Occurrence and Distribution of Potato Bacterial Wilt Disease and Variability of its Causal Agent in Southern Highlands of Tanzania

Abstract

A survey was carried out in three potato growing Districts of the Southern Highland Zone of Tanzania to determine incidence and severity of bacterial wilt of potato caused by Ralstonia solanacearum and characterizing the strains of the causing pathogen. Ten villages from each district and five fields from each village were surveyed. The fields and plants were randomly selected and bacterial wilt symptoms identified. Wilt incidence was recorded based on number of plants showing symptoms and expressed as percentage of the total number of plants observed. Disease severity was recorded by severity score as described by Horita and Tsuchiya (2001). Asymptomatic and symptomatic tubers samples were collected. Asymptomatic for the detection of latent infection whereas symptomatic for characterization of R. solanacearum respectively. The results showed the highest incidence in Mbeya Rural (27.7%) followed by Rungwe (26.7%) and lastly Njombe (19.4%). The highest and lowest bacterial wilt severities recorded were 4.0 and 2.3 both in Mbeya rural fields. Asymptomatic tubers were (18.8%) latently infected after being tested by NCM-ELISA. Molecular characterization confirmed the isolates to be R. solanacearum by species specific and grouped to phylotype 2 by phylotype specific PCR respectively. Upon isolation in TZC medium twenty of thirty isolates were virulent whereas ten were avirulent. Pathogenicity test showed the isolates to be highly virulent on potato and tomato and slightly virulent on eggplant. No symptoms in tobacco and pepper were produced and therefore grouped as race 3. The isolates were grouped to biovar 2 as they oxidized the tested carbohydrates but not the sugar alcohols. The disease was observed to occur in all three surveyed districts. Use of certified and disease-free seeds, area for seed production, development of resistant varieties and training on symptoms and bacterial wilt control were recommended.