2. INTRODUCTION
The contribution of crop improvement to feed the ever-
growing world population is quite eminent.
However, it also generates some undesirable effects like
genetic erosion and vulnerability of the improved
cultivars to diseases and other biotic and abiotic
stresses.
During the co-evolution of plants and pathogens, all the
important crops are damaged unless effective protection
is devised.
Protection may be realized by different means:
chemicals, hygiene, biological control or host plant
resistance (Hogenboon, 1993). 2
3. INTRODUCTION
Host plant resistance is the most preferable means of
crop protection of all kinds as it combines the
advantages of cost effectiveness and ecological
soundness.
However, it has been a challenge to the breeders to
achieve durable resistance.
The exploitation of genes with known durability, the
use of quantitative resistance and the use of gene
deployment can lead to the better ways of achieving
durability of resistance (Burdon et al., 2014).
3
4. GENETIC VULNERABILITY
EVENTS THAT OCCURRED IN THE HISTORY OF GLOBAL
AGRICULTURE
The Irish famine in Ireland, England and continental Europe in 1840s that
occurred due to Potato Late blight (Phytophthora infestans)
Wheat stem rust (Puccinia graminis) in USA in 1917 that wiped out the
wheat fields
The complete elimination of all oats derived from the variety Victoria in the
USA in the mid 1940s by Victoria blight (Cochliobolus victoriae)
The destruction of maize hybrids carrying T-cytoplasms in the USA in 1970-
71 by Southern corn leaf blight (C. heterostrophus)
The great Bengal famine in India in 1943 due to the destruction of rice by C.
miyabeanus .
Adugna, 2004
4
5. CAUSES OF GENETIC VULNERABILITY
Narrow genetic base of cultivars
Devoting a large acreage of land for the cultivation of a
single cultivar
Induced susceptibility with desirable traits
The otherwise resistant varieties can be attacked by the
previously unrecognized strains of pathogens
Failure of vertical resistance
Adugna, 2004
5
6. DURABILITY OF DISEASE RESISTANCE
Disease resistance is the inherent ability of a plant to prevent or
restrict the establishment and subsequent activities of a potential
pathogen (Mehrotra, 2000).
Resistance that remains effective in a cultivar that is widely
grown for a long period of time in an environment favorable to
the disease is said to be durable resistance (Johnson, 1983).
Resistance conferred with major genes has frequently failed to
provide long-term disease control and the use of single major
genes in cultivars grown over a broad area potentially lead to
serious epidemics (Gananamanickam, 1999).
6
8. MECHANISMS OF
DURABLE RESISTANCE
It is emphasized that durability of resistance is not merely a
property of genes; it is the property of the cropping system.
The strategies by which durability of resistance can be achieved
include:
the use of classical R-genes supported by an effective backup
system,
use of known durable resistance,
pyramiding of major genes to present multiple gene barrier,
deployment of combinations of major gene resistance,
use of partial/polygenic resistance,
exploitation of tolerance
the use of biotechnological aids for durable resistance.
8
Adugna, 2004
9. Mechanisms of durable resistance
9
If non-durable major genes are used in combination
it may be more difficult for the pathogen to build up
races with a wider virulence spectrum (complex
races) (Parlevliet,1993).
Such genes can be exploited in combination in two
ways:
1)Through gene deployment
2)Through multiple gene barriers/gene pyramiding
10. WHAT IS GENE DEPLOYMENT
• Gene deployment is a well planned strategy for utilization of
resistance genes for maximum benefits on long-term basis (Chaube
and Pundhir, 2009).
• It requires special considerations of specific or major genes, which
are inherently usable.
• Gene deployment is the guided distribution of genes in space and
time (Parlevliet, 1993).
• The gene deployment strategies can be divided into two broad
categories (Gould, 1983)
1) spatial gene deployment
2) temporal gene deployment
10
11. SPATIAL GENE DEPLOYMENT
GENE DEPLOYMENT AT THE FIELD LEVEL
It includes variety mixtures, species mixtures, multi lines and
multi blends.
Variety mixtures refers to a spatial mixture of different
genotypes of one plant species in a field (Wilhoit, 1992).
Cultivar mixtures have been successfully exploited for
management of powdery mildew in Barley (Chaube and
Pundhir, 2009).
They provide greater genetic heterogeinity to host population.
The concept of mixtures is to shield individuals carrying one
particular resistance with a range of others carrying other
resistance genes (Burdon, 1993).
Disease resistance in multilines and variety mixtures depends
on slowing down the development of the best-adapted race on
each component (Wolfe, 1985).
11
12. VARIETY MIXTURES
Wolfe (1985) reported that good mixtures of spring barley
varieties resulted in up to 80% reduction in powdery mildew
compared with mean disease levels of the components grown as
pure stands.
Variety mixtures have epidemiological advantage in that the
interaction among neighboring plants in terms of spore dispersal
leads to a reduction in disease spread relative to that in pure
stands.
