Soil is a complex, heterogeneous and dynamic natural system. Soil environment is important in ecology as it influences several processes in plants. Plant growth, abundance and distribution are naturally influenced by soil environment. The soil environment can have a significant impact on microbial habitat through its influence on water and gaseous movement in the soil system, which affects microbial diversity, activities and functions. It directly affects the microbes including pathogenic forms, as well as the rates of processes they perform. The degree of pathogenicity of soil-borne micro-organisms observed under laboratory conditions is not usually repeated in the field, where biological and non-biological influences are less controlled. Ecological study of soil borne plant pathogens, thus, includes the detailed information on major aspects of the soil, which helps in a proper understanding of the damage and yield loss caused by soil borne pathogens. ‘Ecology of Soil-borne Plant Pathogens” is being taught as a core course in PG degree programmes at various universities, with a common syllabus. Not a single text book covering the prescribed syllabus is available. Thus, a dire need of such a textbook has been felt for quite some time for the benefit of students specializing in the subject. This book, therefore, has been designed to cover the course outline of “Ecology of Soil-borne Plant Pathogens”, prescribed by the Indian Council of Agricultural Research, New Delhi, India. The book comprises eight chapters covering the latest information on “Soil as an environment for plant pathogens, Nature and importance of rhizosphere and rhizoplane, Soil and root Inhabiting fungi, Types of biocontrol agents, Inoculum potential and density in relation to host and soil variables, Competition, Predation, Antibiosis and Fungistasis, Conducive and Suppressive soils and f inally, Biological control-Concepts & potentialities of bioagents for managing soil borne pathogens”. Appropriate diagrams, convincing tables and suitable graphs / illustrations have been furnished at the right places. A bibliography providing the list of references cited has also been included at the end of each chapter.

Soil is a strange full of wisdom particularly in terms of biodiversity. Soil biota consists of the microbes (bacteria, fungi, archaea and algae), soil animals (protozoa, nematodes, mites, springtails, spiders, insects and earthworms) and plants, living all or part of their lives in or on the soil or pedosphere. Millions of species of soil organisms exist but only a fraction of them have been cultured and identified. Micro organisms (MOs) (fungi, archaea, bacteria, algae and cyanobacteria) are members of the soil biota but are not members of the soil fauna (Sachidanand et al., 2019). The soil fauna is the collection of all the microscopic and macroscopic animals in a given soil. The size of a soil organism can restrict its location in the soil habitat. Smaller members of the microfauna like nematodes are basically aquatic organisms that live in the thin water f ilms or capillary pores of aggregates preying or grazing on other aquatic microfauna such as amoebas. Soil has a direct effect on the environmental conditions, habitat and nutrient sources available to the soil biota. The term pedosphere is often used interchangeably with soil and captures the concept that the soil is a habitat where the integration of spheres occurs. These spheres include the lithosphere, atmosphere, hydrosphere and the biosphere. Numerous biogeochemical processes regulated by soil biota occur in the pedosphere. The physical structure, aeration, water holding capacity and availability of nutrients are determined by the mineral constituents of soil, which are formed by the weathering of rock and the degradative metabolic activities of the soil MOs. Soil microbiology is the study of organisms in soil, their functions, and how they affect soil properties. It is believed that between two and four billion years ago, the first ancient bacteria and MOs came about in earth’s oceans. These bacteria could fix N2 , in time multiplied and as a result released oxygen into the atmosphere. This led to more advanced MOs. Soil biology plays a vital role in determining many soil characteristics. The decomposition of organic matter by soil organisms has an immense influence on soil fertility, plant growth, soil structure, and C storage. As a relatively new science, much remains unknown about soil biology and their effects on soil ecosystems. The soil is home to a large proportion of the world’s biodiversity.

2.1 Roots and the Rhizosphere The rhizosphere, the region of soil surrounding plant roots in which soil and microbes closely interact–plays an important role in soil carbon cycling. The rhizosphere is only about 1-2% of Earth’s soil volume but can store up to 30-40% of Earth’s total soil organic matter. Plants secrete organic compounds such as amino acids or enzymes known as root exudates, which are key in recruiting and selecting relevant beneficial microbes to colonize in the rhizosphere (Julie Bobyock, 2023). Studying these microbial-plant interactions is critical to better understand the relationship between plant growth and the role soils play in keeping carbon from the atmosphere. It is the region that is a few distances (2-80 mm) extended from the root system. Rhizosphere zone is the region of intense microbial activity, and it is isolated from the bulk soil that often called as Edaphosphere or Non-rhizosphere. Plant growth results from interaction of roots and shoots with the environment. The environment for roots is the soil or planting medium, and its importance is often not appreciated beyond the physical and nutritional aspects. Roots do provide structural support for the plant, and they absorb water and nutrients, but they also produce plant growth-regulating substances, such as cytokinins. The cortex of the root is largely starch storage tissue, and represents a transition zone from the soil to the stele. The cortex also provides a niche to be occupied by mycorrhizal fungi. It is not as generally appreciated, however, that roots also support the growth and functions of a complex of microorganisms that can have a profound effect on the growth and survival of plants. These microbes interact in the soil around the root that is influenced by root exudates; this zone is called the rhizosphere. The effects of rhizosphere microorganisms on plant growth or health often are ignored unless the microbes weaken or kill the plant through disease. There are, however, beneficial interactions that can occur and be enhanced if actively managed by the grower. Establishment and maintenance of rhizosphere microbial populations is a dynamic process affected by age and kind of plant, nature and treatment of soil, environmental factors, foliar applications of chemicals, and interactions of microorganisms. It is important to remember that selective or preferential development of rhizosphere populations occurs in the early stages of plant growth and root development when root exudation is maximal but, thereafter, is altered and maintained at some microbial equilibrium by both microbial and plant activity, with both being influenced by edaphic and other environmental factors.

