This document discusses strategies for deploying disease resistance genes in crops to achieve durable resistance. It describes different types of gene deployment including spatial methods like variety mixtures and multiline varieties, which introduce genetic heterogeneity, and temporal methods like rotating varieties with different resistant genes. Regional deployment by creating "gene zones" with cultivars carrying different resistance genes in different geographical areas is also discussed. Gene pyramiding, or stacking multiple resistance genes in cultivars, is described as a way to present a higher barrier to pathogens. The document emphasizes that durable resistance depends on the cropping system and integration of these strategies with whole farm management.

1. Strategies for Gene
Deployment
APP-713
1

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
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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
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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).
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7. The boomand burst cycle
BURST
Adugna, 20047

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.
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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).
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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.
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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).
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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
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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).
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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
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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.
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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
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Burdon, J.J. 1993. Genetic Variation in Pathogen Populations and
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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
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Jensen, N. F. 1952. Intravarietal diversification in oat breeding.
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Johnson, R. 1983. Genetic Background of Durable Resistance. In:
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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
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Wolfe, M. S. 1985. The current status and prospects of multiline cultivars
and variety mixtures for disease resistance. Ann. Rev. Phytopathol., 23:
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