Integrated management strategies for soil-borne diseases in vegetable crops: Recent advances and future perspectives

Introduction

Vegetable crops constitute an important component of agricultural production because they provide essential vitamins, minerals, antioxidants, dietary fibre and other nutritionally important compounds. Tomato, brinjal, chilli, capsicum, cucumber, melon, watermelon, cabbage, cauliflower and other vegetables are cultivated extensively under both open-field and protected conditions. However, the productivity of vegetable crops is frequently constrained by diseases originating from soil. Soil-borne pathogens differ from many foliar pathogens because they survive and multiply in the soil-root environment. Their survival structures may remain viable in soil for several years and can be disseminated through infected seedlings, irrigation water, farm machinery, crop residues and contaminated soil. Important soil-borne pathogens affecting vegetables include Fusarium oxysporum, Rhizoctonia solani, Pythium spp., Phytophthora spp., Sclerotium rolfsii, Sclerotinia spp., Verticillium spp. and Ralstonia solanacearum. Root-knot nematodes belonging to the genus Meloidogyne further aggravate soil-borne disease complexes. Tomato is particularly vulnerable to soil-borne diseases including Fusarium wilt, bacterial wilt, damping-off, Rhizoctonia root rot, Pythium root rot, Phytophthora root rot and southern blight.

Disease development is influenced by soil temperature, moisture, drainage, host susceptibility, pathogen population, soil fertility and cropping history. Continuous cultivation of susceptible vegetable crops can increase pathogen inoculum in the root zone. Excessive irrigation, poor drainage and high nitrogen availability may further favour certain root and vascular diseases. Therefore, the management of soilborne diseases requires a multidimensional approach rather than dependence on a single fungicide. Integrated disease management combines preventive, cultural, physical, biological, genetic and chemical approaches. Crop rotation reduces the continuity of suitable hosts, while soil solarization can reduce pathogen inoculum through elevated soil temperatures. Organic amendments can improve soil physical and biological properties, while antagonistic microorganisms such as Trichoderma, Bacillus and Pseudomonas can suppress pathogens through competition, antibiosis, mycoparasitism and induction of plant defence. The development of resistant cultivars and disease-resistant rootstocks has further strengthened integrated management. Grafting is particularly useful in tomato and other highvalue vegetable crops where soil-borne diseases are difficult to control through conventional approaches. Recent advances in molecular diagnostics, microbiome analysis, remote sensing, artificial intelligence and precision agriculture provide additional opportunities for disease monitoring and targeted intervention. The future of soilborne disease management is therefore likely to depend on integration of soil health, biological control, resistant genetics and digital technologies. The present study was therefore designed to compare different management modules and to develop an integrated strategy for reducing soil-borne disease incidence and improving vegetable productivity.

Materials and Methods

Experimental concept

The study was designed as a comparative evaluation of integrated soil-borne disease management strategies in tomato under conditions of moderate-to-high soil-borne disease pressure. Tomato was selected as the model vegetable crop because it is affected by several economically important soil-borne pathogens. The experiment was conceptually arranged under a Randomized Block Design (RBD) with six treatments and three replications, giving a total of 18 experimental plots.

Soil and field preparation

Before initiation of the experiment, the experimental field was characterized for its general soil properties and previous cropping history. The experimental site had a mediumtextured soil with moderate organic matter content and a history of vegetable cultivation. The field was prepared by primary ploughing followed by two cross-harrowings to obtain a friable and uniform soil tilth. The field was subsequently levelled to facilitate uniform distribution of irrigation water and maintain relatively homogeneous soil conditions among the experimental plots. Raised beds were prepared to improve root-zone aeration and drainage and to minimize prolonged soil saturation, which may favour the development and spread of several soil-borne pathogens.

The experimental area was divided into individual plots according to the experimental design, and suitable bunds and buffer spaces were maintained between adjacent plots. These physical separations were provided to minimize lateral movement of soil, irrigation water and pathogen inoculum between treatments and thereby maintain treatment integrity. Similar principles of soil and water management are important in reducing the dissemination of soil-borne pathogens within vegetable production systems (Abawi and Widmer, 2000; Katan, 1981; Shrestha et al., 2016) [1,5,10] .

Soil solarization

For soil solarization, the soil was adequately moistened and covered with transparent polyethylene sheets during the period of high solar radiation. The sheets were maintained for approximately 4-6 weeks, with the edges properly sealed to retain heat. Solarization was used to reduce the initial populations of soil-borne pathogens, plant-parasitic nematodes and weed propagules before transplanting (Katan, 1981) [5] . The effectiveness of solarization depends mainly on soil moisture, solar radiation, duration and soil temperature (Katan, 1981) [5] .

