Mechanisms of resistance in tropical maize inbred lines to the Spotted Stem Borer
Mechanisms of resistance intropical 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
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.
Damage to maize leaves and ear by C. partellus larvae a bChilo partellus damage on Chilo partellus damage on maizemaize leaves ear
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.
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.
Objective and HypothesisObjective 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.
Materials and MethodsSite● 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.
Materials and MethodsExperimental Design●The lines were planted in single row plots of sevenmeters (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 fivesecond instar C. partellus neonates three weeks afterplanting.
Materials and MethodsSampling●The trial was designed to accommodate destructivesampling of five plants per row.●The plots were marked with five (5) strings asshown below; Number of 1 5 1 10 10 2 plants Treatments Border Destructive Border Borer Borer Border plant sampling plant infested protected plants
Materials and MethodsData collection●Leaf damage score (1-9) - taken on visual ratingscore two (2) weeks after infestation on each of the 10infested plants.●Leaf toughness (kg-force) - taken on five (5)randomly selected plants per row using apenetrometer. Done on the youngest leaf with fullydeveloped ligule.
Materials and MethodsData 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.
Data collection in the field a bCollecting stem samples for Cutting stem for pith extractionsugar measurement
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.
Measuring stem sugar contentExtracting the stem pith using a Brix Refractometer (r2minicork borer for sugar measurement Handheld Refractometer Model)
Materials and MethodsData analysis●All data collected were analyzed using PROC GLMof SAS package (2003).●To secure error control, coefficient correlationanalysis was done for the mechanisms of resistanceand the damage parameters using canonicalcorrelations.
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.
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).
ResultsFigure 1 Relationship between trichome density and selection index in 120tropical inbred lines
ResultsFigure 2 Representation of the different categories of resistance andsusceptibility in 120 inbred lines
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.
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.
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.
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.