Detection, characterization and control of Xanthomonas spp., causal agents of bacterial leaf spot of tomato in Tanzania.

PhD Thesis

Tomato (Solatium lycopersicum L.) is one of the most important cash and dietary vegetable crops grown by small and medium scale farmers in Tanzania. Demand for fresh market and canning for tomato is high, however, the national average yield remains low mainly due to insect pests and diseases. Bacterial leaf spot (BLS) caused by Xanthomonas spp. is considered to be a devastating disease of tomato worldwide and reported to cause yield losses of up to 45% in Tanzania. Diagnosis of BLS disease and differentiation of the different species and strains of the pathogen involved is usually complicated due to similarities of symptoms expression in tomato plants. The problem is further complicated by presence of other bacterial pathogens associated with tomato, such as Pseudomonas spp., which can induce similar leaf spot symptoms. The BLS disease management in Tanzania has been primarily limited to foliar applications with copper based compounds. However, their efficacy can be limited due to inefficient control once disease symptoms are observed, the rapid development of pathogen resistance to these compounds, risk of polluting the environment, lack of available resistance in comercial cultivars, and sometimes unavailability of suitable commercial products to small scale farmers. document the presence of the causal agents of BLS in seed and plant material in Tanzania develop and apply a PCR based tool for detection of xanthomonads associated with BLS of tomato and evaluate plant extracts and essential oils applied as seed treatment in order to provide an alternative to chemical management of BLS. Therefore, the main focus of the present PhD study was to: To document the presence of the causal agents of BLS in tomato in Tanzania, two surveys were conducted in the Northern, Central and South Highlands of Tanzania between 2008 - 2010. The first survey was conducted in May-July 2008 and aimed at assessing the identity, genetic relationship and pathogenic potential of bacterial leaf spot-causing xanthomonads (BLSX) and other yellow pigmented, Xanthomonas-Vike bacteria (XLB) in tomato seeds from 52 samples of 15 cultivars collected from 45 locations. Characterisation of the BLSX and XLB isolates was based on morphological characteristics on semi-selective agar substrates. Biolog identification system, 16S rRNA sequence analysis and pathogenicity tests on tomato and sweet pepper plants. The

The present PhD study was supported by Danish International Development Assistance (DANIDA) ENRECA project No.731.