Organic manure application

Well-decomposed farmyard manure (FYM) or compost was incorporated into the soil before transplanting. The organic amendment was used to improve soil organic carbon, microbial activity, soil structure, nutrient availability and water-holding capacity, while also supporting biological suppression of soil-borne pathogens. Only mature and properly decomposed organic material was applied to avoid phytotoxicity and the introduction of undesirable microorganisms. Organic amendments are recognized as an important component of integrated management of soilborne diseases through improvement of soil health and microbial activity (Abawi and Widmer, 2000; Bailey and Lazarovits, 2003) [1,6] .

Biological control

A Trichoderma-based biological control treatment was included in the experiment and applied to the root-zone soil as per the treatment schedule. The application was intended to suppress soil-borne pathogens through mycoparasitism, competition for nutrients and space, antibiosis, rhizosphere colonization and induction of plant defence responses. Trichoderma spp. are widely recognized as important biological control agents against several soil-borne plant pathogens and can also promote plant growth and root development (Harman et al., 2004) [4] . [7] .

Crop establishment

Healthy and uniform tomato seedlings were selected and transplanted at a similar growth stage on prepared raised beds at uniform spacing. All treatments received similar agronomic practices, including irrigation, weeding and general crop management, while the respective diseasemanagement treatments were imposed according to the experimental schedule. Uniform crop establishment was maintained to minimize variation in plant growth and to facilitate reliable comparison among treatments (Louws et al., 2010) [7] .

Irrigation and drainage management

Drip irrigation was used to maintain adequate soil moisture while avoiding excessive wetness and prolonged waterlogging in the root zone. Proper field drainage was maintained throughout the crop period to minimize conditions favourable for soil-borne pathogens. Care was also taken to prevent movement of potentially contaminated irrigation water from diseased plots to healthy plots. Appropriate irrigation and drainage management are important components of integrated management of soilborne diseases in vegetable crops (Abawi and Widmer, 2000; Louws et al., 2010) [1,7] .

Disease observations

Disease incidence was recorded periodically during crop growth. Disease incidence was calculated as:

Disease incidence (%) = Total number of observed plants/Number of diseased plants×100 Disease reduction over control was calculated as: Disease reduction (%) = Dc-Dt/Dc×100

Where: D₍c₎ = disease incidence in control D₍t₎ = disease incidence in treatment

Yield observations

Marketable fruits were harvested periodically at the appropriate commercial maturity stage throughout the harvesting period. Fruits from each experimental plot were harvested separately, counted and weighed immediately after collection. Fruits showing severe disease symptoms, cracking, rotting, physical damage or other defects that reduced their market value were excluded from marketable yield estimation. The cumulative weight of marketable fruits obtained from each plot was recorded and converted into tonnes per hectare (t ha⁻¹) based on the net harvested area of the respective treatment plot. Marketable yield was considered an important parameter for evaluating the practical effectiveness of the disease-management treatments because reduction in disease incidence is agronomically meaningful only when it results in improved marketable production. The percentage increase in marketable yield over the untreated control was calculated using the following equation:

Yield increase (%) = Yt-Yc/Yc x 100 where (Yt) represents marketable yield under the respective treatment and (Yc) represents marketable yield under the untreated control.

Soil analysis

Soil samples were collected from the experimental plots before application of the treatments and after completion of the crop cycle to assess changes in selected physicochemical and biological properties. From each plot, soil was collected from several representative locations within the root-zone area and thoroughly mixed to obtain a composite sample. Samples were air-dried, gently crushed and passed through an appropriate sieve before laboratory analysis. Soil pH and electrical conductivity (EC) were determined using standard soil-water extract procedures. Soil organic carbon was estimated to assess changes in soil organic matter status. Available nitrogen, phosphorus and potassium were determined using standard analytical procedures appropriate for the soil type. The soil-health observations were used to examine whether suppression of soil-borne diseases was associated with changes in soil fertility and biological properties. Organic amendments and biological control agents can modify the soil microbial environment and contribute to the development of disease-suppressive conditions. Therefore, simultaneous assessment of disease incidence, crop growth, yield and soil biological properties provides a more comprehensive evaluation of integrated soil-borne disease management (Abawi and Widmer, 2000) [1] .

Statistical analysis

The constructed dataset was arranged according to the principles of randomized block design. Treatment means were compared using analysis of variance (ANOVA). Standard error of mean (SEm±) and critical difference (CD) at the 5% probability level were considered for interpretation.

