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The major types of plant-pathogenic fungi:

setting the scene

It is helpful to begin with some definitions:

1 A parasitecan be defined as an organism that gains

all or part of its nutritional requirements from the

living tissues of another organism – the host. In

other words, parasitism describes a nutritional

relationship.

2 A pathogencan be defined as an organism that

causes disease. It is almost always a parasite, but the

distinction between a parasite and a pathogen is

important because parasites do not always cause

obvious or serious disease.

There are many thousands of plant-pathogenic fungi,
so we need to group them in some way that reflects
their fundamental biology. The system we will use is
based on two principal features: (i) the type of nutri-
tional relationship between the pathogen and the
host, and (ii) whether the pathogen has a broad host
range or is host-specialized.
In terms of the nutritional relationship, we dis-
tinguish between necrotrophic pathogens and
biotrophic pathogens. Nectrotrophs kill the host
tissues, usually by directly invading them, or by pro-
ducing toxins or degradative enzymes, and then feed
on the tissues that they kill (Greek: nekros=dead; trophos
=a feeder). By contrast, biotrophic pathogens feed on
living host tissues (Gr. bios=life), often by producing
special nutrient-absorbing structures that tap into
the host’s tissues. A third group – hemibiotrophic
pathogens – initially grow as biotrophs but then
invade and kill the tissues. Some of the Phytophthora
species such as P. infestansbehave in this way.
The secondary distinction is between pathogens of
broad host rangeand those that are host-specialized,
with a narrow host range. Pathogens of broad
host range usually attack immature or senescing
tissues, or plants whose resistance is compromised by
environmental factors. By contrast, host-specialized
pathogens are adapted to overcome the specific
defense mechanisms of plants, such as the fungitoxic
compounds that many plants produce to ward off
infection. The biotrophic pathogens are necessarily
host-specialized.
So, we can categorize almost all pathogens in the
following simple scheme, which will form the basis for
the rest of this chapter. We will discuss representative
examples of the major types of plant disease, to build
up a picture of the diversity of plant-pathogenic fungi
and their activities.

Necrotrophic pathogens of immature or

compromised hosts

A large number of fungi attack the tissues of young

plants, causing seed rots or seedling diseases. Other fungi

typically attack the older, senescing tissues of plants,

and yet others attack plants that are compromised by

drought or other stress factors. Although these fungi

do not have a very sophisticated mode of parasitism,

they can cause serious economic damage. Several ex-

amples are discussed below.

Seed rots and seedling pathogens

A characteristic range of fungi cause seed rots or

seedling diseases – sometimes called damping-off

diseases because they are often associated with wet

soil conditions. For example, many Pythium spp.

(Oomycota) attack the root tips of seedlings, and

strains of Rhizoctonia solanioften attack the young

shoot base, just above or below the soil level. Other

fungi with similar behavior include Fusariumspp.

which cause seed-rots and seedling diseases of cereals.

All these fungi respond rapidly to the presence of seed

or seedling exudates, and invade the young tissues that

have little inherent host resistance. To help counter-

act this problem, seeds are usually coated with broad-

spectrum fungicides such as maneb, thiram, or

dinocap (Chapter 17) which interfere with several

basic metabolic processes of fungi. But there is an

increasing trend to replace fungicidal seed coatings

with biological control agents. Several of these are

now available commercially. They include antibiotic-

producing bacteria such as Bacillus subtilis, used widely

in the USA for cotton and several other field crops, and

Pseudomonas chlororaphis, used widely in Scandinavia

to protect against seedling diseases of cereals. Several

fungi are marketed for biocontrol of seedling diseases,

including Trichoderma spp. and Clonostachys rosea

(under its former name Gliocladium catenulatum).

All these products work well in appropriate condi-

tions; but seedling pathogens can infect rapidly, before

280 CHAPTER 14

Pathogen

Necrotroph Biotroph

Host-specialized Non-specialized Host-specialized