The presentation discusses the role of vertical and horizontal resistance in plant disease management, emphasizing methods of breeding to reduce reliance on pesticides. It defines genetic resistance, differentiates between vertical (race-specific) and horizontal (general) resistance, and outlines their advantages, limitations, and genetic control mechanisms. The conclusion highlights the unstable nature of race-specific resistance compared to the more stable but incomplete horizontal resistance against pathogens.

2. CREDIT SEMINAR
Department of Plant Pathology
Rajasthan College of Agriculture, Udaipur
Speaker : Hansraj Dhakar
M.sc. Plant Pathology
Major Advisor: Dr. R.S.Ratnoo
Associate professor

3. ROLE OF VERTICALAND HORIZONTAL
RESISTANCE IN
PLANT DISEASE MANAGEMENT

4. CONTENT……
 This presentation concerns an unusual method of plant
breeding which is designed to reduce, or even to eliminate,
the need for crop pesticides.
 Presentation will be discussed under following points-
 Definitions
 Different ways in which the resistances can be employed in field.
 Citations of the various works carried out under the theme.
 Advantages and limitations in resistance breeding.
 Conclusion

5.  Genetic resistance refers to those heritable features of a host
plant that suppress or retard development of a pathogen.
 It is controlled genetically by the presence of one , a few or many
genes for resistance in the plant i.e., either nuclear or cytoplasmic
or both in nature.
 Most plants are resistant to most diseases(Non-host resistance) .
 Resistance is the rule and susceptibility is an exception.
 Disease resistance is a relative trait in plants rather than an
absolute quality.
GENETIC RESISTANCE

6. Horizontal resistance Vertical resistance
Multigenic OLIGOGENIC
Race nonspecific Race specific
Durable Less durable
Quantitative resistance Qualitative resistance
Environmentally influenced Environmentally not
influenced
Often effective against
necrotrophs
Often effective against
biotrophs
Partial (r=0 to 1) Complete (r=0 or 1)

7. Protection may be Stable or Unstable
A stable protection does not break down
to new strains of the parasite. It is an
enduring protection.
An unstable protection does break down
to new stains of the parasite. It is an
ephemeral protection.

8. Examples of Unstable Protection
Insecticides
DDT-resistant houseflies
Fungicides
Metalaxyl-resistant blight of potatoes
Host Resistance
Vertical resistance

9. Examples of Stable Protection
Insecticides
Natural pyrethrins
Fungicides
Bordeaux mixture
Host Resistance
Horizontal resistance

10. The Terms ‘Vertical’ and ‘Horizontal’
 These terms were coined by the late J. E. Van der plank
(“Plant Diseases: Epidemics and Control” 1963), who was
possibly the most important plant pathologist who ever lived.
 The terms are derived from his classic diagrams that define
the two kinds of host resistance.

11. VERTICAL RESISTANCE
 According to Van der plank(1963) vertical resistance is that kind of resistance
in plant varieties that effective against some races of pathogen and not against
others.
 Such resistance differentiates clearly between races of pathogen, as it is
effective against specific races of pathogen and ineffective against others .
 It is also known as race-specific, major gene, strong, qualitative , monogenic
or oligogenic, hypersensitive resistance , R-gene resistance , racial resistance ,
pathotype- specific resistance , unstable resistance and differential
resistance.
 Pathotype-specificity denotes that the host carrying a gene for vertical
resistance is attacked by only that pathotypes, which is virulent toward that
resistance gene; to all other pathotypes the host will be resistant.
VERTICAL RESISTANCE

12. 0 1 2 3 12 13 23 123
R2
Resistance
Max
Min
0 1 2 3 12 13 23 123
Max
Min
R1
When the two
diagrams are
compared, it will be
seen that differences
in this kind of
resistance are
parallel to the vertical
axis of the diagram,
and this prompted
Vanderplank to call it
vertical resistance.

13.  According to J.E.van der plank (1963) when the resistance is evenly spread against all races of
pathogen it is called “horizontal” or “lateral’’ resistance.
 All plants have a certain, but not always the same level of possibly unspecific resistance that is
effective against each of their pathogen.
 Such resistance is sometimes called partial, race non specific, general, quantitative, polygenic,
adult-plant, field, additive, durable, stable, non-differential ,Rate-reducing, minor gene ,
incomplete or multigenic resistance.
 In the case of horizontal resistance, reproduction rate of pathogen is never zero, but it is less
than one, i.e., r >0 but < 1.
HORIZONTAL RESISTANCE