Cultivation of appropriate mixtures in an environment with a high
disease risk provides a degree of high stability, which cannot be
obtained from pure stands
Perhaps, the biggest problem with mixtures is finding the correct
combination of varieties. 12
13. MIXED CROPPING
Growing species mixtures in which the important pathogens are
incompatible with one or more of the host species used.
The growers plant simultaneously more than one plant species
such as cereals (wheat+barley), cereals+legumes
(wheat+chickpea or sorghum+pigeon pea), wheat+oilseed
(mustard).
Mixed cropping of cotton species is common in the Malwa plateau
of central India (Browning and Frey, 1969).
Oat+barley mixtures have been suggested as an agro-technical
method for controlling the oat sterile-dwarf virus disease. 13
14. MULTILINE VARIETIES
The term multiline variety was first defined by Jensen
(1952) as a blend of multiple pure lines, each of which is of a
different genotype (Isoline).
Marshal (1977) has given two approaches of multiline
development:
1) the clean crop approach
2) the dirty crop approach
Dirty multilines are mixtures of lines each carrying a unique
resistance gene
Clean multilines are mixtures of lines each resistant to all
prevalent races of the disease.
14
15. MULTILINE VARIETIES
Multiline varieties offer various advantages:
by using multilines we can exploit horizontal resistance,
which is otherwise difficult to achieve because of the
complexities associated with its genetic mechanism
(Borlaug, 1953).
multiline varieties proved to live longer than their
corresponding pure stands because of stabilized selection.
all the component lines are phenotypically similar the only
difference being the gene for resistance. 15
16. REGIONAL DEPLOYMENT OF
RESISTANT GENES
CREATION OF GENE ZONES
Vertical resistance governed by major genes is more
effective in heterogenous host populations where inoculum
produced on one host genotype is ineffective on other host
cultivar (carrying different resistant gene).
This principle has been suggested for the creation of gene
zones.
Gene zones are geographical areas planted with cultivars
having particular resistant genes.
Gene zones have been used for the management of rust
pathogens (Nelson,1972; Chaube and Pundhir, 2009).
16
17. CREATION OF GENE ZONES
Reddy and Rao (1979) suggested a strategy for control of
leaf rust (Puccinia recondita) of wheat in India by regional
deployment of resistant genes.
It was proposed that India could be divided into three
zones, viz.
a) Central plains
b) North Himalayan Region
c) Southern Nilgiri and Palani hills
Region (a) (Lr9 or Lr19 or Lr10+Lr15)
Region (b) (Lr1+Lr107, Lr 1b or Lr3ka+Lr10+Lr17)
Region (c) (Lr3ka+ Lr20 or Lr1+Lr10+Lr17
17
18. TEMPORAL GENE DEPLOYMENT
Sequential release of resistance genes where by each
variety is used until populations reach the breakdown
population level and is immediately replaced by another
variety (Wilhoit, 1992).
Variety rotations from season to season or recycling of
resistance genes.
Rotation of varieties with different resistances prevents
selection of compatible isolates in populations of soil-born
pathogens (Fry, 1982).
18
19. GENE PYRAMIDING
It is the stacking of R genes in cultivars or
simultaneous introduction of more major genes in
host cultivars.
The purpose of combining genes in host plant is to
create a situation where pathogen could not
develop matching genotype.
Higher the number of resistant genes greater would
be the longevity of resistance provided.
Chaube and Pundhir, 2009
19
20. Conclusion
The careful use of all of these different strategies can
reduce short-term epidemic development and the
probability of longer-term evolutionary change in the
pathogen.
Successful execution of these strtegies requires the
development of agriculturally realistic models based
on evolutionary principles and guided by detailed
knowledge of pathogen biology and life history.
Finally, it is essential to ensure that resistance
strategies are integrated into whole crop and farm
management systems.
20
21. REFERENCES
Adugna, A. 2004. Alternate approaches in deploying genes for
disease resistance in crop plants. Asian Journal of Plant
Sciences. 3(5): 618-623.
Borlaug, N.E. 1953. New approaches to the breeding of wheat
varieties resistant to Puccinia graminis tritici. Science. 13: 467-
467.
Burdon, J. J., Barrett, L. G., Rebetzke, G., & Thrall, P. H. 2014.
Guiding deployment of resistance in cereals using evolutionary
principles. Evolutionary applications. 7(6): 609-624.
Burdon, J.J. 1993. Genetic Variation in Pathogen Populations and
Its Implications to Adaptation to Host Resistance. In: Durability of
Disease Resistance, Jacobs, T. and J.E. Parlevliet (Eds.). Kluwer
Academic Publishers, Dordrecht. pp: 41-56.
Chaube, H. S., & Pundhir, V. S. 2009. Crop diseases and their
management. PHI Learning Pvt. Ltd. 703p.
Fry, W.E. 1982. Principles of Plant Disease Management. Academic
Press, New York, London.
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22. REFERENCES
Garrett, K. A., Andersen, K. F., Asche, F., Bowden, R. L., Forbes, G.
