Introduction

Root rot and damping-off pathogens often cause severe problems both in extensively grown field crops and in protected crops such as vegetables and ornamental plants. New control measures are being sought both for environmental reasons and since the possibilities for chemical control are limited. Among these, biological control (Sewell, 1965) seems promising: it is generally believed that the use of biological control will be beneficial and that risks and unwanted side effects are limited. In this chapter, the use of fungal antagonists as biocontrol agents (BCAs) will be discussed.

Most fungi used for biological control of damping-off and root rot are Hyphomycetes, and among these the genera Penicillium, Trichoderma and Gliocladium have received most attention (Kommedahl and Windels, 1981). Mycoparasitic Pythium spp. (Paulitz and Baker, 1987; Lewis et al, 1989; Hockenhull et at., 1992) and nonpathogenic Fusarium spp. (Alabouvette, 1990; Komada, 1990) and other species are also potential BCAs. Although the potential for using fungal antagonists for biological control is well documented (e.g. Papavizas, 1985), there are as yet only a few examples of their use on a commercial scale. However, BCAs such as non-pathogenic Fusarium oxysporum (Rajnchapel-Messai, 1990), Pythium oligandrum (Vesely, 1989), Gliocladium virens (Lumsden et al 1991; see Lumsden and Walter, Chapter 25) and Trichoderma harzianum (Harman and Tronsmo, 1992) are examples of agents that are expected to be on the market in the near future.

This review will be focused mainly on the use of Trichoderma spp. and Gliocadium spp. for the control of root rot and damping-off caused by soilborne pathogens. Possibilities for using biological control for seed-borne diseases will also be mentioned briefly.

Introduction

Today, about 1200 viruses are known to infect insects (Martignoni and Iwai, 1986). These do not form a taxonomic unit but belong to a range of different virus families including the Iridoviridae, Parvoviridae, Poxviridae, Reoviridae and Baculoviridae. Most of these families also have representatives that infect not only insects, but also vertebrates and even plants. There is one exception to this rule: viruses that belong to the family Baculoviridae have so far only been isolated from arthropod hosts. More than 60% of all insect viruses known today belong to this family. Some baculoviruses have been found in shrimps and mites, but most of them have been isolated from insects, in particular lepidopterans.

Baculoviruses are characterized by doublestranded circular DNA, which is included in rodshaped nucleocapsids (Federici, 1986). They are formed mainly in the nucleus of the host cells. In common with many insect viruses from other virus families, the virions of most baculoviruses are contained within proteinacious paracrystalline formations, the so called occlusion bodies, which are often polyhedral (hence the name polyhedrosis virus for one group of the baculoviruses). The thick layers of polyhedral protein provide protection against adverse physical and chemical factors within the environment, allow the viruses to survive outside the host cell, and enable them to kill their host rapidly, without jeopardizing their own existence (Jaques, 1977). It is obvious that protection of the virus particles by the occlusion bodies is a great advantage for the use of these viruses as biological insecticides. The natural infection process is by ingestion of food contaminated with virus.

Introduction

Species of the genus Sitona Germar (Col., Curculionidae) have been recorded on all continents and all develop at the expense of a number of Leguminosae. In their larval stages, they attack the root system and may be especially injurious to the nodules (Aeschlimann, 1986), whereas the longlived adults consume the stems and foliage of their host plants. The four palaearctic representatives which comprise the S. humeralis group of species all depend upon Medicago spp. as host plants (Aeschlimann, 1984). One of these, S. discoideus Gyllenhal, was accidentally introduced into the southern hemisphere, where it has become a pest of economic importance to both perennial cultivated lucerne and volunteer annual Medicago species used for pasture throughout southeastern Australia and subsequently in the south island of New Zealand.

