Introduction to Plant Pathology by Richard N. Strange



Bibliographic Information:
Title: Introduction to Plant Pathology
Editor: Richard N. Strange
Edition: 1st
Publisher: Wiley
Length: 497 pages
Size: 7.95 MB
Language: English



Pathogenic agents which infect plants are many, both in kind and number, and
they are in direct competition with us for our food and our cash crops. They are
therefore a cause of malnutrition, starvation and poverty. They also deplete our
food reserves by spoiling stored produce and ruin our environment by destroying
vegetation.
‘Know your enemy’ is a good adage in most walks of life and is particularly
apposite to that of the plant pathologist. The range of organisms which cause
plant disease and the techniques for their identification are therefore introduced
in the first two chapters of this book together with accounts of some of the worst
epidemics they have caused. Those with an historical inclination may know that a
disease of potato was the cause of a great famine in the 1840s in which Ireland
lost over a quarter of its population through starvation and emigration. Similarly,
death by starvation of an estimated two million people living in Bengal
(now the Province of West Bengal in India and Bangladesh) is thought to have
been caused by a disease of rice in the 1940s. As an example of the devastation of
a cash crop, the growing of coffee in Sri Lanka became uneconomic in the last
part of the 19th century owing to coffee rust; in consequence, the farmers turned
to growing tea – and the British, obligingly, to drinking it! With regard to the
environment, parents and grandparents of today’s students may remember that
elm trees were a distinctive and attractive feature of parts of the USA and the
southern part of England but that in the 1970s they rapidly died out owing to
‘Dutch Elm Disease’. Finally, of the many organisms that cause spoilage of
produce in storage, fungi of the Aspergillus flavus group are perhaps the most
notorious; not only are the infected or infested plant products such as maize and
peanuts toxic, they are also highly carcinogenic.
Once the identity of a pathogen is known and there are adequate techniques for
its rapid identification (Chapters 1 and 2), the study of its dissemination and the
losses it causes become practicable (Chapter 3 and 4, respectively). Chapter 5
addresses the control of inoculum of the pathogen since prevention is not only
better than cure but is also, apart from growing resistant plants, virtually the only
feasible option for poorer parts of the world. Richer countries may be able to
afford pesticides but concerns about their adverse effects on human health and
the environment have led to several of them being phased out. Emphasis is
therefore placed on biological control as a potentially less hazardous and more
environmentally friendly way of reducing inoculum.
Plant pathogens have evolved many ingenious ways of locating and effecting
entrance into their hosts, often involving the recognition of host topography or
chemical signals. In some instances, entrance is achieved through natural openings
and wounds, but in others direct penetration of the cell wall is achieved by
the secretion of enzymes or the development of high turgor pressure in special
organs (appressoria). Enzymes are also required by many pathogens for the
colonization of the host and the release of metabolizable products and their
action may give rise to a considerable amount of cell death (necrosis). Location,
penetration and colonization are the topics discussed in Chapter 6.
Some pathogens subvert the metabolism of their hosts by altering hormone
levels, the ultimate level in sophistication in this regard being species of Agrobacterium.
These organisms are genetic engineers since they insert genes into
plants that encode enzymes for the synthesis of two important plant hormones,
as well as other genes that harness the metabolism of infected plants to produce
compounds which can only be used by the pathogen (Chapter 7). Some pathogens
influence the metabolism of their hosts and may kill them by the production
of phytotoxic compounds (Chapter 8).
Faced with this onslaught from pathogens, it is not surprising that plants
have developed mechanisms for fighting back. Some of these are constitutive
(Chapter 9) while others are induced by the challenging organism (Chapter 11).
The induced responses are often triggered by recognition of the invader, the
genetic elements responsible on the part of the plant and the pathogen being
termed resistance and avirulence genes, respectively. This gene-for-gene relationship
is discussed in Chapter 10 together with structures of the genes involved and
the means by which their products may be brought together and interact.




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