A., Kulakow, P. A., & Zhou, B. (2017). Resistance genes in global
crop breeding networks. Phytopathology. 107(10): 1268-1278.
Gnanamanickam, S.S., V.B. Priyadarisini, N.N. Narayanan, P.
Vasudevan and S. Kavitha, 1999. An overview of bacterial blight
disease of rice and strategies for its management. Current
Science. 77: 1435-1444.
Hogenboon, N. G. 1993. Characteristics and Aspects of Durability
of Resistance. In: Durability of Disease Resistance, Jacobs, T.
and J.E. Parlevliet (Eds.). Kluwer Academic Publishers,
Dordrecht. pp: 5-9.
Jensen, N. F. 1952. Intravarietal diversification in oat breeding.
Agronomy Journal. 44: 30-34.
Johnson, R. 1983. Genetic Background of Durable Resistance. In:
Durable Resistance in Crops, Lamberti, F., J.M. Waller and N.A.
van der Graaff (Eds.). Plenum Press, New York, pp: 5-26.
Marshal, D. R. 1977. The advantages and hazards of genetic
homogeneity. Ann. N. Y. Acad. Sci., 287: 1-20. 22
23. REFERENCES
Mehrotra, R. S. 2000. Plant Pathology. Tata McGraw-Hill Publishing Co.
Ltd., New Delhi.
Nelson, R. R. 1972. Stabilizing racial populations of plant pathogens by use
of resistant genes. J. environ. Qual., 1:220-227.
Parlevliet, J. E. 1993. What is Durable Resistance: A General Outline. In:
Durability of Disease Resistance, Jacobs, T. and J.E. Parlevliet (Eds.).
Kluwer Academic Publishers, Dordrecht. pp: 23-40.
Reddy, M. S. S. and Rao, M. V. 1979. Resistance genes and their
deployment for control of leaf rust of wheat. Ind. J. Genet Plant Breed.,
39: 359-365.
Wilhoit, L. R. 1992. Evolution of Herbivore Virulence to Plant Resistance:
Influence of Variety Mixtures. In: Plant Resistance to Herbivores and
Pathogens, Fritz, R.S. and E.L. Simms (Eds.). The University of Chicago
Press, Chicago, London, pp: 92-119.
Wolfe, M. S. 1985. The current status and prospects of multiline cultivars
and variety mixtures for disease resistance. Ann. Rev. Phytopathol., 23:
251-273.
23
The crop improvement practices have definitely led to an increase in the yield of crops and economic benefits but despite many adavantages various undesirable consequences have emerged which include genetic erosion and vulnerability of the improved cultivars to diseases and other biotic and abiotic stresses. During the co-evolution of plants and pathogens, all the important crops are damaged unless effective protection is devised. Plants can be protected from diseases by various ways including chemical control, field sanitation, biological control or host plant resistance (Hogenboon, 1993).
Host plant resistance is the most preferable means of management of plant diseases as it is a cost effective as well as environmentally sound method. However breeders have been facing challenging situations when it comes to developing cultivars with durable resistance. The exploitation of genes with known durability, the use of quantitative resistance and the use of gene deployment can lead to the better ways of achieving durability of resistance (Adugna, 2004).
There have been many events in the history of global agriculture that occurred due to the monoculture practice, growing a crop over large area and lack of heterogeneity in the crop. These events include the Irish famine in Ireland, England and continental Europe in 1840s that occurred due to Potato Late blight (Phytophthora infestans), Wheat stem rust (Puccinia graminis) in USA in 1917 that wiped out the wheat fields, The complete elimination of all oats derived from the variety Victoria in the USA in the mid 1940s by Victoria blight (Cochliobolus victoriae), The destruction of maize hybrids carrying T-cytoplasms in the USA in 1970-71 by Southern corn leaf blight (C. heterostrophus) and the great Bengal famine in India in 1943 due to the destruction of rice by C. miyabeanus. These events created awareness for the need to develop more knowledge on the cause and managemnet of plant diseases.
The major causes of genetic vulnerability include
Narrow genetic base of cultivars
Devoting a large acreage of land for the cultivation of a single cultivar
Induced susceptibility with desirable traits
The otherwise resistant varieties can be attacked by the previously unrecognized strains of pathogens
Failure of vertical resistance
All of the above factors are responsible for making the crop vulnerable to the attack of pathogen. The major concern is to develop durably resistant genotypes which can provide resistance against the pathogen for longer period of time.
The popularity of vertical resistance based crops cultivars is self destructive or advantages of VR are diluted or lost as cultivar becomes more popular. Once a new VR gene is introduced in an area, the cultivar looks attractive and becomes popular among the farmers. The popularity of the VR genes put selection pressure on pathogen population, which strikes back sooner or later by developing suitable or matching virulence. As a result, the whole area under that particular genotype becomes dramatically susceptible. Thus popularity of VR becomes self destructive. This is known as the boom and burst cycle.