As a consequence of the importance of S. discoideus, surveys were carried out from 1973 to 1985 over most of the Mediterranean basin (Aeschlimann, 1980) with the aim of identifying efficient natural enemies of the various stages of S. discoideus that would be suitable for deliberate introduction into Australia. These investigations demonstrated that the parasitoid Microctonus aethiopoides Loan (Hym., Euphorinae) was the most promising biological control agent for use against adult Sitona weevils. Based on these findings, several biotypes of this natural enemy were imported between 1975 and 1979 from various parts of the western Mediterranean region to Australia for mass-rearing in quarantine and subsequent field release against S. discoideus (Aeschlimann, 1983a). Initial establishment followed by spreading was recorded in Australia 2 years after the start of the release programme (Aeschlimann, 1983a).

Under suitable aeration and temperature conditions non-dormant plant seeds need only access to water to be able to germinate, but since orchid seedlings are heterotrophic to varying degrees their seeds often have several requirements from outside besides water. These are undoubtedly met by the endophyte when germination takes place in nature, or indirectly by the substrate that the fungus lives on.

Some other requirements for germination of orchid seeds are associated with dormancy. The currently available evidence suggests that several types of dormancy, both exogenous and endogenous, occur in the seeds of holarctic orchids. In spite of the apparently delicate nature of the translucent testa its impermeability to water represents a form of physical dormancy that can be overcome by prolonged soaking or by scarification. There are different types of physiological dormancy that can be broken by darkness, temperature regime, atmospheric conditions, or certain external chemical signals. Finally, morphological dormancy, expressed as a requirement for the embryos to after-ripen, probably also occurs in orchid seeds.

The treatments and substrate additives that are necessary to obtain germination in vitro can thus be divided into those that are needed for seeds to germinate in symbiotic culture, i.e. such that break dormancy, and the more extensive set of conditions that must be met if the seeds are to germinate asymbiotically. The difference between these sets would seem to directly represent the effects of the fungus, but this conclusion may not be valid, since several external stimuli may elicit the same physiological response, for instance when different metabolites influence different steps in the same group of reactions.

Abstract

Three failure modes of trees are described, selected from a more extensive study, because of their practical importance. These are: failure of hollow trees, axial splitting of hazard beams, and windthrow.

Failure of hollow trees by cross-sectional flattening

The failure mechanism observed in nature in hundreds of hollow trees is shown in Fig. 10.1. Hollow trees will fail when a certain ratio of wall thickness t to stem radius R is reached. The failure mechanism by cross-sectional flattening is due to lateral forces which increase with the curvature of the ‘pipe’ due to bending. At a certain degree of flattening the hoop stresses exceed the value of circumferential strength resulting in axial splitting. When the hollow tree collapses into individual timber boards the stiffness is dramatically reduced and overall breakage will normally happen. It has been shown in a field study (Mattheck et al., 1993) with more than 700 trees that failure can start if 68–70% of the stem radius is hollow or decayed wood (Figs. 10.1, 10.2). This failure has to be expected if the crown of the tree is not reduced and therefore the full canopy (sail) area is under wind loading. If, on the other hand, the crown volume is reduced, it is possible that trees with even much smaller t/R ratios will resist the wind, with a much smaller canopy.

Axial splitting of hazard beams

Lateral forces are also responsible for axial splitting of hazard beams.

Abstract

Two field studies are presented on the in situ dynamic loading of mature coniferous trees planted on wet mineral soils. The studies consisted of examining the behaviour of a complete tree under natural wind loading and a number of trees with truncated stems under forced dynamic loading. The test site has a history of tree instability and the mineral soil under the rootplates consists of a clayey silty sand. The rootplates of the tested trees were shallow and the main roots had an asymmetrical radial distribution about the tree stem centre. The dynamic loading caused an increase in soil pore water pressure, and in the forced loading tests it led to hydraulic fracturing of the rootplate.