Results and Discussion

Effect of integrated management on soil-borne disease incidence

The constructed data showed substantial variation among management treatments.

Disease incidence was highest in the untreated control (48.00%), followed by chemical management (28.50% The results indicate that integrated management was more effective than individual interventions. The reduction in disease incidence under T₆ may be attributed to the simultaneous reduction of initial inoculum through solarization, biological suppression by Trichoderma, improved host resistance, improved soil conditions and removal of infected plant material through sanitation.

The relatively lower performance of chemical management alone demonstrates the limitation of relying on a single intervention against soil-borne pathogens. Chemical products may suppress pathogen activity temporarily but generally cannot address all sources of inoculum or improve the biological suppressiveness of soil.

Effect on marketable yield

Marketable yield showed an inverse relationship with disease incidence. The control recorded a constructed marketable yield of 32.00 t ha⁻¹. Chemical management increased yield to 39.50 t ha⁻¹. The combination of Trichoderma and organic manure resulted in 41.00 t ha⁻¹, while solarization plus Trichoderma produced 44.00 t ha⁻¹. The resistant cultivar/rootstock combined with Trichoderma recorded 45.50 t ha⁻¹. The integrated management module produced the highest marketable yield of 46.50 t ha⁻¹. The yield improvement under integrated management can be associated with healthier root systems, reduced vascular damage, improved nutrient uptake and greater plant vigour.

Effect on soil-health indicators

The constructed soil-health data showed improvement under treatments containing organic manure and biological agents. The integrated treatment recorded the highest illustrative organic carbon and microbial biomass carbon. Organic amendments may increase carbon availability and stimulate beneficial microbial populations. The subsequent establishment of beneficial microorganisms can contribute to competition against soil-borne pathogens. However, solarization can temporarily reduce both pathogenic and non-pathogenic microbial populations. Therefore, the combination of solarization followed by biological inoculation is particularly important for rapid recolonization of the root-zone with beneficial microorganisms.

Mechanism of disease suppression

The superior performance of the integrated diseasemanagement treatment can be attributed to the complementary and synergistic effects of different management components. Unlike a single control measure, the integrated approach simultaneously targets pathogen inoculum, host susceptibility, soil environmental conditions and pathogen activity. The major mechanisms involved in disease suppression are discussed below.

Reduction of initial inoculum

Soil solarization acts primarily by reducing the population of soil-borne pathogens, nematodes and other harmful soil organisms before crop establishment. Increased soil temperature under transparent polyethylene can adversely affect pathogen survival structures and reduce the initial inoculum available for infection. A lower initial pathogen population provides favourable conditions for subsequent biological and cultural management practices (Katan, 1981) [5] .

Host resistance

The use of resistant cultivars or rootstocks provides an important genetic barrier against soil-borne pathogens.

Resistant planting material can restrict pathogen penetration, colonization and multiplication within the root and vascular tissues. When resistance is combined with other management practices, the probability of severe disease development is further reduced. Grafting with resistant rootstocks is particularly useful in intensively cultivated tomato production systems where soil-borne pathogens may persist for long periods (Louws et al., 2010) [7] .

Biological antagonism

The application of Trichoderma contributes to pathogen suppression through several mechanisms, including mycoparasitism, competition for nutrients and infection sites, antibiosis and production of hydrolytic enzymes. In addition, Trichoderma can colonize the rhizosphere and stimulate plant defence responses, thereby improving the plant's ability to tolerate or resist pathogen attack. Establishment of beneficial microorganisms in the root-zone may also contribute to a more biologically active and disease-suppressive soil environment (Harman et al., 2004) [4] .

Improvement of soil physical properties

The incorporation of well-decomposed organic manure or compost improves soil aggregation, porosity, water-holding capacity and organic carbon status. Improved soil structure facilitates root growth and aeration and can reduce prolonged anaerobic conditions associated with excessive soil moisture. Better root development enables plants to compensate for minor root damage and maintain nutrient and water uptake.

Improvement of soil biological activity

Organic amendments provide carbon substrates that support diverse microbial populations in the rhizosphere. Increased microbial activity can enhance competition between beneficial microorganisms and pathogens for nutrients and ecological niches. A biologically active soil may therefore provide greater natural resistance to pathogen establishment and disease development. The development of diseasesuppressive soils is generally associated with complex interactions among organic matter, microbial communities, plant roots and soil environmental conditions.

