Breeding for CMD resistance

1. Cassava breeding in
SE Asia:
disease resistance
Hernán Ceballos
The bad
The good
The future

2. With the turn of the century, cassava in SE Asia has
faced several health issues
The first problem was the emergence of Witches Broom Disease (WBD)
more than ten years ago

3. In fact, these
farmers in Eastern
Cambodia had
negligible root yields
because of WBD

4. With the turn of the century, cassava in SE Asia has
faced several health issues
Then came the mealybug...

5. With the turn of the century, cassava in SE Asia has
faced several health issues
… but also, its biological control !!!
Then came the mealybug...

6. With the turn of the century, cassava in SE Asia has
faced several health issues
Finally CMD landed in the region…

7. Cassava breeding in
SE Asia:
disease resistance
Hernán Ceballos
The bad
The good
The future

8. SOURCES OF RESISTANCE AND GENETIC VARIABILITY
BREEDING FOR RESISTANCE TO DISEASES AND PESTS REQUIRES
APPROPRIATE BREEDING STRATEGY

9. SOURCES OF RESISTANCE AND GENETIC VARIABILITY
BREEDING FOR RESISTANCE TO DISEASES AND PESTS REQUIRES
The germplasm collection at CIAT is available. We can screen for sources of
resistance to different biotic problems
The collection has already proven to offer
a wealth of useful traits:
Resistance to CBB
Resistance to SED
Resistance to CBSD
Molecular evidence of resistance to CMD
Resistance to whiteflies
Resistance to thrips
Resistance to mites
Starch quality traits (e.g. waxy starch)

10. SOURCES OF RESISTANCE AND GENETIC VARIABILITY
BREEDING FOR RESISTANCE TO DISEASES AND PESTS REQUIRES
A copy of the core germplasm collection is already at Rayong FCRC in Thailand

11. SOURCES OF RESISTANCE AND GENETIC VARIABILITY
BREEDING FOR RESISTANCE TO DISEASES AND PESTS REQUIRES
The migration of the CMD2
source of resistance from
IITA to CIAT
CTCRI in India received the
CMD2 source of resistance and
successfully used it to produce
highly resistant clones which are
currently grown by farmers
We should be proud of pre-emptive
actions to introduce CMD2 resistance
into SE Asia in 2011
Many years before the arrival of CMD
to the region we already had a source
of resistance
Crosses have been made and at least 5
clones areconfirmed to carry CMD2

12. We already have experience introgressing
single gene traits in cassava (waxy starch)
BREEDING FOR RESISTANCE TO DISEASES AND PESTS REQUIRES
APPROPRIATE BREEDING STRATEGY
F2
(Wx Wx)
(Wx wx)
(wx Wx)
(wx wx)
AM 206-5
Rayong 60
CM 3306-4
MCOL 1505
HMC1
CM 523-7
SM 1219-9
….
CM 4843-1
CM1-1
CM1-2
CM1-n
…
CM2-1
CM2-2
CM2-n
…
CM3-1
CM3-2
CM3-n
…
CM4-1
CM4-2
CM4-n
…
CM5-1
CM5-2
CM5-n
…
CM6-1
CM6-2
CM6-n
…
CMx-1
CMx-2
CMx-n
…
25% of segregating progeny
producing waxy starch and
negligible inbreeding depression
This scheme was developed to allow
for the expression of waxy starch
trait which is recessive
Introgressing CMD2 should be
much easier because it is dominant
(Wx Wx)
(Wx wx)
(wx wx)

13. SELECTING WAXY STARCH GENOTYPES IN A SEEDLING NURSERY
Screening is fast, cheap and reliable
We screened & selected
about 2000 genotypes/day
We have molecular markers
to track CMD2 in
segregating populations

14. HBxw09-19-2
27.4 t/ha (FRY)
17.7 % starch
4.8 t/ha DMY
HBxw09-635-4
29.5 t/ha (FRY)
16.6 % starch
4.9 t/ha DMY
FIRST GENERATION OF WAXY STARCH CASSAVA
KU50
39.5 t/ha (FRY)
27.1 % starch
10.7 t/ha DMY
Rayong 1
28.6 t/ha (FRY)
19.4 % starch
5.5 t/ha DMY

