THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN CROP SCIENCE SOKOINE UNIVERSITY OF AGRICULTURE. MOROGORO, TANZANIA. 2016This study was conducted in Mbeya, Morogoro, Kilimanjaro, Shinyanga and Dar es Salaam regions. The overall objective of the study was to establish the magnitude of rice blast disease in Tanzania and its contribution to yield losses. Rice is an important staple food crop and is affected by blast disease with suspected high yield losses of up to 100%. Surveys were conducted in two years (2012 and 2014) to observe disease prevalence of rice blast disease in two different ecosystems in Mbeya, Morogoro, Kilimanjaro and Shinyanga regions of Tanzania. Results showed that the incidence and severity of blast disease varied considerably across the surveyed regions. The highest rice blast incidence (74.38%) and severity (87.62%) were observed in the rainfed ecosystem in Mbeya and Morogoro, respectively. The lowest rice blast incidence (19.38%) and severity (41.5%) occurred in the irrigated ecosystem in Kilimanjaro region. Another study was conducted in the screen-house to assess yield loss caused by Pyricularia oryzae using ten rice varieties Jaribu 220, Supa, Kalamata, Shingo ya mwali, Mwarabu, Mbawambili, Kihogo, IR 64, TXD 306 and TXD 85. The varieties were sown in completely randomised design and inoculated to evaluate the effect of rice blast disease on yield. Results showed that most of the rice varieties were susceptible to P. oryzae and caused grain yield losses of between 11.9 and 37.8% per hectare. Identification and characterization of pathogenic variation of P. oryzae causing rice blast disease were conducted in the laboratory and screen-house at Sokoine University of Agriculture, Morogoro. Ten P. oryzae isolates collected from Shinyanga, Kilimanjaro, Mbeya and Morogoro were tested with a set of ten rice blast differentials viz; IRBLk-Ka, IRBLkm-Ts, IRBLb-w/co, IRBLkp-K60, IRBLz-Fu, IRBLa C, IRBLi-F5, IRBLta2-Pi), St and IRBLt-K59. The experiment was laid out in a completely randomized design with three replications. Twenty one-day-old seedlings grown in pots were inoculated with a P. oryzae spore suspension (2 x 105 ml -1) with the hand sprayer in the evening. Disease assessment was done in seven to ten days after inoculation. Considerable pathogenic variations among the tested isolates were observed. The International rice blast disease differentials IRBLk-Ka, IRBLkp-K60, IRBLa-C, IRBLi-F5, IRBLta2-Pi, St and IRBLt-K59 were resistant to all isolates, except that IRBLb-w/co, IRBLkm-Ts and IRBLz-Fu (Piz) were susceptible. Further studies were conducted in the biotechnology laboratory at Mikocheni Research Institute in Dar es Salaam to characterize P. oryzae using molecular techniques. DNA was extracted from seven isolates of P. oryzae. Polymerase chain reaction (PCR) amplification was carried out in a thermocycler. The results of DNA amplification showed that the isolates collected from Morogoro, Shinyanga, Mbeya and Kilimanjaro regions were genetically similar. The study recommends that research on diversity of P. oryzae pathotype should be conducted further in order to develop rice varieties resistant to the disease.