Chapter 14: Fungi as pathogens of plants

When early humans gave up their nomadic hunter-gatherer existence and turned to agriculture to solve their food problem they would quickly have been challenged by the fungi. Early farmers must have learned very rapidly that crops are very uncertain resources, prone to variations in weather, fire, floods, weeds, insect pests, and those troubles which came to be referred to collectively as ‘blights’ which were due to various sorts of plant disease.

Great plant losses, caused by any of these factors, can be suffered in natural ecosystems but by bringing the crops together into fields in the first place, the early agriculturalist created ideal conditions for the spread of plant disease. And the more selective his farming, the closer his crops came to being true monocultures, the greater the extent of agricultural losses due to any single agency like a particular plant disease, so in this Chapter we look at fungi as pathogens of plants.

Fungi are the main disease organisms of plants, being responsible for major losses of world agricultural production. Because of the number that exist, we can only give a few specific examples, so we limit these to the headline crop diseases: the Rice Blast fungus (Magnaporthe oryzae), the Bootlace or Honey fungus (Armillaria), rusts, mildews and smuts (pathogens that produce haustoria that penetrate the plant cells), Leaf Spot (Cercospora). Dutch elm disease (Ophiostoma), and Black stem rust of wheat (Puccinia graminis).

Following these specific examples we discuss the basics of plant disease; the disease triangle, differences between necrotrophic and biotrophic pathogens of plants, and the effects of pathogens on their hosts. In the final Sections of the chapter we describe how pathogens attack plants, comparing penetration through stomatal openings with direct penetration of the host cell wall, involving both physical and enzymatic penetration of the host. Finally, we discuss the defence mechanisms of plants and the co-evolution of disease systems which match the genetic variation of pathogens to that of their hosts.

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Updated July, 2019