15. LESSONS FROM THE FIRST GENERATION OF WAXY STARCH CASSAVA
Introgressing a single gene trait in cassava is VERY inefficient
 we lost 40 years of breeding (went back to Rayong 1 yields!!!)
A key problem is the limited number of meiotic events leading to the
“F2” plants evaluated  undesirable linkages remain unbroken
Selected waxy genotypes did not have a nice plant architecture and
DMC was low  New crosses made among selected plants at the
seedling stage, which originated the 2nd generation
of waxy starch clones

16. SECOND GENERATION OF WAXY STARCH CASSAVA
1st generation 2nd generation
18-19% starch 24-25% starch
5 years

17. LESSONS FROM THE SECOND GENERATION OF WAXY STARCH CASSAVA
Cassava breeding is not necessarily “difficult, inefficient and slow”
 After just 1-2 additional meiotic recombinations and few years of
selection, problems of plant architecture and DMC were solved
Many traits (DMC, plant type, carotenoids content, resistance to pest
and diseases, etc.) are relatively easy to breed for
IT IS NOT TRUE THAT “CASSAVA BREEDING IS DIFFICULT AND
SLOW”… WE NEED TO SPECIFY THE TRAIT WE HAVE IN MIND
 cassava can be
very responsive to selection for many different traits

18. MORE GOOD NEWS: WE CAN NOW INDUCE FLOWERING IN CASSAVA
Normal Illuminated
Normal Illuminated
“RED LIGHT DISTRICT”
PLANT GROWTH REGULATORS

19. NO PRUNED
NO PRUNED
PRUNED
PRUNED
Just after
pruning
A month later
We can induce flowering
in “shy” clones (e.g. four
months after planting
instead of 9-14)
We can then obtain
seeds from the first
flowering (usually
sterile)
We can get seed 6 MAP
in clones that would
normally give us few
seeds 24 MAP

20. Cassava breeding in
SE Asia:
disease resistance
Hernán Ceballos
The bad
The good
The future

21. STRATEGIES TO DEAL WITHT THE PROBLEM OF CMD
RIGHT NOW
We can teach farmers and large enterprises to apply the “corner of fortune”
principle to produce relatively clean planting material
MID TERM (2-3 YEARS)
CR41 and CR36 (released in Nigeria) have high starch. They originated in CIAT so
they have never seen CDM or CBSD.
We can bring in commercial clones carrying CMD2 developed in India (which is
“safe” as they have the same single isolate of the virus present in SE Asia).
Third best option would be to bring materials from Africa, but it is a bit riskier.
Test the five hybrids produced in Thailand which have been confirmed to carry
CMD2 resistance and carry “Asian blood”
LONG TERM (5-10 YEARS)
Carry out a full breeding process to locally generate CMD resistant germplasm

22. HOW TO INTROGRESS RESISTANCE TO CMD IN ASIA
Source of
resistance
to CMD
(R r)
Elite
clones (r r)
Clone 1
Clone 2
Clone 3
…..
Clone n
1
Full sib 1
Full sib 2
Full sib 3
…..
Full sib n
Resistant (R r)
Susceptible (r r)
Resistant (R r)
Susceptible (r r)
Resistant (R r)
Susceptible (r r)
Resistant (R r)
Susceptible (r r)
Self-
pollinate
Self-
pollinate
Self-
pollinate
Self-
pollinate
2
3
Resistant (R R)
Resistant (R r)
Susceptible (r r)
Resistant (R R)
Resistant (R r)
Susceptible (r r)
Resistant (R R)
Resistant (R r)
Susceptible (r r)
Resistant (R R)
Resistant (R r)
Susceptible (r r)
If markers are
efficient, resistance
to CMD is fixed.
Select for resistance to CMD,
harvest index, and other
high heritability traits
Also select for
tolerance to inbreeding
depression (reduce
genetic load)
Three meiotic events to break undesirable linkages
Partial inbreeding allows reducing genetic load
Resistance is fixed and there is no further need to use markers

23. THE REAL PROBLEM IS STILL WBD
We have not (yet) identified a source of
resistance or tolerance to WBD
We have all the reasons to believe that the
germplasm collection holds resistance to WBD
There is an absolute priority to start screening
the germplasm collection in SE Asia
We will most probably find:
 Resistance to WBD
 New sources of resistance to CMD
We will also widen the genetic variability of
cassava in SE Asia

24. Thank you !!!