14.  According to Ghost gene hypothesis(Martin and Ellingboe, 1976), HR represents the
cumulative effect of VR genes, which have succumbed to the corresponding virulence
genes of the pathogen, i.e., ‘ghost’ or ‘residual’ effect of vertical genes.
 Horizontal resistance, therefore , does not prevent the development of symptoms of
the disease , but it slows down the rate of spread of the disease in the population
thereby slowdown spread of epidemics in the field.
 Thus van der plank defined these terms genetically but shown their effect
epidemiologically.
contd……

15. 0 1 2 3 12 13 23 123
Max
Min
A
B
C
This diagram represents three potato cultivars that have no vertical resistance genes at
all. Consequently, they are susceptible to all races of the blight fungus. However,
cultivar A is less susceptible (or more resistant) than cultivar B, which is more resistant
than cultivar C. Differences in this kind of resistance are parallel to the
horizontal axis of the diagram and this prompted Van der plank to call it horizontal
resistance.

16. Why Two Kinds of Resistance?
Because there are two kinds
of infection

17. Primary (allo)infection
means that the parasite
originates away from its
host, and that it has to
travel to that host. This is
the equivalent of an
immigrant arriving in an
island. Clearly, the first
infection of any host must
be an
primary-infection

18. Secondary (Auto) infection means that
the parasite originates on
the host that it is infecting.
It has no need to travel to
that host. This is the
equivalent of an immigrant's
progeny populating an island.
Clearly, secondary-infection can
occur only after there has
been a successful primary-infection.

19. Think of primary(allo)infection being
the equivalent of cross-pollination
Think of secondary(auto)infection being
the equivalent of self-pollination

20. Vertical resistance can control
primary(allo)infection only
Secondary(Auto)-infection can be
controlled only by horizontal resistance
But horizontal resistance can
also control primary(allo)-infection

21. This is because all plants
have horizontal resistance.
But many plants have no
vertical resistance.

22. Every host suffers auto-infection
It follows that every host has
horizontal resistance to all
of its parasites
But the level of horizontal
resistance is low in most
modern cultivars due to the
vertifolia effect

23. VERTIFOLIA EFFECT
 The vertifolia effect was discovered by van der plank(1963) .
 The process of neglecting and losing horizontal resistance in the causes of
breeding for vertical resistance will be called vertifolia effect.
 In such cases, when the vertical resistance fails, an epidemic is the result.
 The Vertifolia potato variety furnishes a striking example of this where loss of
HR to potato blight (Phytophthora infestans) causes an epidemics during1943
in Ireland.

24.  This was proposed by Pristley (1970).
 From farmer Pt of view: VR variety become popular – increase in area but
matching virulent race will appear. So, there is association of Boom& Brust
Cycle of popularity- Cycle of repetitive plant breeding in which a new cultivar is
produced to replace one whose VR has been broken down.
BOOM AND BRUST CYCLE
BRUST

25. Genetics of vertical resistance
Vertical resistance is most likely a reflection of
gene for gene system in operation, in which
hypersensitive response is the result following the
infection by avirulent strain of pathogen.

26. Gene for gene hypothesis
 Proposed by (H.H.Flor, 1945)
• Acc to him For resistance (incompatibility) to
occur complementary pairs of dominant genes must
be present in the host and pathogen. These
dominant genes are referred to as Resistance (host)
and Avirulence (pathogen). Altering either of these
genes leads to compatibility (disease).
• The mechanisms of resistance most likely involves
interaction between the Avr protein (an elicitor) and
the R-gene product (the receptor).
1927-1991

27. MANAGEMENT OF VR GENES / GENE MANAGEMENT
Strong genes for vertical resistance may be employed in the following
ways to check epidemics:
1. Multilines / ML varieties
2. Gene cycling
3. Gene deployment
4. Gene pyramiding / multigenic varieties
The proposed basis of success of these strategies is the introduction of
vertical resistance genes into the host population in such a way that
they will function as horizontal resistance.

28. GENE-FOR-GENE HYPOTHESIS
QUADRATIC CHECK
(Agrios 2007)
Avirulence or
Virulence genes in the
pathogen
Plant Resistance or susceptibility genes
In plants
R (Resistant) r (susceptible)
A (Avirulent) AR (-) Ar(+)
a (virulent) aR(+) ar(+)
where, – = Resistance/compatible
+ = Susceptible

29. 1. Multilines : One of recent approaches for long term control of typical Compound
interest diseases. First suggested by Jensen (1952) for control of rust of oats.
Simultaneously, this was advocated by Borlaug & associates (1953, 1958) for
control of Rust of wheat.
 Useful for self pollinated crops only.
Def.: A multiline is mixture of phenotypically similar but genotypically dissimilar pure
lines / isogenic lines.
 Important characteristics of multilines are:
1. Genotypic diversity w.r.t. V.R. genes and phenotypic uniformity w.r.t. all
agronomic characteristics
2. Each V.R. gene in a multiline must be strong to reduce exodemic .
3. Each line in a multiline should preferably have H.R. to reduce esodemic.

