Identification and Characterization of Causative Agents of Brown Leaf Spot of Cassava and Phenotypic Response of Elite Cassava Genotypes to Infection by the Pathogens

Abstract

Cassava is one of the most important food crops globally and is a major staple food for more than 700 million people across the tropical and sub-tropical world. Cassava is adversely affected by diseases (caused by viruses, bacteria, nematodes and fungi), pests and adverse environmental constraints. Current research activities in cassava diseases are focused more on viral and bacterial diseases than on fungal diseases, yet phytopathogenic fungi play an important role in causing devastating disease epidemics leading to significant annual yield losses. One of the most important fungal diseases is the cassava brown leaf spot (BLS) disease. This study aimed at identification and characterization of the causative agents of BLS disease in Kenya and the reaction of cassava genotypes to infection by the pathogens. Experimental materials were sourced from the Kenya Agricultural and Livestock Research Organization (KALRO) Biotechnology Research Center, KALRO Kakamega and from a confined field trial of the Virus Resistant Cassava for Africa (VIRCA) project located at KALRO Kandara in Murang’a County. Fungi were isolated and purified on antibiotic-amended PDA media from the symptomatic leaf samples. Identification of the fungal pathogens was based on cultural and morphological characteristics of pure fungal cultures coupled with molecular characterization of individual pathogens. Results of the study delineated three pathogens from the genera Colletotrichum, Cladosporium and Alternaria, working in synergism to produce brown leaf spot disease as observed in the fields. The three fungal pathogens were used, in combination, to challenge five farmer-preferred cassava genotypes TME 204, TME 14, TME 7, Ebwanatereka 1 and Ebwanatereka 2. Symptoms were observed over a period of 56 days (eight weeks) at intervals of seven days, after which disease progress was determined. Analysis of Variance (ANOVA) was carried out using GenStat software, 15th Edition while molecular data was analyzed using Geneious Prime and MEGA 11 softwares. The cassava plants had varied responses depending on genotype. The highest area under disease progress curve (AUDPC) was 178.5 in TME 204 and lowest at 103.8 in Ebwanatereka 2. With regard to symptom severity, Ebwanatereka 2 exhibited a relatively slow response to infection compared to the other genotypes throughout the assessment period. On the other hand, TME 204 maintained a high infection response therefore indicating high level of susceptibility to cassava brown leaf spot disease. Findings of this study will add to the knowledge gap in the management approaches to cassava brown leaf spot disease. More research should be carried out to identify sources of resistance to the disease.