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1984 | Buch

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Plant Pathogens and their Control in Horticulture

verfasst von: G. R. Dixon, B.Sc. (Horticulture), Ph.D.

Verlag: Macmillan Education UK

Buchreihe : Science in Horticulture Series

Enthalten in: Professional Book Archive

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Inhaltsverzeichnis

Frontmatter

1. Pathogens and Diseases

Abstract

The term pathogen indicates an ability on the part of a micro-organism to cause disease which results from an intimate relationship with the host plant. In this process the pathogen derives energy for growth and reproduction without any benefit to the host. Disease is, in consequence, a change in the normal functioning of plants and results in poorer growth and lower yields. It is of primary consideration for those concerned with the control of plant pathogens to establish the amount and timing of crop loss associated with particular levels of pathogen infection. This permits an evaluation of the methods for pathogen control discussed in chapters 5–10 from which a balanced judgement can be achieved of their costs and benefits. This type of analysis has been done with considerable precision for some temperate cereal crops. For instance, Large and Doling (1962) established that the square root of the percentage leaf area infected with Erysiphe graminis (powdery mildew) is related to grain yield lost in tonnes per hectare. Horticultural crops, however, have a combination of yield characters which often makes such direct estimations difficult. The value of these crops frequently rests on quality characters such as visual appeal as with flowers, fruit, pot plants, trees, shrubs and vegetables. Alternatively, value may lie in an estimation of future productivity as with transplanting material sold for growing-on or forcing.

G. R. Dixon

2. Fungi

Abstract

Fungi are living organisms that are devoid of chlorophyll, obtaining energy either as parasites or saprophytes. Simple organic compounds such as amino acids and monosaccharides are absorbed directly. More complex materials are degraded first by fungal-produced extracellular enzymes and then absorbed. Parasitic fungi must be capable of overcoming host resistance mechanisms in order to obtain supplies of energy. Many fungi are filamentous and multicellular although some are single cellular. Fungal nuclei are small rounded bodies, usually 2–3 µm in diameter (with some species they are up to 30 µm in diameter) and bounded by a double layer of membranes with conspicuous pores. Characteristically, the nuclear membranes persist throughout nuclear division and do not disappear at prophase as in other organisms. The nucleolus is also retained during division in some fungi. Typically, a vegetative fungal thallus (a tissue of cells that has little or no differentiation) is formed of filaments which branch in all directions. Each filament is known as a hypha and an aggregation of hyphae is known as a mycelium. Hyphae are thin transparent tubes containing protoplasm and bounded by a rigid cell wall. The walls of fungal hyphae are composed of many layers (multilaminate) each consisting of variously orientated fibrils. Characteristically, fungal cell walls contain chitin.

G. R. Dixon

3. Viruses

Abstract

Man has for centuries recognised the symptoms caused by viruses without realising their cause. The garish colour breaks in tulip flowers so highly prized in seventeenth-century Europe resulted from virus infection spread during vegetative propagation. It was not until the late nineteenth century, however, that evidence was produced that such symptoms were caused by pathogens. It was then shown that mosaic disease to tobacco could be transmitted to healthy plants by inoculation with sap from an infected plant. Proof that such symptoms were the result of particles smaller than those of bacteria was obtained when sap from mosaic-infected plants was passed through a bacteria-proof filter and it was shown that the resultant eluant was still infective. From this Beijerinck proposed the theory of contagium vivum fluidum, from which the present term virus is derived. The requirement for vectors to transmit viruses began to be realised in this period. Workers in Japan showed that dwarf disease of rice was spread by a leaf hopper (Nephotettix nigropictus), while in the USA it was demonstrated that a single leaf hopper (Circulifer tenellus) taken from a sugar beet plant infected with curly top and placed for five minutes on a healthy beet plant could transmit the organism causing disease. Following this there has been intensive work to isolate and identify viruses.

G. R. Dixon

4. Bacteria

Abstract

Bacteria are single-cellular organisms that are very variable in size and shape; often a particular species possesses more than one shape (pleomorphic). Together with the blue-green algae they form a group known as prokaryotes whereas the fungi and other plants are known as eukaryotes. There are three general morphological forms of prokaryote cell: cocci, which are ellipsoidal or spherical, bacilli, which are cylindrical or rod-like and spirilla, which are spiral or helical. Included within spirilla are the vibrio shape, consisting of a rudimentary helix. Some other shapes are found; of particular interest to pathologists are the Actinomycetes which have a filamentous structure analogous to that of some fungi (section 4.4). Typical features of eukaryotic cells such as cytoplasmic streaming, mitochondria, chloroplasts, Golgi apparatus, endoplasmic reticular and membrane-bound vacuoles are absent in bacteria. The cytoplasm is surrounded by a triple-layered cytoplasmic (cell) membrane which consists of two electron-dense layers surrounding an electron-transparent layer. This structure is typical of semi-permeable membranes found in other organisms; it does, however, contain part of the respiratory and, in some cases, photosynthetic apparatus. Noticeably, sterols are absent from this membrane. Mitosis has not been observed within the nuclei of prokaryotes. The nucleus is a diffuse area containing fibrous material but lacking a nuclear membrane.

