Characterization of Ralstonia solanacearum infecting tomato (Solanum lycopersicum L.) along the Coast of Tanzania and reaction to lines bred for resistance

A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN CROP SCIENCE OF SOKOINE UNIVERSITY OF AGRICULTURE, MOROGORO. TANZANIA. 2017Ralstonia solanacearum is a bacterium recognized as a major wilt causing phytopathogen with an unusual broad host range. This study aimed at determining the distribution of the bacteria along the coast of Tanzania and environmental factors influencing incidence and disease severity. It also aimed to define the population structure of the strains in terms of its biology and phylogenetic make up in order to effectively screen for variety tolerance. The study revealed that contaminated irrigation water and temperature are the key environmental factors determining incidence and severity. Bacterial wilt was found in Tanga, Pwani, Unguja ukuu and Morogoro where temperature varied within the range of 25°C to 32°C. R. solanacearum was detected in river water by DAS-ELISA. At a temperature of 18 20°C, no infected plants were found in Mtwara and Lindi. Due to high diversity in its genotypic and phenotypic traits, Ralstonia solanacearum is considered a species complex. Strains from different locations vary in virulence, hence its control using resistant varieties is usually strain specific. The study also included characterization of R. solanacearum isolates in order to determine strains present in the selected study areas. The strains were tested for their ability to utilize hexose sugars and alcohols and found to be biovar 3. Multiplex PCR showed that strains found along the Tanzanian coast are phylotype I which are predominantly Asian. A representative strain from each region was chosen and inoculated on tomato lines from AVRDC bred for resistance to R. solanacearum. Lines AVTO 1010, 9221, 0301, and 1226 had varying levels of resistance against different strains but were significantly resistant to the pathogen compared to the susceptible variety Cal J. These lines can be disseminated appropriately for farmers' seed testing and used to improve yield. The collection of strains can be used for selection purposes when screening for available local sources of resistance and mapping genes strongly linked to strain virulence