Chapter 14
Fungi as plant pathogens
This chapter is divided into the following major sections:- the major types of plant-pathogenic fungi
- necrotrophic pathogens of immature or compromised
hosts - pathogens of fruits: the roles of pectic enzymes
- host-specialized necrotrophic pathogens
- vascular wilt diseases
- the smut fungi
- fungal endophytes and their toxins
- Phytophthoradiseases
- biotrophic pathogens
Fungi are pre-eminent as plant pathogens. Roughly
70% of all the major crop diseases are caused by
fungi, or the fungus-like Oomycota. One of the most
notorious examples is potato late blightcaused by
Phytophthora infestans(Oomycota), which devastated
potato crops in Ireland in the 1840s leading to
widespread famine. It is estimated that more than half
a million people died of starvation in that period, and
a similar number emigrated to the rest of Europe and
North America. Potato blight is still a potentially seri-
ous disease wherever potatoes are grown, despite
many attempts to control it by plant breeding and
fungicides (Chapter 17). A more recent example is the
Great Bengal Famineof 1943, which is estimated
to have cost the lives of some 2 million people due
to failure of the rice crops caused by a leaf-spot
pathogen, Helminthosporium oryzae(now variously
known as Bipolaris oryzaeor Cochliobolus miyabeanus).Fungi also cause many serious diseases of landscape
and amenity trees. Dutch elm disease, caused by
Ophiostoma ulmiand the related species O. novo-ulmi,
swept repeatedly across North America, Britain, and
continental Europe over the last century, decimating
the elm populations. Similarly, chestnut blight,
caused by Cryphonectria parasitica, has devastated
the native American chestnut tree, Castanea dentata.
This disease swept across much of the eastern part of
the USA, after it was first recorded in the New York
Zoological Garden in 1904. The magnificent American
chestnut forests have now virtually disappeared and
are represented only by a shrub-like understorey
layer. Cinnamomi root rot, caused by Phytophthora
cinnamomi, is another example of an introduced
pathogen, currently threatening large tracts of natural
eucalypt forest in Australia. And, at the time of writing,
sudden oak deathcaused by Phytophthora ramorum
is ravaging the natural oak woodlands in the coastal
fog belt of northern California and southern Oregon.
This “new disease,” first recorded in 1996, has now
spread to many parts of Europe and is causing serious
alarm. It has the potential to destroy many of Britain’s
ancient woodlands. These are just a few examples
of many that could be cited. Taken together, the
epidemics in crop plants, plantations, and natural
woodlands have caused inestimable damage. In this
chapter we focus on the activities of plant-pathogenic
fungi and their several adaptations for causing disease.
In Chapter 17 we will consider how these diseases can
be controlled.