Plant Pathology and Plant Diseases

Anne Marte Tronsmo, David B Collinge, Annika Djurle, Lisa Munk, Jonathan Yuen, Arne Tronsmo

• Publisher: CAB International
• Year: 2020
• ISBN:
  9781789243208
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

This textbook provides a comprehensive introduction to all aspects of plant diseases, including pathogens, plant-pathogen interactions, their management, and future perspectives. Plant diseases limit potential crop production and are responsible for considerable losses in agriculture, horticulture and forestry. Our global food production systems are under increasing pressure from global trade, climate change and urbanization. If we could alleviate the losses due to plant diseases, we would be able to produce roughly 20% more food - enough to feed the predicted world population in 2050. Co-authored by a group of international teachers of plant pathology who have collaborated for many years, this book gives expert and seamless coverage. Plant Pathology and Plant Diseases: Addresses major advances in plant-pathogen interactions, classification of plant pathogens, and the methods of managing or controlling disease Is relevant for a global audience; it covers many examples of diseases with an impact worldwide but with an emphasis on disease of particular importance in a temperate context Features over 400 striking figures and colour photographs It is suitable for graduate students and advanced undergraduates studying plant pathology, biology, agriculture and horticulture.

Part I General Plant Pathology

The first part of the book introduces the concept of plant disease and how it is studied. The reader is introduced to the terminology of plant pathology, which is important in all kinds of communication that involves plant diseases. This is the first step in developing the skills to talk and think as a plant pathologist.

Chapter 1 describes the causes of disease and introduces the concepts of symptoms and signs. The disease triangle is introduced to explain the conditions needed for disease to develop.

Although plant diseases have existed for thousands of years, plant pathology is a young science compared to many others. Chapter 2 describes how our understanding of plant disease has evolved, leading to the concept of pathogens as causal agents of disease. There have been additional paradigm shifts from the early discoveries of plant pathogens and their roles to the recent advances in biological science and technology. This has led to an increasingly improved knowledge of the biology of interactions between plants and pathogens and how we now approach and study plant diseases.

Chapter 3 starts with a presentation of the different lifestyle strategies exhibited by interactions between pathogens and microorganisms in their interaction with plants, ranging from the beneficial to the harmful. The next section describes a generalized disease cycle and the chain of events that lead to development of a disease. Knowing the different processes in the disease cycle and the cycle as a whole is, in essence, the prerequisite for understanding the development of diseases in time and space, as well as their management. The variation in the biological processes used by different pathogens employing different lifestyle strategies in the disease cycle illustrates the breadth and complexity in the science of plant pathology.

1 What is a Plant Disease?

Anne Marte Tronsmo, Lisa Munk, Annika Djurle, Jonathan Yuen and David B. Collinge

Plant diseases limit potential crop production and are responsible for considerable losses in agriculture, horticulture and forestry. Plant diseases are caused by infectious agents: plant pathogens. What we observe as a disease is the outcome of the interaction between the host plant, the pathogen and the environment. The concept is described as the disease triangle. This chapter gives an introduction to these factors and their interactions that may or may not result in a plant disease. The host plants, the pathogens and the environment (including all non-pathogenic organisms) in a distinct geographical unit constitute a phytobiome.

Introduction

Deviations from normal growth of a plant may be caused by an infectious agent, such as a microorganism, virus, viroid or a nematode. The deviation may also be a disorder caused by abiotic stress factors (non-infectious factors). A short definition of a plant disease is ‘abnormal functioning of a plant due to a pathogen’, and plant pathology (or phytopathology) is the study of disease in plants. Etymologically, pathology originates from the Greek words ‘pathos’, meaning suffering, and ‘logia’, meaning study. The importance of plant disease cannot be overestimated since diseases of plants limit potential crop production and are responsible for considerable losses in agriculture, horticulture and forestry. Diseases can affect yield quantity as well as quality of the product. The focus of this book is plant diseases caused by fungi and fungal-like organisms, bacteria, viruses and nematodes.

Box 1.1. The difference between a pathogen and a disease

A pathogen is the causal agent of a disease and is usually a microorganism, virus or nematode. The pathogen has a scientific binomial (essentially Latin) name that is written in italics, or if it is a virus, an abbreviation. The pathogen’s name should not be used as a synonym for the disease’s name. For example:

• The fungus Blumeria graminis causes the disease powdery mildew in cereals.

• The bacterium Erwinia amylovora causes fireblight in fruit trees.

• The barley yellow dwarf virus (Barley yellow dwarf virus, BYDV) causes the disease barley yellow dwarf.

A disease is caused by a pathogen. The disease generally has a trivial name. Sometimes it is part of the scientific name, but not written in italics or with capital letters (unless it contains the name of a person or a place). Examples include:

• Powdery mildew in cereals is caused by the fungus Blumeria graminis.

• Fireblight in fruit trees is caused by Erwinia amylovora.

• Barley yellow dwarf is a viral disease caused by the BYDV.

