Variability arises in plant pathogens through various genetic mechanisms such as mutation, hybridization, and recombination. This variability allows pathogens to evolve new races or strains that can infect resistant host varieties and overcome plant resistance. The document discusses several mechanisms that generate variability in fungi, bacteria, and viruses, including mutation, transformation, transduction, conjugation, heterokaryosis, parasexualism, and recombination, which allow pathogens to adapt to new environments and hosts. Understanding pathogen variability is important for breeding disease-resistant crop varieties.

1. VARIABILITY IN PLANT
PATHOGENS
PRESENTED BY
Arushi Arora
A-2016-30-050

2. GENETICS AND VARIABILITY OF PLANT
PATHOGENS
• Dr. E.C. Stakman (U.S. Deptt. Of Agriculture)
defined need and use of variability in fungi
• Pathogen population mutates with time and
environmental conditions like stress
• In nature variability in organisms may be due
to:
HYBRIDIZATION MUTATION

3. • When progenies show variations in
characterstics from parents it is called as a
VARIANT.
• Physiological specialization: – with in the
species of a pathogen there exist certain
individuals that are morphologically similar
but differs w.r.t their physiology, biochemical
characters and pathogenicity and are
differentiated on the basis of their reaction on
certain host genera or cultivars.

4. • Physiological race: – a group of population
within a species which have ability of infecting
a particular genotype and do not differ in their
morphology but have physiological differences
such as a specific host or food type or
pathogenicity
• Variability: it is the property of an organism to
change its characters from one generation to
the other

5. • With in species or f.sp. There are further
subgroups of individual that infect different
varietes of the host– such subgroups are
called RACES/ STRAINS
• A sub-group within a species or race,
characterized by a common possesion of
single of few new characters and differ only in
few minor characters from parents are called
BIOTYPES

6. BREAKDOWN OF RESISTANCE:
• This term is used when a previously resistant
variety suddenly develops disease.
• It implies that the host has changed, that the
resistance mechanisms no longer work.
• New pathogen races have developed because
of selection pressure that was put on
population by the host resistance mechanism

7. Races are defined by ability to develop on specific host
genotypes (varieties, cultivars, hybrids, etc.).
Races are identified by ability to cause disease on members
of a set of 10 differential varieties that contain specific
resistance genes.
V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 Race
x x x 14
x x x 27
x x 31

8. VARIABILITY IN FUNGI
MUTATION
HERTEROKARYOSIS
PARASEXUALISM
RECOMBINATION
CYTOPLASMIC
ADAPTATION

9. HYBRIDIZATION
• Mating of dissimilar strains or species of fungi
• 2 haploid nuclei (1N) with different genetic
material combine to form a diploid
nucleus(zygote)
• Eg. Basidiospores( haploid) from different races
often infect the same leaf or plant
Chances of dikaryotization high

10. HERTEROKARYOSIS
Hyphae or parts of hyphae contain nuclei,
which are genetically different, generally of
two different kinds. This condition is known as
heterokaryosis .
 The phenomenon is commonly brought about
by hyphal anastomosis between mycelia of
two parental genotypes.
 In Ascomycotina and Basidiomycotina

11. PARASEXUALISM
• First demonstrated by Pontecorvo (1956) in
Aspergillus nidulans.
Parasexualism is the process by which genetic
recombination can occur with in fungal
heterokaryon :
• Stage 1: heterokaryosis
• Stage 2: recombination

12. PARASEXUALISM
• Dissimilar nuclei to fuse and produce diploids
known as mitotic recombination.
• Recombination without sexual cycle
• There is no fine coordination between
plasmogamy, karyogamy and haplodisation
• This sequence of events has been described in
the parasexual cycle

13. PARASEXUALISM IMPORTANCE
• In rust fungi as P. graminis tritici, mitotic
recombination may represent a most
important method of generating new races
especially in countries such as India where
sexual stage of the fungus is rare due to
scarcity of the alternate host, the barberry.
• Common in deutromycotina

14. RECOMBINATION
• When two haploid nuclei (1N) containing
different gnentic maeterial unite to form diploid
(2N) nucleus (Zygote,) when under go meiotic
division produce new haploid .
• Recombination of genes occurs during meiotic
division of zygote as a result of cross over in
which part of chromatid of one chromosome of a
pair are expressed with that of the other
• Important in fungi Puccinia graminis.

16. Cereal rust fungus variability
• Races of rust pathogen Puccinia graminis
differ from each other in shape and size of
uredospores but principle diffrence between
them is
• Preference for groups of host in different
members of graminae
Eg. Race tritici, secalis, avenae etc

17. MUTATION
Sudden heritable change in genetic material of an
organism
 Mutation represent change in sequences of the
bases in DNA either by – substitution or by
deletion or addition
Mutations are spontaneous
May occur due to:
1. Improper cell division
2. Abnormalities during division
3. Physical radiations
4. Mutagenic chemicals

18. CYTOPLASMIC ADAPTATION
• Pathogens develop capicity to perform
biochemical reactions which were not present
in them earlier
• Can utilize protoplasm of unfavourable host.
• Adaptation to new cytoplasm

19. Three types of cytoplasmic
adaptatibility
I. Pathogen may aquire tolerance to toxic
materials
II. Utilization of new types of cytoplasm
III. Change in virulence

20. VARIABILITY IN BACTERIA
Transformation transduction
Conjugation

21. CONJUGATION
• Transfer of DNA from one bacterial cell to
another Donor cell (F+) transfers DNA to
recipient cell (F- )
• In this two compatible bacteria come in
contact and exchange the portion of plasmid
or chromosome through conjugation bridge or
pilus

23. TRANSFORMATION
DNA taken up from external environment by
absorption

25. TRANSDUCTION
Transfer of bacterial genes with a bacteriophage

28. VARIABILITY IN VIRUSES
Recombination
• May results from mixed infection of two
strains of the virus
• Occurs mostly during replication
Mutation – Results from nucleotide changes
in the coding regions due to addition or
deletion or replacement. – Ultimately leads to
functional changes in the genes.

29. REFRENCES
• R.S. Singh, INTRODUCTION TO PRINCIPLES OF
PLANT PATHOLOGY, Oxford & IBH Publishing
CO. pg.200-208
• http://agriinfo.in/default.aspx?page=topic&su
perid=3&topicid=2131
• http://www.columbia.edu/itc/hs/medical/pat
hophys/id/2008/Notes.pdf