Genetic diversity and resistance to cassava brown streak disease in central Uganda

Abstract

Cassava (Manihot esculenta Crantz), a common food security crop in many sub-Saharan African countries is being threatened by cassava brown streak disease (CBSD) which causes up to 100% yield loss in susceptible varieties. Cassava brown streak disease is caused by two distinct Ipomoviruses namely Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV Cassava brown streak disease causes symptoms characterised by chlorotic patches on leaves, dark brown "streaks" on stems, cracked constricted corky, yellow-brown necrotic root tubers of cassava. However, variability of symptoms exists within cassava genotypes and CBSV strains depending on the environmental conditions. Currently in central Uganda, there is limited information on CBSV genetic and symptom diversity, as well as sources of resistance to CBSV. Yet development of CBSD control strategies requires knowledge of CBSV genetic diversity, virus-host interaction and availability of resistant cassava germplasm to CBSD. The objective of this study was therefore to 1) assess genetic diversity and distribution of cassava brown streak disease causal viruses in central Uganda. 21 unravel the symptom diversity of CBSD causal viruses on selected cassava genotypes. 3) identify potential sources of resistance to cassava brown streak disease in central Uganda. Symptomatic leaf samples collected from the four districts in central Uganda were used for total RNA extraction using CT AB protocol. The purified samples were subjected to PCR amplification of specific DNA sequence by in vitro DNA synthesis done as prescribed by Mullis and Faloona (1988). PCR products were separated using electrophoresis and the sequencing of PCR products was done in Netherland at Macrogen laboratory and the sequenced products were edited using Edit Seq in DNA Star, aligned using clustal logarithm MEGA 6 software and coat protein nucleotide sequence data analysed using AMOVA. The results revealed that there was variation in nucleotide sequence and the coat protein among and within CBSV strains with isolates clustering into two groups. Additionally, CBSV was the dominant strain, followed by UCBSV and last mixed infection among 18 out of 20 symptomatic leaves sampled. Symptoms diversity and sources of resistance were identified by inoculating disease free cassava grown in screen house with CBSV strains singly and in combinations using chip bud method of grafting. Symptom diversity was identified based on the dominant symptoms from symptoms appearance at 1 MAI to 3 MAI. For resistance, incidence and severity was assessed. Later severity scores were used to compute area under disease progress curve (AUDPC). Findings showed that there was significant difference (P<0.001) in reaction of cassava genotypes to CBSV viral strains. In addition, significant variety x viral strains interactions (P<0.001) was observed. The highest CBSD incidence and severity was recorded from Kwatampola and the lowest in NASE14. Overall, NASE 14 and NAROCASS were resistant while Aladu, NASE12, Njule, Bam_unanika and Bukalasa were moderately susceptible. Symptom diversity and severity was scored visually from symptom appearance and continued at monthly intervals for three months using a scale of 1 - 5. Findings showed Variation in symptoms of CBSD existed on leaves and stems of cassava genotypes inoculated with CBSV, UCBSV and CBSV+UCBSV strains. Infact, the symptoms varied from chlorotic spots on leaf tips to mild chlorosis, mottling of secondary to primary veins and mild stem streaks. It was concluded that CBSV and UCBSV were dominant strains of CBSV. Additionally genotype NASE 14 was most resistant to CBSD. Accordingly CBSD symptoms varied from chlorotic spots to mild stem streaks. This study has an implication on the management of CBSD in Uganda. However another study to assess the relationship of CBSV viral load, CBSD symptom severity and resistance of cassava genotypes needs to be conducted.