The Host Plant

Plants are advanced organisms with the essentially unique ability to convert CO₂ to sugars and carbohydrate polymers, by utilizing solar energy in the process of photosynthesis. This means that they are autotrophic organisms, i.e. organisms that are able to synthesize all components that are essential for growth. Plants are the primary source of all food, feed and energy we depend upon. In the ideal healthy plant, all physiological processes, such as photosynthesis, respiration, translocation of water and minerals, phloem transport of photosynthetic products and reproduction, function optimally. If basal functions of a plant are disturbed by plant pathogens, the plant may become diseased. The physiological processes are impaired compared to the full genetic potential of the plant. The normal function of cells and tissues of the plant are compromised by pathogens and may be reduced or arrested completely. One or several of the essential processes in the plant may be affected, resulting in specific symptoms on the plant. As to which physiological functions are affected depends on which tissue or part of the plant that is affected initially. A longer and more detailed definition of a plant disease is therefore ‘any disturbance of the normal physiological and biochemical development of a plant’ caused by a plant pathogen.

A successful infection will lead to the development of visible symptoms of disease or signs of the pathogen itself (Fig. 1.1). Eventually, growth and development of the plant will be inhibited, and the final outcome may be that the plant dies. Symptoms and signs may be found on all parts of a plant (i.e. roots, stems, leaves or flowers) and on the seeds and fruits that the plant produces.

Fig. 1.1. Examples of symptoms of diseases and signs of plant pathogens. (A) Various leaf spots on cereals. (B) Fusarium foot rot in wheat (Triticum aestivum). (C) Sclerotia of ergot in rye (Secale cereale). (D) Grey “fur” of grey mould conidia in strawberry (Fragaria × ananassa). (E) Necrotic tissue and bacterial slime due to ring rot in potato tuber (Solanum tuberosum). (Drawings: H. Karlsen, courtesy of NIBIO.)

Signs mean that structures of the pathogen itself are visible. This could be in the form of fungal spores or mycelia, different resting structures of the pathogen (such as sclerotia), nematode cysts or galls and bacterial slime. Pathogen structures are described further in Chapter 9 (see Fig. 9.2).

Symptoms are visible or otherwise detectable abnormalities arising from a disease or a disorder. Symptoms of plant diseases can be described further as necrosis, wilt, rot, discolouration or malformation (see Chapter 9, Fig. 9.1 for examples). Symptoms, being the reaction of the host plant, reflect the physiological or biochemical disturbances in the plant, as exemplified in Table 1.1.

Table 1.1. Symptoms and underlying causes of pathogen damage on a host plant.

Symptom, observed effects	Underlying cause (affected function in plant)	Examples
Necrosis: black or brown spots, sometimes whitish, on leaves or other plant parts, or as dry rots	The plant tissue is dead	Black spot on roses, leaf blotch diseases
Discolouration and yellowing (chlorosis): plant cells are not dead. Chlorosis can develop into necrosis at later stages	Plant cell function is disturbed Photosynthesis is affected: • Reduction of CO2 uptake in plant cells • Damage to chloroplasts	Several viral diseases, including barley yellow dwarf virus and tobacco mosaic virus Bacterial wilt of forage grasses
‘Water-soaked’ tissue (hydrosis)	Plant cells leak electrolytes due to impaired cell membranes, caused by pathogen enzymes, toxins or other pathogenicity factors	Bacterial pathogens, as Erwinia, Pseudomonas and Xanthomonas species
Wilting: the plant has lost turgor	The water transport and balance is disturbed due to clogging of vascular tissues, rots, necrosis or malformation of the plant roots	Pathogens in the vascular system, take-all, clubroot in crucifers
Rots (soft rots)	A result of enzymatic disintegration of cell walls (middle lamella) by the pathogen	Soft rots of potatoes caused by Pectobacterium spp. or Dickeya spp.
Dry rot	When the cells in rotting tissue die, the rot can dry out and the tissue becomes necrotic	Potato tubers infected with the late blight pathogen
Malformations: misshape of plant or plant organs; cankers, tumours, galls, etiolation, dwarfism, and gigantism	Hormonal disturbances	Clubroot in crucifers, witches broom
Accelerated senescence: early maturation, leaf abscission, early ripening of fruits	Senescence induced by the pathogen. Usually a hormone induced effect (ethylene, ABA)	Sclerotinia stem rot in oilseed rape, dry eye rot in apples

The symptoms of the disease may be local or the result of systemic effects. We therefore distinguish between primary symptoms that are at, or close to, the infection site, for example rotten tissue; and secondary symptoms that occur somewhere else on the plant than the actual infection site and are caused by a systemic stress. An example of that is wilting or white heads due to infection in the roots, as in the disease take-all of cereals caused by Gaeumannomyces tritici. Different symptoms can be expressed as the disease develops in the plant and more than one symptom can be expressed simultaneously. One example of how symptoms can change over time is potato late blight caused by Phytophthora infestans. The primary symptoms are seen on the leaves and the secondary symptoms appear on the potato tubers in the soil. All characteristic symptoms of a disease over time are termed a disease syndrome (i.e. the whole disease picture).

Signs and symptoms are important characteristics in diagnosing a disease. See Chapter 9 for further information.

The Pathogen

Most plant pathogens are microorganisms and belong to the same phyla as those that cause disease in humans and animals, but are very rarely the same species. In contrast to plants that are autotrophic organisms, most microorganisms, except for a few bacteria, depend on organic nutrients that other organisms have produced; they are heterotrophic, like animals and humans. Almost all live as saprophytes, which means that they feed on dead organic material, as parasites, which feed on other living organisms, or endophytes, which live inside plants without causing visible damage. These microorganisms are bacteria and phytoplasma, fungi and fungal-like organisms.

Other groups of plant pathogens are viruses, viroids and nematodes. Since nematodes are not microorganisms but belong to the animal kingdom, in many countries they are considered as pests on plants rather than as pl...