigaspora margarita, Acaulospora laevis and
Sclerocystis dussii eliminated the damaging effects of
Sclerotium rolfsii.
Glomus mosseae prevented infection of soybean plants against
P. syringae. P. cinnamomi was managed by
Glomus sp. in avocado
(Davis et al., 1978), Dematophora necatrix was managed by
Glomus spp. (
Bharat and Bhardwaj, 2001) and
P. cactorum managed by
G. intaradices (
Utkhede and Smith, 2000) in apple and pathogen
Rhizoctonia sp. was managed by ectomycorrhizal
Suillus granulatus in
Pinus excelsa (
Corte, 1969).
Compost
Compost is used effectively in nursery, high value crops, potting soil mixtures for control of root rot diseases. Compost act as a food source and shelter for the antagonists that compete with plant pathogens. Two or more common beneficial biocontrol agents (strains of
Trichoderma and
Flavobacterium) were added to compost for suppressing
Rhizoctonia solani. There is a direct relationship between compost level of decomposition and its suppression of
Rhizoctonia. Trichoderma harzanium acts against a broad range of soil borne fungal crop pathogens by producing antifungal exudates. Immature compost, raw manure is conductive to diseases at first and becomes suppressive after decomposition.
Mulching
The proper mulching improves bulk density of soil, saturated hydraulic conductivity, soil moisture, soil temperature and consequently soil functions and improvement in biological activities.
Zhang et al., (2008) reported abundance of earthworms under wheat straw mulch field as compared to non-mulch field. In addition, mulching increases the total carbon in bulk soil and helps to conserve moisture, control surface erosion and reduce airborne particulates
(Karlen et al., 1994).
Disease suppressive soils
Soil population comprises of microbes that suppress the activity or growth of phytopathogens without any chemical usage (
Timmusk, 2003). This ability is naturally borne by the soil, which is actual functional site (anatagonistic activity) for beneficial microbes
(Weller et al., 2002). Antagonistic microbes like
Trichoderma, Penicillium, actinomycete
s,
etc. are the inhabitants of such soils producing sufficient amount of antibiotics, which restrict soil-borne pathogens like
Fusarium, Pythium, etc. Plants cultivated under such soils are healthy and rarely infected with diseases or attacked by insects. The mechanisms
viz., induced resistance, direct parasitism, nutrient competition, direct inhibition through antibiotics secreted by beneficial organism are involved in disease suppressive soils. There are two types of suppressive soil a). General suppression (non-specific, against many pathogen): termed as the widespread but confined ability of soils to inhibit the activity or growth of soil borne pathogens. It can as well be termed as nonspecific antagonism or biological buffering
(Weller et al., 2002), e.g.
Phytophthora root rot of papaya was managed by planting papaya seedlings in suppressive soil by putting in holes in the orchard soil that was infested with the
P. palmivora (
Rani and Sudini, 2013); b). Specific suppression (one organism operates against only certain types of pathogens): arises from a direct inhibition of a known pathogen by one organism. There are incidences where an agent of biological control is introduced into the soil for the specific reduction in occurrence of the disease. Specific suppressiveness owes its activity to the impacts of individual or select groups of microorganisms. Specific suppressiveness has been described for Fusarium wilts
, Gaeumannomyces graminis var.
tritici, Phytophthora spp.,
Pythium spp.
, Rhizoctonia solani and
Thielaviopsis basicola. In take-all suppressive soils (
G. graminis var.
tritici.) activity of certain antibiotic-producing fluorescent pseudomonas or parasitism by
Trichoderma spp
. resulted in disease suppression. Production of antibiotics including phenazine-1-carboxylic acid, 2,4 diacetylphloroglucinol (2,4-DAPG), pyoluteorin and pyrrolnitrin played an important role in the biological control of this soil borne pathogen by certain strains of fluorescent
Pseudomonas spp.
(Berendsen et al., 2012).