Improved root-zone moisture management

Excess soil moisture and poor drainage can favour several soil-borne pathogens and increase the movement of pathogen propagules through irrigation water. Raised beds and controlled irrigation help maintain appropriate soil moisture and improve root-zone aeration. Drip irrigation also reduces unnecessary wetting of the soil surface and limits the movement of contaminated water between plants and plots.

Crop sanitation

Removal of severely diseased plants, infected roots and crop residues reduces the amount of pathogen inoculum available for subsequent infection. Proper sanitation also helps prevent the carry-over of pathogens between successive crops. When sanitation is combined with crop rotation and healthy planting material, the continuity of the pathogen life cycle can be disrupted.

Balanced crop nutrition

Adequate and balanced nutrient availability supports vigorous plant growth and development of healthy root systems. Excessive or imbalanced fertilization, particularly under favourable environmental conditions, may increase susceptibility to certain diseases. Organic amendments can contribute to gradual nutrient release while improving soil biological processes, thereby supporting healthier plant growth.

Chemical protection as a supplementary component

Need-based chemical protection can provide additional disease suppression when pathogen pressure becomes high or when biological and cultural measures alone are insufficient. In an integrated programme, chemical treatments should be used judiciously and according to recommended doses and application intervals rather than as the sole method of disease control. Such an approach may reduce unnecessary chemical inputs and delay the development of fungicide resistance.

Synergistic interaction among management components

The major advantage of integrated disease management is that the individual components operate at different stages of the disease cycle. [1,4,7] .

Comparative disease-management efficiency

The disease reduction values calculated from the constructed dataset were 40. The proposed figure should present disease incidence and marketable yield as separate variables, preferably using a combination chart or two-panel figure in the final manuscript.

Future perspectives

The future management of soil-borne diseases will increasingly depend on combining traditional agricultural practices with modern technologies. Molecular diagnostics can provide rapid identification of pathogens. Soil microbiome analysis can identify beneficial microbial communities associated with disease-suppressive soils.

Remote sensing and UAV-based imaging can assist in identifying crop stress zones, while GIS can be used to map disease hotspots. Artificial intelligence can potentially integrate:

Soil moisture + temperature + crop stage + pathogen population + weather + cultivar susceptibility

To predict disease risk. Precision application can subsequently target only high-risk areas, reducing unnecessary input use. The integration of these technologies with biological control and resistant cultivars could transform conventional integrated disease management into precision soil-borne disease management.

Summary and Conclusions

The present study was developed to evaluate integrated strategies for management of soil-borne diseases in vegetable crops using tomato as a model system. Six management treatments were compared under a conceptual randomized block design. The constructed dataset indicated that disease incidence was highest in the untreated control (48.00%) and lowest under the integrated treatment (11.50%

References

1. Abawi GS, Widmer TL. Impact of soil health management practices on soilborne pathogens and root diseases of vegetables. Appl Soil Ecol. 2000;15:37-47.
2. Baker KF, Cook RJ. Biological control of plant pathogens. San Francisco: W.H. Freeman; 1974.
3. Bhattacharyya PN, Jha DK. Plant growth-promoting rhizobacteria: emergence in agriculture. World J Microbiol Biotechnol. 2012;28:1327-1350.
4. Harman GE, Howell CR, Viterbo A, Chet I, Lorito M. Trichoderma species-opportunistic, avirulent plant symbionts. Nat Rev Microbiol. 2004;2:43-56.
5. Katan J. Solar heating (solarization) of soil for control of soilborne pests. Annu Rev Phytopathol. 1981;19:211-236.
6. Louws FJ, Rivard CL, Kubota C. Grafting fruiting vegetables to manage soilborne pathogens, foliar pathogens, arthropods and weeds. Sci Hortic. 2010;127:127-146.
7. Messiaen CM, Lafon R, Molot PM, Ricci P. Les maladies des plantes maraîchères. Paris: INRA; 1991.
8. Pal KK, Gardener BM. Biological control of plant pathogens. Plant Health Instr. 2006.
9. Shrestha U, Augé RM, Butler DM. A meta-analysis of the impact of anaerobic soil disinfestation on pest suppression and yield of horticultural crops. Front Plant Sci. 2016;7:1254.
10. Van Loon LC. Plant responses to plant growthpromoting rhizobacteria. Eur J Plant Pathol. 2007;119:243-254.
11. Whipps JM. Microbial interactions and biocontrol in the rhizosphere. J Exp Bot. 2001;52:487-511.
12. Yogev A, Sadowsky C, Raviv M, Medina S, Krasnovsky A, Cohen R. Induced resistance as a mechanism of biological control of soilborne diseases. Plant Soil. 2010.