30. 2. Gene cycling: Obligate/near obligate pathogens. Such pathogens in which soil
survival is limited. There is temporary discontinuity in the utilization of Vertical
resistance gene.
A three-gene system of variety rotations to minimize genetic vulnerability.

31. 3. Gene Deployment:
Def. A system of assigning specific resistance genes to a specific
geographic area to control disease.
• This was advocated by Van der plank(1968) exploited by Frey
et. al. (1977) for the control of crown rust of Oats (Puccinia
coronata) in U.S.A.
• For disease as rust – where epidemic route of Pathogen is known
i.e. where epidemic starts and where ends, Start in hills – ends
in field (plains).

32. 4. Gene pyramiding/ multigenic variety:
• This method aimed at assembling multiple desirable genes from multiple
parents into a single genotype.
• Increases the utility of a variety and makes it durable or stable particularly
for pathogens or diseases caused by Fungi imperfecti or other pathogens
where sexual stage is not known/ absent or ultimately the variability is less.
• So pathogen will take more time to overcome the broad genetic base.
• Useful in control of Stem rust in countries like India, Kenya, Australia,
where alternate host of rust pathogen are not active.

33. Pyramiding of genes
Backcross 1 Backcross 2 Backcross 3 Backcross 4
Gene 1 Gene 2 Gene 3 Gene 4
Variety A x
Source 1 source 2 source 3 source 4
6-8 years
x x
Variety A 1+2
Variety A 3+4
6-8 years
x
Variety A 1+2+3+4
6-8 years
Time consuming, sanctity of variety may be lost, new varieties would come

34. EXAMPLE OF VERTICAL RESISTANCE:
Potato Phytophthora
infestance
Late blight R1 -gene Leonard ,et.al,1992
Wheat Puccinia striformis
f.sp.tritici
Yellow rust Yr 18 Kumar, et.al., 1999
Cotton Verticillium dahliae wilt Variety
S9063 with
R gene
Popov, et.al, 1981
Wheat Puccinia recondita Leaf rust Lr9,Lr19,
Lr24 and
Lr38
Yuan, et. al., 2011
Wheat Erysiphe graminis
tritici
Powdery mildew Pm4a Szunics,1995

35. EXAMPLES OF HORIZONTAL RESISTANCE
CROP PATHOGEN DISEASE RESISTANCE
CULTIVAR
REFERENCE
Bell pepper Phytophthora
nicotianae var
nicotianae
Leaf blight and
fruit rot
HC 201 and
Feroz
Gupta, et. al.,
2007.
Red gram Phytophthora
drechsleri f.sp.
cajani
Phytophthora
blight
ICP 7065,
KPBR 80-1
AND KPBR
80-2
Agrawal,S.C.
and
Khare,M.N,
1987
Wheat Erysiphe
graminis
Powdery
mildew
Dominique,
Carre and 0541
japan 2003
Dobrev,D. and
Tufa,F.1995.

36. o Temporary in nature
o valuable in annual grain crop only
o VR is less valuable against compound interest diseases.
o Valuable against pathogen with a higher vertical mutability.
o Vertical resistant is less effective in open season crops.
o VR is likely to be less valuable against a disease transmitted by the
propagating material of the host.
Limitations of Vertical resistance :

37. Advantages of horizontal resistance
 Breeding for horizontal resistance is cumulative: A good cultivar is replaced
only with a better cultivar.
 HR is present universally in wild and domesticated plants and operates against all
races of a pathogen, including the most pathogenic ones.
 Horizontal resistance can be domesticated just as yield and quality have already
been domesticated.
 An inadequate level of horizontal resistance can be increased by further
breeding.

38. Limitations of horizontal resistance
 Breeding for resistance is problematic in most of the cases.
 The chief difficulty relates to an accurate and reliable assessment of the
level of resistance.
 Bringing many gene together into a single variety require meticulous
planning, elaborate execution and far greater effort than that required in
breeding for single resistance.
 The cost of artificial control of diseases may be preferable to the
difficulties of developing adequate horizontal resistance.
 Linkage between different gene create problem in breeding for desirable
attributes in plant.

39. CONCLUSION
۞Varieties with race specific resistance generally show unstable but
complete resistance to a specific pathogen under most environmental
condition, but a single or a few mutations in the pathogen may
produce a new race that may infect the previously resistant variety.
۞Varieties with horizontal resistance show stable but incomplete
resistance against different pathogen races and it vary in their reaction
to the pathogen under different environmental condition, and a
pathogen will have to undergo many more mutations to completely
break down the resistance of the host.