G. R. Dixon

5. Control: Host Resistance

Abstract

Control of plant pathogens is sought by five main avenues: biological, chemical, cultural (including post-harvest control), host resistance and plant health regulation. Each of these forms of control aims at a different facet of the host-parasite-environment-time interaction. Resistance is directed at the host in an attempt to produce plants that are unaffected by or can withstand the pathogen per se; chemical control aims at the pathogen in an attempt to destroy it before or immediately after invasion and to prevent extensive colonisation and sporulation; cultural control aims at changing the environment that is encountered by the pathogen, either inside or outside the host, thereby providing conditions conducive to host growth but adverse to the pathogen; plant health regulation aims to prevent pathogens becoming endemic and to provide the producer with sources of healthy planting material; biological control aims to use negative interactions between micro-organisms such as competition, antibiosis or antagonism. Crop scientists have been largely preoccupied with the use of resistance, chemicals and plant health regulation. The grower has often used cultural controls with success but without any rationale for their operation. Since cultural controls are a multidimensional approach, the factors involved are less clearly drawn and less well comprehended, thereby lacking the conceptual cleanliness of other forms of crop protection and appealing less to those who seek to interpret disease control by the scientific method.

G. R. Dixon

6. Control: Chemicals

Abstract

There are four main avenues for the chemical control of plant pathogens.

Protection of healthy hosts from infection (prophylaxis).

Cure or therapy of diseased plants (chemotherapy).

Destruction of pathogen propagules and resting stages present on the surfaces of dormant host tissue such as seeds or on the surfaces and in crevices of structures such as glasshouses (disinfestation).

Elimination of pathogens present in growing media such as soils and composts (fumigation or sterilisation).

G. R. Dixon

7. Control: Spray Application

Abstract

Spraying is the predominant method by which chemicals are used to control plant pathogens (section 6.4). This chapter describes, in nonengineering terms, the major components of a sprayer and some factors that affect their effectiveness. Basically a sprayer is a mechanical device whereby the manufacturer’s product is diluted by mixing with water or other fluid in a reservoir vessel and then forced by hydraulic or air pressure through one or more nozzles. These break up the liquid into droplets which are impelled to a target crop. Choice of sprayer depends on the total volume of spray to be applied, the area of crop to be covered, the habit of the crop—some sprayers are suited to low-growing vegetable crops rather than to trees or bushes—and crop location, that is, whether the crop is grown under protection or in the open field.

G. R. Dixon

8. Control: Plant Health Regulation

Abstract

National and international trade in horticultural produce and planting material carries with it the risks of disseminating pathogens. Control of these risks by using plant health regulation is attempted by domestic legislation and international co-operation. The latter is fostered under an International Plant Protection Convention of 1951. Three forms of plant health regulation are described in this chapter, which although they are treated separately, are in practice closely interrelated.

G. R. Dixon

9. Control: Cultural and Biological Methods

Abstract

The basic approach with cultural control is to invoke those aspects of husbandry that promote sound crop growth and inhibit or otherwise obstruct pathogenic growth. This should avoid, delay or lessen the impact of disease. Conversely those practices that operate in an opposite direction should be eliminated. More than any other form of control, cultural methods emphasise that the objective of horticulture is to produce fruitful, high-yielding crops rather than simply to control plant pathogens. These controls act largely in a preventive manner and are applied in advance of invasion. They do not lend themselves to control pathogens that occur in a sporadic manner. It should be firmly borne in mind that advocating adjustments to crop environment is a highly risky business where knowledge of the effects of environmental change on crop economics is imperfectly understood.

G. R. Dixon

10. Control: Post-Harvest Pathogens

Abstract

Storage of sound high-quantity horticultural produce curtails its exposure to air and soil-borne pathogens in the outdoor environment and should therefore reduce losses. Pathogens present as latent infections on produce going into storage and more frequently new infections from air-borne pathogens present within a store will cause disease on crops as they are stored. Disease development is encouraged by biochemical and physiological changes which begin once crops are harvested. A primary aim of storage should be to limit such changes for as long as possible. Clamps and barn types of storage provide cool conditions for preserving commodities such as cabbage, carrots, celery, onions and potatoes. The economic margins to be gained by storage are often such as to permit the use of more sophisticated techniques, rather than these traditional methods, whereby losses due to pathogens may be reduced. Storage may be short-term—a few days—as for soft fruit and loose leafy vegetables in order to remove field heat, thereby delaying the onset of wilting and disease and permitting transit over long distances; this is achieved by vacuum, forced air, hydro or ice-bank cooling. Storage may be long-term—up to several months—by means of refrigeration or controlled atmospheres.

G. R. Dixon

Backmatter

Titel: Plant Pathogens and their Control in Horticulture
verfasst von: G. R. Dixon, B.Sc. (Horticulture), Ph.D.
Verlag: Macmillan Education UK
Electronic ISBN: 978-1-349-06923-1
Print ISBN: 978-0-333-35912-9
DOI: https://doi.org/10.1007/978-1-349-06923-1

Springer Professional

Inhaltsverzeichnis

Frontmatter

1. Pathogens and Diseases

2. Fungi

3. Viruses

4. Bacteria

5. Control: Host Resistance

6. Control: Chemicals

7. Control: Spray Application

8. Control: Plant Health Regulation

9. Control: Cultural and Biological Methods

10. Control: Post-Harvest Pathogens

Backmatter