Genetic Diversity and Resistance to Bacterial Blight Among Kenyan Cassava (Manihot Esculent Crantz) Germplasm

Abstract

Cassava (Manihot esculenta Crantz) has high carbohydrate content and is grown in Africa as an alternative staple crop for food, feed, and bioethanol production. However, the production of cassava encounters limitation due to abiotic and biotic stresses. Of these, the bacterial blight of cassava (CBB) affects most cassava germplasm grown by farmers. Breeding for disease resistance and knowledge of the genetic constitution of the germplasm is key in improving cassava production worldwide. This study aimed to investigate the genetic variation present in the 15 cassava germplasm of Kenya using morphological and molecular markers, while assessing their reactions to CBB. The study involved field and greenhouse experiments. Field trials were carried out at Kakamega and Kiboko research facilities of Kenya Agricultural and Livestock Research Organization (KALRO) using a randomized complete block design (RCBD) with three replications. The cassava plants were spaced 1 m apart within plots, which were spaced 1.5 m apart. Every plot comprised 10 plants, with six plants tagged for data collection. Data for morphological traits were collected at early growth stage, mid-growth stage (reproductive phase), and late growth stage (first branching to maturity). Total DNA was extracted from the 15 cassava varieties using a modified Cetyl Trimethyl Ammonium Bromide (CTAB) protocol. Then, 30 Start Codon Targeted (SCoT) markers were utilized to screen for polymorphism, and seven polymorphic SCoT primers were selected for subsequent analysis. In this study, SCoT markers were preferred because they have a wide genomic coverage. In addition to the field experiments, a greenhouse experiment was established at KALRO Kabete using a completely randomized design (CRD) replicated three times. Differential responses to CBB were observed in both field and greenhouse conditions at 30- and seven-days interval, respectively. The study identified seven clusters based on morphological traits and five clusters based on molecular markers, indicating genetic diversity within the cassava germplasm. Notably, four CBB-resistant varieties independently clustered in sub-cluster A of cluster I using molecular markers, whereas morphological markers did not differentiate between resistant and susceptible varieties. This suggests that morphological traits alone are not indicative of CBB resistance, and resistance may be associated with genetic factors. The identification of CBB-resistant varieties plays a crucial role in developing and deploying improved cassava genotypes, ultimately benefiting farmers and enhancing food security in Africa and beyond.