3.1 Soil Inhabiting Fungi There’s a wild and wonderful world that remains hidden for most of us, at least most of the time. It’s an amazing ecosystem filled with fascinating creatures interacting with one another to create an intricate, dynamic web of life. It’s right under our feet: the soil ecosystem! Many organisms make up this ecosystem, and some of the most important ones are the fungi. Healthy soil is alive and teeming with an array of fungus species, each playing a vital role in its environment. Healthy soils are alive with an especially diverse array of fungal species, and many are essential to the health of the plants growing in that soil. In fact, a soil devoid of fungi is a dead soil, and is unlikely to support thriving plants. Some soil fungi are single-celled and visible only through a microscope. Others form large, underground networks of filaments that can cover an area the size of a football field or larger. Most of us are familiar with the above-ground parts of some soil fungi, the mushroom. Fungi are one of the key microbial groups in terrestrial ecosystems that enabled colonization of land by plants and facilitated development of soil that supports most of the biota on Earth. The kingdom Fungi is one of the most diverse groups of life with an estimated 1.5–6 million species that represent heterotrophic mutualists, pathogens, and saprotrophs (Tedersoo et al., 2017). The 70,000–100,000 currently recognized species are distributed among 156 orders, 46 classes, and 12 phyla. Fungi have traditionally been identified and classified based on morphological characters of fruiting bodies and living cultures. Similar to bacteria and archaea, merely <1% of fungal species have been cultivated with established protocols, which renders large taxonomic groups undescribed and virtually unknown to science. Roughly 80% of all soil-inhabiting fungal taxa cannot be identified at the species level, and 20% cannot be reliably assigned to known orders.

Biological control is defined broadly as the “use of natural or modified organisms, genes, or gene products” to reduce the effects of pests and diseases. The many approaches to biological control can be categorized conventionally into regulation of the pest population (the classical approach); exclusionary systems of protection (a living barrier of microorganisms on the plant or animal that deters infection or pest attack) and systems of self-defense (resistance and immunization) (Anchal Sharma et al., 2013). The agents of biological control include the pest- or disease-agent itself (sterile males or a virulent strain of pathogens), antagonists or natural enemies, or the plant or animal managed or manipulated (immunized) to defend itself. Principles of plant health care are offered, know the production limits of the agro-ecosystem, rotate the crops, maintain soil organic master, use clean planting material, plant well-adapted, pest resistant cultivars, minimize environmental and nutritional stresses, maximize the effects of beneficial organisms and protect with pesticides as necessary. Biological control is the control of one organism by another. This control may be expressed as either a longer population of the pest or as a restriction or prevention of the severity or incidence of pest damage without regard to the pest population (Cook and Baker, 1983). Biological control depends on knowledge of biological interactions at the ecosystem, organism, cellular, and molecular levers and often is more complicated to manage compared with physical and chemical methods. Biological control is also likely to be less spectacular than most physical or chemical controls but is usually also more stable and longer lasting. In spite of biological controls having been used in agriculture for centuries, as an industry biological control is still in its infancy. Biological control is now being considered for an increasing number of crops and managed ecosystems as the primary method of pest control. One reason for its growing popularity is its record of safety during the past 100 years considered as the era of modem biological control (Anchal Sharma et al., 2013). No microorganism or beneficial insect deliberately introduced or manipulated for biological control purposes has, itself, become a pest so far as can be determined, and there is no evidence so far of measurable or even negligible negative effects of biocontrol agents on the environment. The new tools of recombinant DNA technology, mathematical modeling, and computer technology combined with a continuation of the more classical approaches such as importation and release of natural enemies and improved germplasm, breeding, and field testing should quickly move bio-control research and technology into a new era.

