This study investigated mechanisms of resistance to the spotted stem borer in 120 tropical maize inbred lines. Trichome density, leaf toughness, stem hardness, and pith sugar content were measured. While trichome density, leaf toughness, and sugar content differed significantly, resistance was found to be germplasm-dependent with no single mechanism conferring resistance. Trichome density best discriminated lines but some lines with high or low values did not correlate with resistance. Multiple mechanisms likely interact to provide resistance in different genotypes. Further study of specific trichome types and sugars is needed.

1. Mechanisms of resistance in
tropical maize inbred lines to the
Spotted Stem Borer
Munyiri S. W., Mugo S., Otim M. and Okori P.
Presented at the 3rd RUFORUM Biennial Conference,
Entebbe Uganda, 24-28th Sep, 2012

2. Background
● Lepidopteran stem borers are serious pests of
maize in SSA. In Kenya, they cause about 13.5%
yield losses.
● Chilo partellus Swinhoe (Lepidoptera: Crambidae),
a highly invasive stem borer, has become the most
important maize stem borer in Kenya since its
introduction from Asia in the 1930s.
● A large collection of tropical inbred lines could have
great diversity and could capture 80% of the allelic
diversity in landraces, thus may hold a wide variety
of resistance mechanisms.

3. Damage to maize leaves and ear by C. partellus
larvae
a b
Chilo partellus damage on Chilo partellus damage on maize
maize leaves ear

4. Background
● Trichome density, leaf toughness and stem
hardness are important forms of physical
resistance (antibiosis).
● Stem sugars have been reported to promote stem
borer feeding on cereals.
● Developing maize with durable resistance to stem
borers could be enhanced by identifying different
mechanisms of resistance acting in different
genotypes and pyramiding the mechanisms into
high yielding genotypes.

5. Statement of the problem &
Justification
● Maize stem borer is one of the major limitations to
maize production in Kenya causing yield loss of an
average 13.5%.
● The International Maize and Wheat Improvement
Center (CIMMYT) has over the years developed
maize inbred lines with superior traits including
resistance to field and post-harvest insect pests.
● Identification of different mechanisms of resistance
to the stem borer is critical for development of
durable resistance in maize.

6. Objective and Hypothesis
Objective
To determine the effects of leaf trichome density,
leaf toughness, stem hardness, and pith sugar
content on resistance to Chilo partellus stem borers
in tropical maize inbred lines.
Hypothesis
Leaf toughness, trichome density, stem hardness
and pith sugar content influence resistance to Chilo
partellus in tropical maize inbred lines.

7. Materials and Methods
Site
● Kenya Agricultural Research Station – Kiboko field
sub-Station which lies in the mid-altitude dry zones
of Kenya, Max/Min daily of temperature of 14 oC and
35oC & 950m asl.
Germplasm
● One hundred and twenty (120) inbred lines, which
included drought tolerant, low-N tolerant, stem
borer resistant, post harvest pest resistant and
others.

8. Materials and Methods
Experimental Design
●The lines were planted in single row plots of seven
meters (29 hills per row) spaced 0.75mx0.25m.
●Alpha lattice design with 3 replications was used.
Stem borer infestation
●Ten (10) plants per plot were each infested with five
second instar C. partellus neonates three weeks after
planting.

9. Materials and Methods
Sampling
●The trial was designed to accommodate destructive
sampling of five plants per row.
●The plots were marked with five (5) strings as
shown below;
Number of 1 5 1 10 10 2
plants
Treatments Border Destructive Border Borer Borer Border
plant sampling plant infested protected plants

10. Materials and Methods
Data collection
●Leaf damage score (1-9) - taken on visual rating
score two (2) weeks after infestation on each of the 10
infested plants.
●Leaf toughness (kg-force) - taken on five (5)
randomly selected plants per row using a
penetrometer. Done on the youngest leaf with fully
developed ligule.

11. Materials and Methods
Data collection
●Trichome density (count) - At on-set of flowering:
 the leaf below the first ear was cut at the center of the
blade from10 randomly selected leaf samples per plot
were taken from the protected plants.
 A cork borer, 1cm diameter was used to punch a disk of
the leaf for trichome density count using a dissecting
microscope.