Introduction

Windthrow is a major source of economic loss in Irish and United Kingdom forests and crop instability imposes important restrictions on silviculture. This chapter presents details of two field studies (Rodgers et al., 1990; McMenamin, 1992) on the dynamic loading of mature Sitka spruce trees planted on double mouldboard plough ridges. The objectives of the studies were:

1. To develop a mechanical rocking device and a high-speed data logging system which could be used to assess the stability of trees in the field.

2. To study the behaviour of trees and rootplates subjected to forced dynamic loading.

3. To study the behaviour of a tree under natural storm conditions.

4. To identify soil properties which are important for tree stability.

5. To investigate the stiffness values and Young's moduli of the trees.

6. To monitor the changes in damping that occur during forced dynamic loading.

Wind not only causes extensive damage to trees in many parts of the world, it also has more subtle effects on their growth, form and ecology. This, the first symposium volume on the topic, contains a selection from the papers presented at a conference, Wind and Wind-Related Damage to Trees, held at Heriot-Watt University, Edinburgh, in July 1993. The conference, which was initiated by Chris Quine, was held under the auspices of the International Union of Forestry Research Organisations, and brought together about a hundred people from seventeen countries.

Wind damage to trees has historically been the province of the silviculturist, but increasing recognition of the importance and complexity of the subject has more recently involved people from many other discplines, and this has resulted in a greatly increased understanding of the main processes involved. The conference served to bring together the new approaches and methodologies; it enabled discussion between physicists, aerodynamicists, foresters, engineers, physiologists, ecologists, pathologists and modellers, and led to a remarkable cross-fertilisation of ideas between scientists who would not normally meet.

For convenience the papers have been gathered into five parts, each beginning with a review. The parts are: Airflow over topography and in forests; Mechanics of trees under wind loading; Tree physiological responses; Impacts of wind on forests and ecology; Risk assessment and management response. The coverage of different aspects of the subject is decidedly uneven. For example, tree root development and the way in which roots and soil anchor the tree have received less attention than what is going on above ground, and the contents of this book reflect such disparities.

Abstract

The response of four trees to wind loading in a dense spruce plantation has been investigated. Tree movement is well correlated with the passage of coherent gusts over the forest which can be identified by applying the variable-interval time-averaging (VITA) method to the momentum flux signal within the canopy. The gusts are relatively long lived but with a small spatial scale (˜1 tree height). The trees do not resonate with turbulent wind components close to their natural frequency but behave like damped harmonic oscillators responding to the intermittent impulsive loading during gust passage. The presence of trees severely modifies the turbulence spectra within the canopy by short-circuiting the normal energy cascade process and this may be an adaptive strategy developed by trees to efficiently lose energy absorbed from the wind.

Introduction

Strong winds associated with Atlantic depressions in Northern Europe, hurricanes in North America and the Caribbean or cyclones in Japan, New Zealand and the Pacific Islands cause extensive damage to forests world-wide every year (Savill, 1983). Considerable research has been carried out to understand the physical processes involved in order to improve silvicultural practices and to predict better the likelihood of damage.

It was soon realised that trees blow down at windspeeds considerably lower than those predicted from static pulling tests under calm conditions (Fraser & Gardiner, 1967; Oliver & Mayhead, 1974; Blackburn & Petty, 1988). One explanation is that trees are dynamic structures capable of resonating at their natural sway frequency with the turbulent wind field.

The primary concept of this Series of books is to produce volumes covering the integration of plant and microbial biology in modern biotechnological science. Illustrations abound, for example the development of plant molecular biology has been heavily dependent on the use of microbial vectors, and the growth of plant cells in culture has largely drawn on microbial fermentation technology. In both of these cases the understanding of microbial processes is now benefiting from the enormous investments made in plant biotechnology. It is interesting to note that many educational institutions are also beginning to see things in this way and integrating departments previously separated by artificial boundaries.

Having set the scope of the Series, the next objective was to produce books on subjects which had not been covered in the existing literature and, it was hoped, to set some new trends.