5.1 Important Terms i. Inoculum: The infective pathogen propagules coming in contact with the host constitutes the inoculum. The inoculum in the liquid state is a significant factor in anaerobic digestion in solid phase because it is very useful to convey the micro organisms, the water of recirculation. Depending on the particular inoculant formulation, the inocula can be used for seed coating, for dipping seedlings, direct application to the furrow, or as foliar application. An appropriate inoculum consists of copious amounts of active microbes having the potential to convert the organic matter to biogas and the S/I ratio is essential for the rate of degradation of a recalcitrant substrate since it governs not only the biogas composition, digestion period and reactor. Spread of inoculum may occur by wind, water, infected plant material, insects, animals, birds, and people. Plant diseases may have more than one source of inoculum. The infection cycle of some economically important soil-borne plant pathogens involves a combination of primary infection from particulate inoculum residing in the soil and secondary infection as disease is spread from infected to susceptible hosts. The infectivity of inoculum can be quantified by the pathozone profile which measures changes in the probability of infection when inoculum occurs at different distances from the host. For instance, the germinability of inoculum, the growth of the mycelial colony and the infectivity of mycelium at the surface of the host combine to dictate the shape of the pathozone profile for Rhizoctonia solani on radish. The ultimate shape of the pathozone depended on inoculum type and was particularly sensitive to changes in the density and distribution of the mycelium in the fungal colony. Mycelium from an infected radish plant grew much further and at a higher density than that from particulate inoculum (mycelial discs). This results in pathozone profiles that differed in shape. For particulate inoculum the profile rise and fall with distance whilst for an infected plant the decay is sigmoidal.

6.1 Competition Soil microorganisms in particular are perceived as active competitors that have evolved diverse strategies to facilitate resource acquisition. Activities including motility, coordinated behavior, and antibiotic production can tip the competitive balance, resulting in numerical dominance or local extinction of competing populations. Functional consequences of microbial social behaviors have been studied extensively and demonstrate the importance of factors such as relatedness, kin discrimination, enforcement of cooperation, and competition among relatives to microbial survival and f itness in the environment (Adil Essarioui et al., 2017). However, most investigations have focused on interactions among closely related taxa, often among species or genera. Yet, in natural environments, coexistence of individuals from diverse microbial phyla and kingdoms is common. In particular, bacteria and fungi commonly coexist in soil, but there has been relatively little study of the role of species interactions in mediating the dynamics of specific bacterial and fungal populations in soil. Moreover, we have little understanding of the extent to which the ecological context, and specifically plant host or plant community diversity, may influence bacterialfungal interactions in soil. Such studies are needed to begin to establish a more complete understanding of the ecological and coevolutionary dynamics of soil microbial populations. Microbes have coevolved to engage in complex networks of interactions with coexisting kin and non-kin individuals in the soil and in association with plants. Microbial antagonism and resource competition within complex communities have been the focus of many studies that have explored the effects of diverse factors on the ecological outcomes of these interactions. However, the concurrent impacts on these processes of plant species richness and plant host, which jointly structure the environment within which rhizosphere microbes interact, have received far less attention. This is despite the fact that both plant community richness and plant species have been shown to impact microbial community composition and function in rhizosphere soil, with potential implications for microbial interactions and pathogen suppression in natural and agricultural habitats.

7.1 Conducive Soils Without a conducive environment, plant disease does not occur. Soils that favour the disease/ and the pathogen are generally referred to as conducive soils. Inoculum of powdery scab is the either the spore ball (an aggregation of thick walled, tough, resting spores, comprising an average of 700 spores) or the swimming spore (zoospore) that each spore of the spore ball releases. Spore balls of powdery scab have been shown to persist for long periods (Brierley et al., 2008). Exactly, how long they can survive has not been fully determined, but experimentally and anecdotally, it is much longer than a typical rotation. Inoculum may be seed-borne or soil-borne and whatever the source spore balls are able to cause infection and disease. Seed tubers may carry spore balls within powdery scab lesions or may carry spore balls symptomlessly where they have been picked up from adjacent infected tubers or contaminated machinery (e.g. on grading lines). In addition, recently, it has been found that tubers may be latently infected. That is where tubers are infected but symptoms are not expressed. Latent infections may be associated with conditions that are unfavourable for disease development, such as sub-optimal temperatures. i. Soil Moisture: A high soil moisture content both encourages swimming spore release and facilitates movement to the host. It is generally accepted that swimming spores are unable to move through the soil without free water within the soil matrix (i.e. soil saturation). Many studies confirm the importance of free water in soil as a requirement for infection. The occurrence of wet soil conditions prior to tuber initiation, enabling inoculum multiplication, has been cited as important for high levels of powdery scab. Alternatively, prolonged wet soil conditions after tuber initiation can lead to high levels. In experiments investigating soil moisture, there is disagreement over the importance of constant versus fluctuating soil moisture conditions in powdery scab development. However, it is difficult to make clear comparisons because soil moisture levels have not always been measured and factors that may affect disease development (e.g. soil type, initial inoculum level) will have differed from experiment to experiment. Some studies have shown that constant high soil moisture has suppressed powdery scab development possibly due to low oxygen/high carbon dioxide levels rendering swimming spores inactive.