Agriculture Reference
In-Depth Information
The true fungi are classified in their own kingdom, the
Fungi. Now we know that the true fungi are more closely
related to animals than to plants. The four major groups
(phyla) of true fungi are the Ascomycota, Basidiomycota,
Chytridimycota and Glomeromycota (includes traditional
Zygomycota). The first two groups contain most of the
plant pathogenic fungi.
Some important and widespread plant pathogens known
as oomycetes were once considered part of the fungal
kingdom. Their name derives from the round oospores
produced by sexual reproduction and serve as thick-walled
survival spores. Oomycetes are adapted to living in moist
environments and produce asexual zoospores with f lagella
that allow them to swim. The best-known oomycetes are
the plant pathogens
Phytophthora
,
Pythium
and the downy
mildews. Structural, molecular and biochemical studies
show that oomycetes are more closely related to diatoms,
kelps and golden-brown algae. These organisms are now
placed into the Kingdom Stramenopila.
The main characteristics of the plant pathogenic fungi
and oomycetes are listed in Table 1.2.
the cell, which assists in plant infection. Members of the
genera
Agrobacterium, Clavibacter, Erwinia,
Pseudomonas, Xanthomonas, Ralstonia
and
Xylella
account for most species that cause diseases in plants.
Most bacterial plant pathogens are facultative parasites that
adapt readily to different environments and can usually be
cultured easily in the laboratory. Some bacteria that infect
plants have never been cultured or only with great
difficulty, using specialised media, and are described as
fastidious bacteria. These include the xylem-limited
Xylella
fastidiosa
, the cause of Pierce's disease of grapevine, and the
phloem-limited
Spiroplasma citri
, the cause of citrus
stubborn disease.
Bacteria can survive for some time on plant surfaces as
epiphytes, becoming active when conditions favour their
development. The organisms can also survive in soil and
crop debris, and in seeds and other plant parts. With the
exception of
Streptomyces
, plant bacterial pathogens do
not form spores.
Bacteria spread in infected seed and propagating material,
water splash and wind-driven rain. Overhead irrigation is
often an important means of spreading bacteria within a
crop. Bacteria also spread with insects and with workers
and machinery moving through a crop that is wet from
rain or dew. Some species such as
Xylella
have specific
insect vectors.
Bacteria and phytoplasmas
Bacteria are tiny, single-celled organisms that lack
chlorophyll. Cells reproduce rapidly by dividing into
two (fission). Many bacteria produce extracellular
polysaccharides that form a slime layer or capsule around
Table 1.2 Fungal and fungal-like pathogens
GROUP (PHYLA)
DISTINGUISHING FEATURES
REPRESENTATIVE MEMBERS
True fungi
Chytridiomycota
(Chytridiomycetes)
Glomeromycota (includes
traditional Zygomycota)
Motile oospores; survive as resting sporangia;
most species saprobic
Sexual spores: zygospores
Asexual spores: sporangiospores from sporangia
Physoderma, Synchytrium, Olpidium
Rhizopus; Mucor
Ascomycota (Ascomycetes)
Sexual spores (ascospores) form in an ascus
Asexual spores are conidia Septate hyphae
Powdery mildews,
Cercospora, Guignardia, Meliola,
Taphrina, Septoria, Venturia
Basidiomycota
(Basidiomycetes)
Sexual spores (basidiospores) produced in a basidium
Clamp connections develop at hyphal septa
Rusts, smuts, mushrooms,
Armillaria
Fungal-like organisms
Stramenopila (Stramenopiles)
Oomycota (Oomycetes)
Non-septate hyphae cell wall contains cellulose
and glucans instead of chitin as in true fungi
Sexual spores are oospores
Asexual spores are zoospores with two flagella, produced
in a sporangium
Intracellular in algal, fungal or plant hosts
Develops a multinucleate, unwalled plasmodium within
the host cell
Produces flagellate zoospores
Common in soil and aquatic habitats
Phytophthora, Pythium, Albugo
; downy mildews
Protozoa
Plasmodiophoromycota
Plasmodiophora brassicae
- club rot of brassicas