Two of the first books of this Series addressed the opportunities of protein engineering (Peter Shewry and Steve Gutteridge) and transformation (Kan Wang, Alfredo Herrera-Estrella and Marc van Montagu), while another concerned the release of genetically engineered and other microorganisms (John Fry and Martin Day). One of the major targets of agricultural technology is the biological control of pests and diseases. Some of the debates on genetic engineering have clouded the issue that biological products could reduce the chemical load on the environment while providing more effective products that are less prone to resistance. On the other hand, introduction of any exotic, whether it is genetically engineered or not, into the environment should only be sanctioned when the risks are fully evaluated.

Introductions of organisms in general

Introductions of organisms into new environments can be intentional, accidental, or migratory and may encompass practically any kind of living material. Intentional introductions fall into two broad categories: crop plants and domestic animals, and agents for the biological control of pests or other beneficial purposes, such as microbes to degrade toxic compounds. Benefits from the introduction of crop plants and domestic animals are obvious, but good economic assessments of these introductions are rare. Rigorous evaluations have been done for some classical biological control programmes (see Cullen and Whitten, Chapter 26; Greathead, Chapter 5; and Pimentel, Chapter 2), and overall, a return to investment of 30:1 has been estimated. An example of outstanding economic success is the biological control of Rhodes grass scale in Texas, which cost only US$ 0.2 million, but from which annual economic benefit has been estimated at about US$ 200 million. Other benefits of introducing biocontrol agents include the avoidance of unwanted side-effects associated with chemical pesticides, important from environmental, occupational health and social points of view.

Introductions of living material are always associated with a risk of unwanted side-effects (see Pimentel, Chapter 2; and van Lenteren, Chapter 3). Thus, of all intentional introductions of species some 25–68% result in permanent establishment, and 0–2% of these organisms become pests. In contrast, only about 5% of unintentional introductions result in species establishment, but 7% of those become pests in their new environment. In total, 128 intentionally introduced crop and ornamental plants have become weeds in the USA (2.2% of all introductions of plants), including Johnson grass, Missisippi chick corn, goatsrue and crotalaria (crop plants); and multiflora rose, water hyacinth and lantana (ornamentals).

Introduction

Biological control of arthropod pests using arthropod natural enemies has been practised for centuries (Flint and van den Bosch, 1981), and although it has been widely appreciated that these natural enemies can have adverse effects on the environment, it has been largely assumed that they are either absolutely small or small relative to the benefits of biological control. For example, Samways (1988) suggested that there are no quantified cases where the introduction of an arthropod agent has been shown to have harmed a specific conservation programme or has been conclusively damaging to native fauna. This suggestion has proved difficult to test under natural conditions. For example, many natural enemies have been released to control forest Lepidoptera pests. Some of these enemies kill non-target, nonpest insects, and it has been suggested that such unintended mortality might destabilize these nonpest populations thus causing them to become sporadic pests. Recently, Pimentel et al. (1984) reviewed the evidence that biological control agents can cause adverse environmental effects. Their review clearly documented that biological control can entail environmental risk and that categorical dismissal of this concern is unwarranted. More recently, Howarth (1991) argued that the introduction of biological control agents into Hawaii and New Zealand is one of the major causes of extinctions of the native, insular, endemic arthropod faunae associated with these two islands. In most of those cases, vertebrate natural enemies were implicated, but Howarth (1991) developed a reasonable argument that arthropod natural enemies could also affect the native faunae.

Abstract

The influence of wind on tree growth and development is interpreted within the context of stress and strain relationships. The primary stress is the force of the wind applied to the tree. The fluttering of leaves and branches, the back and forth swaying motion of the stem, the displacement or wind-induced lean of the stem and failure of the stem or roots, resulting in windthrow, are the viable, mechanical strains manifest by the tree. Secondary stresses include the influence of gravity due to displacement, and changes in the atmospheric conditions around leaves. As the magnitude of the stress (windspeed) increases, so do the resulting strains, resulting in a cascade of physiological strain responses. The physiological responses range from rapid changes in transpiration and photosynthesis at the foliar level, to reduced translocation, callose formation and ethylene production in the phloem and cambial zone. Long-term developmental and structural changes occur in canopy architecture, leaf, stem and root morphology, and modifications of cell structure and biomechanical properties of the xylem. The interaction between acute and chronic wind stress, and dynamic and static loading stresses, with their influence on physiology and development is discussed.

