It highlights the types of parasitism of plant pathogens

1. Parasitism and variability

2.  Fungi are essentially heterotrophic or dependant on other
organisms as they are chlorophyll less in nature.
 Moreover, they require predigested food to live and
therefore they obtain food from either living (Parasites) or
non living sources (Saprophytes).
1) Parasite
 An organism that lives on or in some other living organism
(host) and derive nutrition are called as parasites. The
relationship between the parasite and the host is called as
parasitism.
 The parasites are of different types.

3. i) Ectoparasite
The parasite may live on the external surface of the host
(eg) Erysiphe polygoni.
ii) Endoparasite
The parasite may grow inside the host cell (intracellular) or in between
the cells (inter cellular) of the host plant and draw nutrition.
(eg) Leveillula taurica.
iii) Endobiotic parasite
Parasite which produces their entire body within a host cell is called
endobiotic Eg:- Synchytrium endobioticum
Facultative parasites
They usually live as saprophyte but can attack living tissue under certain
condition. i.e., they normally grow as saprophyte but also have the
ability to parasitise.
These fungi are having saprophytic phase with a short parasitic phase
under suitable condition (eg) Pythium sp., Rhizoctonia sp

4. Facultative saprophytes
They usually live as parasite but can grow saprophytically
i.e., they have long parasitic phase and with a short
saprophytic phase in their active life cycle (eg)
Phytophthora sp., Venturia inequalis
Obligate parasite
They invariably occur as parasites in nature and cannot be
grown in axenic culture media under in vitro condition
which are called as 'biotrophs’
(eg) Downy mildew, Powdery mildew, Rusts, Viruses,
Viroids

5. Hemibiotrophs
These pathogen attack living tissues and grows in or on them
in the same way as that of biotrophs but continue to grow
and reproduce after the tissue is dead (eg) leaf spotting
fungi.
Perthotrophs
They are so called less specialised parasite which can cause
immediate and severe damage to their host tissue. They
usually obtain nutrients from cells killed in advance (eg)
Rhizopus, Pythium, Rhizoctonia, Fusarium, Sclerotium rolfsii
They kill the tissue before actually entering and feeding on
them.
Necrotrophs
There are some intermediate parasites which invade and kill
host cell (eg) Phytophthora, Claviceps, Venturia.

6. Synergism
It means the ability of two kinds of organisms to grow better or produce
greater effect conjointly than either one could alone
(eg) Diplodia natelensis and Colletotrichum gleosporioides together
produce much greater effect on citrus than either one of the pair alone
can produce.
Symbiosis :- In some cases of parasitism, as with the root nodule bacteria
of legume plants and the mycorrhizal infection of feeder roots of most
flowering plants, both the plant and the microorganism benefit from the
association. This phenomenon is known as symbiosis.
Mycorrhiza
It is one kind of mutualism between the fungi and roots of higher plants.
The name 'mycorrhiza' literally means "fungus root" and it was coined by
Frank in 1885
Two types
Ectomycorrhiza
Endomycorrhiza

7. Variability in Plant Pathogens

8. Variability:-
The property or ability of an organism to change its
characteristics from one generation to the other.
General Mechanisms of Variability
Mutation and Recombination
 A mutation is a more or less abrupt change in the genetic
material of an organism, which is then transmitted in a
hereditary fashion to the progeny.
 Mutations represent changes in the sequence of bases in the
DNA either through substitution of one base for another or
through addition or deletion of one or many base pairs.
 On average, one mutation occurs for every million copies of a
gene per generation

9. Recombination occurs primarily during the sexual reproduction of
fungi, and nematodes whenever two haploid (1N) nuclei,
containing genetic material that may differ in many loci, unite to
form a diploid (2N) nucleus, called a zygote.
The zygote, sooner or later, divides meiotically and produces new
haploid cells (gametes, spores, mycelium).
Recombination of genetic factors (different genes or alleles of the
same genes) occurs during the meiotic division of the zygote as a
result of genetic crossovers in which parts of chromatids (and the
genes they carry) of one chromosome of a pair are exchanged with
parts of chromatids of the other chromosome of the pair.

10. Specialized Mechanisms of Variability in
Pathogens
 Fungi
 Heterokaryosis
 Heterokaryosis is the condition in which, as a result of
fertilization or anastomosis, cells of fungal hyphae
parts of hyphae contain two or more nuclei that are
genetically different
 Eg:- P. graminis tritici, the fungus causing stem rust of
wheat,

11. Specialized Mechanisms of Variability in
Pathogens
 Variability in fungi
 Parasexualism
 Parasexualism is the process by which genetic
recombinations can occur within fungal heterokaryons.
This comes about by the occasional fusion of the two
nuclei and formation of a diploid nucleus.
 During multiplication, crossing-over occurs in a few mitotic
divisions and results in the appearance of genetic
recombinants as the diploid nucleus progressively and
rapidly loses individual chromosomes to revert to its
haploid state

12. Heteroploidy
Heteroploidy is the existence of cells tissues or whole
organisms with numbers of chromosomes per nucleus.
 Heteroploids may be haploids, diploid, triploid or
tetraploids i.e. have one or more extra chromosomes from
normal euploid number
e.g.N+1
o This represents a normal situation in eukaryotes
Physiological race – individuals within the species of a pathogen
that morphologically similar but differ with respect to their
pathogenicity on particular set of host varieties.
Formae specialis (f. sp.) – individuals within the species of a
pathogen that morphologically similar but differ with respect to
their pathogenicity on particular host genera.
E.g. Puccinia graminis f.sp. tritici

13. Specialized Mechanisms of Variability in
Pathogens
 Variability in bacteria
(1) Conjugation occurs when two compatible bacteria
come in contact with one another and a small portion
of the chromosome or plasmid from one bacterium is
transferred to the other through a conjugation bridge
or pilus.
(2) In transformation, bacterial cells are transformed
genetically by absorbing and incorporating in their own
cells genetic material secreted by, or released during
rupture of, other bacteria.

14. (3) In transduction, a bacterial virus (phage) transfers
genetic material from the bacterium in which the phage
was produced to the bacterium it infects next.

15. Recombination
• May results from mixed infection of two strains of the virus
• Occurs mostly during replication
Reassortment
• It is defined as the exchange of intact genes within the entire
segment, which occurs during coinfection. Reassortment has been
observed in Bunyaviridae, Reoviruses
Mutation
• Results from nucleotide changes in the coding regions due to
addition or deletion or replacement.
• Ultimately leads to functional changes in the genes.
Variability in virus