RESISTANCE TO THE SPOTTED STEM BORER AND AFRICAN MAIZE STEM BORER IN TROPICAL MAIZE

Abstract

In sub-Saharan Africa (SSA), maize (Zea mays L.) is the staple food for about 50% of the population. However, lepidoptera stem borer poses a major threat to sustained food sufficiency in SSA causing annual yield losses of 15% and particularly in Kenya where they cause losses estimated at 13.5%. The potential to manage insect pests using host-plant resistance exists, but has not been adequately exploited. The goal of this study was to determine the resistance levels in tropical maize to the African (Busseola fusca Fuller) and the spotted stem borer (Chilo partellus Swinhoe) to support breeding efforts for improved maize productivity. Specific objectives were; i) Determine the variability in resistance to spotted and African stem borers in tropical maize; ii) Investigate the mechanisms of resistance in CIMMYT tropical maize inbred lines and; iii) Map the quantitative trait loci (QTL) associated with resistance to C. partellus and B. fusca stem borers using a tropical maize population. Two hundred and ninety five (295) germplasm which included 120 inbred lines, 75 landraces, 100 hybrids and open pollinated varieties (OPVs) were evaluated for two seasons at the Kenya Agricultural Research Institute (KARI)-Kiboko and KARI-Embu field stations. Data were recorded on leaf damage on a 1-9 scale, number of stem borer exit holes, stem tunnel length (cm) and grain yield. A selection index was computed using the damage traits leaf damage, number of stem borer exit holes and cumulative tunnel length to categorize genotypes into resistant and susceptible categories. Germplasm with selection index of below 0.8 were regarded as highly resistant, 0.8-1.00 as moderately resistant, 1.0-1.2 as moderately susceptible and above 1.2 as highly susceptible. Evaluations for mechanisms of resistance were carried out for two seasons at KARI-Kiboko on 120 inbred lines. Data were recorded on leaf toughness, stem penetrometer resistance, trichome density and pith sugar content. To map the QTL for stem borer resistance, a population of 203 F2:3 individuals were developed between 2009 and 2011. Field evaluations were carried out at six sites, three for each stem borer species. Data were recorded on leaf damage, number of stem borer exit holes and cumulative tunnel length as putative stem borer resistance traits. The mapping population was genotyped with 152 single nucleotide polymorphism (SNP) molecular markers. Phenotypic data were subjected to ANOVA using PROC GLM of SAS 2007 and means separated using Fisher’s protected LSD (P<0.05). Variability for resistance to maize stem borers was identified in the genotypes evaluated. Top ten highly resistant landraces against C. partellus were GUAT 1050, GUAT 280, GUAT 1093, GUAT 1082, GUAT 1014, CHIS 114, GUAT 1034, GUAT 1038, CAQU 321 and GUAN 34. Topmost highly resistant commercial hybrids and OPVs to C. partellus were DH01, PH1, ECA-STRIGOFF-VL-102-#-, KDV1-1-#, KDV1-2- #, and PH3253 while, KDV1-3-#, EEQPM-8-EA-#, DH02, KDV1-2-#, DKC8053, POOL15QC, KDV1-1-#, WH403, EEQPM-9-EA-#, and PH4 were highly resistant to B. fusca among others. Open pollinated varieties KDV1-1-#, KDV1-2-#, KDV1-3-#, EEQPM-8-EA-#, POOL15QC and EEQPM-9-EA exhibited high resistance to both stem borer species. Most CIMMYT MBR lines exhibited high resistance levels, with CKSBL10008, CKSBL10005, CKSBL10025, CKSPL10273 and CKSBL10027 being the top five highly resistant lines to B. fusca and CKSBL10039, CKSBL10025, CKSBL10026, CKSBL10014 and CKSBL10004 the top five highly resistant to C. partellus. Dual and high resistance to both stem borer species was found in CIMMYT MBR lines CKSBL10025, CKSBL10026, CKSBL10027, CKSBL10034, CKSBL10014 and CKSBL10039 among other inbred lines. Trichome density was the best mechanism in discriminating genotypes into resistant and susceptible categories, followed by leaf toughness and stem sugar content in that order. Number of stem borer exit holes and cumulative tunnel length were the most consistent traits in assessing resistance. A linkage map spanning 1248.01 cM on 10 chromosomes with an average 8.21cM was constructed. Several QTL for putative resistance traits were detected on chromosomes 1, 2, 3, 4, 5, 6, 7 and 9 based on data from both individual sites and different species. In the combined B. fusca sites analysis, one QTL for stem tunnelling was revealed on chromosome 4 (LOD 2.86) while in the C. partellus combined sites, one QTL for reduced stem tunnelling on chromosome 4 (LOD 2.81), and another QTL for reduced number of borer exit holes was revealed on chromosome 5 (LOD 2.53). Individual sites analyses revealed five QTL for reduced stem tunnelling, three for stem exit holes and two for leaf damage. Phenotypic variances explained by each QTL ranged from 6 to 10% suggesting a need to validate these QTL using a larger population and in different environments. Variability for resistance against maize stem borers was identified and germplasm identified as highly resistant are recommended as novel sources of resistance for stem borer resistance breeding in SSA. Information on resistant commercial hybrids and OPVs should to be disseminated to farmers in the relevant ecologies for adoption to curb grain yield losses. Trichome density, leaf toughness and stem sugar content could be adopted as satisfactory indicators of resistance mechanisms and used for pyramiding of resistance genes for high and durable resistance. Quantitative trait loci for the three putative resistance traits were detected in the CIMMYT tropical population studied. Overall, this study identified new sources of resistance to spotted and African stem borers in tropical maize germplasm that could be used as new varieties and/or used as sources of resistance in breeding for resistance to stem borers.