Introduction

Since Metzger (1893) first proposed wind as the most significant or ‘Massgebender’ factor affecting growth of trees, the literature on wind stress has been intertwined within the fields of ecology, physiology and forestry. The ecological literature has focused on canopy and leaf responses, originally interpreting wind-formed trees as a pathological condition of induced nutrient and dehydration stress (assumed as increased transpiration), leading to mechanical damage (Schimper, 1903; Shreve, 1914; Daubenmire, 1959; Odum, 1970; Weaver et al, 1973).

Introduction

Although many mermithid, tylenchid, aphelenchid and rhabditid nematodes are known to be important antagonists of insects, very few nematode species have so far been used for biological control. The mermithid Romanomermis culicivorax was used in field trials to control malariatransmitting mosquitos (Petersen and Cupello, 1981). However, with the discovery of the larvicide Bacillus thuringiensis var. israelensis this nematode lost its significance. In Australia, the neotylenchid parasite Deladenus siricidicola is successfully released to control the introduced European sawfly Sirex noctilio (Bedding, 1984). Other species have never gone beyond the scientific research level, with the exception of rhabditid nematodes of the genus Steinemema and Heterorhabditis and their bacterial symbionts Xenorhabdus spp., which are widely used for biological control of insect larvae in cryptic environments.

Biology

The nematode's life cycle consists of a free-living and a propagative phase. The only stage occurring outside the host insect is the infective third stage dauer juvenile (Fig. 11.1), which is developmentally arrested and morphologically and physiologically adapted for long-term survival under detrimental environmental conditions in the soil. The dauer juveniles (DJs) do not take up nourishment and carry cells of their bacterial symbiont Xenorhabdus sp. in the intestinal lumen (Fig. 11.2) (Bird and Akhurst, 1983; Endo and Nickle, 1991). They actively seek out suitable host insects and penetrate the haemocoel where they find favourable conditions for propagation. The DJs start feeding, release the symbiotic bacteria into the haemolymph and enter into the propagative phase of the life cycle. Provided that the insect's humoral and cellular defence mechanisms do not succeed in the elimination of the nematodebacterium complex, the host will die within 2 days after infection.

Practical cultivation of terrestrial orchids beyond the seedling stage is thoroughly described elsewhere (Fast, 1980; Cribb & Bailes, 1989), but since these plants are not subjected to large-scale production the present culture methods rely largely on a relatively small amount of accumulated experience. With more effective in vitro techniques for seedling production more plants will be available for culture experiments in the future. It would be desirable to have a more objective foundation on which to construct rational production schemes.

In this brief account I will focus only on the establishment in soil of the earliest life stages and on such aspects of cultivation as soil requirements and phenology that have a relation to the mycotrophic lifeform of the plants.

Weaning of seedlings from in vitro culture

Transfer of seedlings to the soil is perhaps the most challenging step in the cultivation of terrestrial orchids (Fast, 1982). Before they are removed from the protected environment of the culture vessel the seedlings should be as large and vigorous as possible, but there is usually an economic incentive to reduce the length of time they are kept in vitro and the number of transfers undertaken.

When seedlings are transferred to a fresh medium in vitro the growth rate always increases, particularly when the seeds have been sown densely (Fast, 1982). Wide spacing allows the seedlings to develop more rapidly (Tsutsui & Tomita, 1989; Rasmussen et al., 1989). This advantage should be balanced against the greater number of culture vessels and amount of substrate necessary.