12. Data collection in the field
a b
Collecting stem samples for Cutting stem for pith extraction
sugar measurement

13. Materials and Methods
● Stem penetrometer resistance (kg-force) – taken
at silking using a Penetrometer (a FHT-803 fruit
firmness tester software) with a fabricated needle.
● Five plants per row were punctured at the center
of the second inter-node below the primary ear,
and the force recorded in maximum kg-force.
● Stem pith sugar content (brix) - taken using a Brix
Refractometer at silking.
 The second inter-node below the primary ear was cut and a
1cm radius cork borer used to extract the pith.

14. Measuring stem sugar content
Extracting the stem pith using a Brix Refractometer (r2mini
cork borer for sugar measurement Handheld Refractometer Model)

15. Materials and Methods
Data analysis
●All data collected were analyzed using PROC GLM
of SAS package (2003).
●To secure error control, coefficient correlation
analysis was done for the mechanisms of resistance
and the damage parameters using canonical
correlations.

16. Results
● ANOVA showed significant differences in all traits
measured (P<0.05).
● According to canonical discriminant analysis, most
important resistance mechanisms in discriminating
the inbred lines were trichome density, leaf
toughness, and stem sugar content.
● Most important damage parameter was leaf
damage; followed by number of exit holes, and
tunnel length.
● Canonical correlation indicated that the two different
sets of variables were not correlated.

17. Results
Table 1 Canonical discriminant analyses for mechanisms of resistance to
maize stem borer and damage traits in tropical inbred lines
Total Pooled Betwe R2 – Variation
Mechanisms Traits SD SD en SD Value Pr>F CAN1 CAN2 CAN3 Attributed
Leaf toughness 0.044 0.036 0.036 0.66 0.002 -0.264 1.230 -0.219 24
Sugar content 2.474 2.193 1.929 0.61 0.008 -0.073 0.361 1.079 16
Stem hardness 0.734 0.707 0.538 0.53 0.211NS -0.142 -0.390 0.211 12
Trichome density 5.046 3.19 4.521 0.80 <.0001 1.594 0.121 0.029 46
Leaf damage 0.805 0.594 0.643 0.64 <.0001 1.251 -0.535 -0.152 59
No. of exit holes 1.387 1.228 0.959 0.48 <.0001 0.135 1.130 -0.988 24
Tunnel length 3.89 3.587 2.575 0.43 0.002 0.247 -0.049 1.417 17

18. Results and discussion
● There was no linear relationship between damage
parameters and mechanisms of resistance.
● The three damage parameters were highly
correlated.
● Relationship between trichome density and
selection indices was negative, but weak.
● The selection index categorized the inbred lines into
33 resistant (<0.80), 29 moderately resistant (0.8-
1.0), 31 moderately susceptible (1.0-1.2) and 27 as
susceptible (>1.2).

19. Results
Figure 1 Relationship between trichome density and selection index in 120
tropical inbred lines

20. Results
Figure 2 Representation of the different categories of resistance and
susceptibility in 120 inbred lines

21. Results and discussion
● The most resistant lines were CKSBL10039 and
MBR CKSBL10025 with indices of 0.50 and 0.52,
respectively.
● CML395 and CML312 were the most susceptible
with indices of 1.85 and 1.65, respectively.
● Trichome density, leaf toughness and stem sugar
content showed high and significant differences (P
< 0.0001).
● Canonical discriminant analysis showed trichome
density was the best in discriminating the inbreds
lines among the other traits.

22. Results and discussion
● In a few notable exceptions, some lines with high
trichome densities were susceptible while some with
low trichome densities were resistant.
● The relationship between trichome density and
selection index was low and weak (scatter plot).
● Some inbred lines with low sugar content were
moderately susceptible, and a few with high sugar
content were resistant or moderately resistant.

23. Results and discussion
● These findings suggest that one or several
mechanisms of resistance may act together in
different genotypes to confer resistance, these
mechanisms are germplasm-dependent.
● There was no clear cut trend found, suggesting
that resistance and susceptibility were
germplasm-dependent.

24. Conclusions
● Several mechanisms of resistance exist in tropical
inbred lines, trichome density is the most promising
indicator of resistance followed by leaf toughness
and stem sugar content.
● More research is needed to classify the specific
types of trichomes and the specific sugars present
in both resistant and susceptible inbred lines.
● Research to identify other/more mechanisms
existing in tropical inbred lines will add onto the
current knowledge and help breeders in setting
breeding objectives in future.

25. Thank
You