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Biological Control of Plant-parasitic Nematodes: Soil Ecosystem Management in Sustainable Agriculture. 2nd Edition

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Plant-parasitic nematodes are one of multiple causes of soil-related sub-optimal crop performance. This book integrates soil health and sustainable agriculture with nematode ecology and suppressive services provided by the soil food web to provide holistic solutions. Biological control is an important component of all nematode management programmes, and with a particular focus on integrated soil biology management, this book describes tools available to farmers to enhance the activity of natural enemies, and utilize soil biological processes to reduce losses from nematodes.

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SECTION I: SETTING THE SCENE

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1

Ecosystem Services and the Concept of

‘Integrated Soil Biology Management’

Plant-parasitic nematodes are important pests of most of the world’s crops. Estimates of crop loss usually range from 5% to 15%, but higher losses sometimes occur, and there are situations where nematodes are a major

­factor limiting the production of a particular crop. Although numerous methods are available to either reduce populations of plant-feeding nematodes or enable crops to better tolerate the damage they cause,

­biological control is generally perceived as playing little or no role in current nematode management programmes. This book will demonstrate that the regulatory mechan­ isms forming the basis of biological control are actually operating in many agricultural systems (although usually at sub-optimal

­ levels), and that their effectiveness can be enhanced through appropriate management.

It argues that modern agriculture must not only be highly productive, but also provide a full range of ecosystem services, including pest and disease suppression; and that this is achievable by incorporating practices into the farming system that increase and sequester carbon, enhance soil biological activity, minimize soil disturbance, improve soil health and ensure long-term sustainability.

 

SECTION II: THE SOIL ENVIRONMENT, SOIL ECOLOGY, SOIL HEALTH AND SUSTAINABLE AGRICULTURE

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2

The Soil Environment and the Soil–Root Interface

Any discussion of biological control of plantparasitic nematodes would be incomplete without some consideration of the soil envir­ onment. Plant-parasitic nematodes spend most of their lives in soil, but since they also develop an intimate relationship with roots during the feeding process, it is perhaps more correct to consider them as occupying the soil–root interface rather than the bulk soil mass. Eggs of some plant-parasitic nematodes hatch in response to substances that diffuse from roots, and feeding takes place in non-suberized areas of the root system near root tips and root hairs, and in regions where lateral roots emerge. The bodies of ectoparasitic nematodes remain in the thin layer of soil no more than 2 mm thick surrounding the root; adult females of some sedentary endoparasites protrude into this zone; and the eggs of many species are aggregated on the root surface. Such distinctions about the habitat of plant-parasitic nematodes may seem trivial but they are vitally important when population regulation and biological control are considered. Essentially, biological control is concerned with interactions between pests and antagonists and it is important to define the battlefield where this ‘biological warfare’ occurs. During fallow periods between host crops, this battlefield is localized microsites within the bulk soil mass. However, once a crop is planted, plant-parasitic nematodes

 

SECTION III: NATURAL ENEMIES OF NEMATODES

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Nematophagous Fungi and Oomycetes

Of all the natural enemies of nematodes, the nematophagous fungi are the most diverse. They are found in many different taxonomic groups within the fungal kingdom, and use a variety of mechanisms to capture and kill nematodes. Hundreds of species have been described, and this chapter makes no attempt to cover them all. Instead, it focuses on the most widely studied groups, discusses how they parasitize or prey on nematodes, and reflects on the ecological characteristics most likely to affect their capacity to suppress nematode populations. Further general information on this group of fungi can be found in Barron (1977), Gray (1987, 1988),

Morgan-Jones and Rodriguez-Kabana (1988),

Jansson and Nordbring-Hertz (1988),

Siddiqui and Mahmood (1996), Chen and

Dickson (2004a) and Nordbring-Hertz et al.

(2006).

One issue that impacts on any discussion of the nematophagous fungi is the many taxonomic changes that have occurred in the last

15 years. DNA analysis is now used routinely in fungal systematics, and the results of such analyses have often challenged historical groupings of species at the genus level, which were previously based on morphology. A list of fungal names, together with commonly

 

SECTION IV: PLANT–MICROBIAL SYMBIONT–NEMATODE INTERACTIONS

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Arbuscular Mycorrhizal Fungi, Endophytic

Fungi, Bacterial Endophytes and Plant

Growth-promoting Rhizobacteria

Some plant-parasitic nematodes are aboveground parasites, but most obtain sustenance from roots and spend their entire lives below ground. Their root-feeding habit brings them into contact with a unique microbial environment that is quite different from the bulk soil mass. Between 25% and 40% of the energy captured by plants during photosynthesis is liberated as root exudates, and since the carbon compounds they contain are a rich source of nutrients for soil microorganisms, the root– soil interface (generally known as the rhizosphere) is by far the most microbially active zone in soil (see Chapter 2). It may seem surprising that plants would release a large proportion of their photosynthates into soil, but we now know that one of the reasons is to prime a defence system that provides protection from root-feeding herbivores. Large numbers of beneficial microorganisms utilize root exudates as a food source, and one of their functions is to interfere in some way with pests and pathogens of roots. Plants also have other ways of using soil microorganisms for their benefit. Some rhizosphere-inhabiting bacteria have plant growth-­promoting properties; certain bacteria and fungi live endophytically in root cells and play a role in plant defence mechanisms; and symbiotic relationships are established with bacteria and mycorrhizal fungi to provide plants with nitrogen and to aid nutrient uptake. Thus,

 

SECTION V: NATURAL SUPPRESSION AND INUNDATIVE BIOLOGICAL CONTROL

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Suppression of Nematodes and

Other Soilborne Pathogens with

Organic Amendments

Soilborne pathogens are an insidious problem in all agricultural crops. Numerous fungi, bacteria, oomycetes and nematodes debilitate root systems; cause wilt, root rot and dampingoff diseases; and are associated with poor growth, yield decline and replant problems.

In many crop production systems, losses from soilborne diseases are the norm, and preferred crops can only be grown successfully if specific management practices (e.g. crop rotations, disease-resistant varieties, soil fumigation) are implemented to limit damage.

However, there are also situations where disease severity is not as high as expected, given the prevailing environment and the level of disease in surrounding areas. Sometimes this is due to subtle variations in soil physical or chemical factors (e.g. sand or clay content, pH or nutrient status), but in other cases it is a biological phenomenon. In these situations, naturally occurring soil organisms interact in some way with the plant or pathogen and either protect the plant from disease, or minimize disease severity.

 

SECTION VI: SUMMARY, CONCLUSIONS, PRACTICAL GUIDELINES AND FUTURE RESEARCH

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Biological Control as a Component of Integrated Nematode Management:

The Way Forward

Plant-parasitic nematodes are important pests of agricultural crops and are generally managed by crop rotation, resistant cultivars or chemical nematicides, although a range of other tactics may be employed in some situations (see Table 11.2). Biological control is generally perceived as playing little or no role in current nematode-management programmes, largely because natural enemies are seen as constituents of products that are deployed in much the same way as chemical nematicides. This narrow view of biological control ignores the fact that: (i) the soils used for agriculture contain a vast array of natural enemies of nematodes; (ii) predacious or parasitic activity will be found in agricultural soils if attempts are made to detect it; (iii) nematode populations are always affected to some extent by the soil microfauna and fauna, as evidenced by the way nematode populations respond when their natural enemies are eliminated; and

 

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