Status and management of rice blast disease caused by Pyricularia oryzae cav. In upland rice in selected regions in Tanzania

PhD Thesis

This study aimed to enhance effective rice blast disease management by the establishment of the current status of the disease, its effect on yield of selected upland rice genotypes and the use of environmentally friendly methods such as bio-agents and hot water seed treatment. Two surveys were conducted in 2017/2018 and 2018/2019 rice growing seasons to investigate farmers’ knowledge of rice blast disease and its management and to establish the incidence and severity of rice blast disease in farmers’ rice fields in Morogoro and Tanga regions. Data were collected through face to face interviews using semi structured questionnaire. Rice blast disease assessment in farmers’ fields was conducted by diagonal transect walk using 1.0 x 1.0 quadrant. Results indicated that about 46.3% of the farmers interviewed were not aware of the cause and means of spread of rice blast disease. The majority of farmers (92.3%) planted local upland rice varieties and about 54.0% did not apply any management method due to lack of knowledge, inability to afford the cost of buying fungicides and unavailability of effective blast disease control measures. Results from the surveys indicated that the highest rice blast disease incidence and severity were recorded in the 2017/2018 rice growing season. In this season, Mvomero and Korogwe districts had higher blast disease severity of 100% and 98.8%, respectively, than Morogoro Rural (88.1%) and Muheza (87.3%) districts. In vitro evaluation of microbial agents, indicated that Trichoderma asperellum and Bacillus subtilis had over 75% inhibition of radial growth of P. oryzae compared to fungicide Linkimil 72 WP (21 - 23%) and the control (0%). In vivo evaluation showed that rice blast disease incidence was reduced by 70% in plants treated with T. asperellum followed by 51.5% in B. subtilis treated plants and 26.5% in Linkimil 72 WP treated plants. There was a decrease in blast disease severity by 35.6% in rice plants treated with T. asperellum and 29.1% in B. subtilis treated rice plants, suggesting that T. asperellum and B. subtilis used in this study had high antagonistic capacity against P. oryzae. Completely randomized design (CRD) experiments in the laboratory and screen house were conducted to investigate the efficacy of T. asperellum, B. subtilis and hot water seed treatments on rice blast disease. Results indicated significant reduction (P ≥ 0.05) of the percentage of infected rice seeds when T. asperellum, B. subtilis and hot water treatment were used. Rice blast disease incidence and severity were significantly reduced (P ≥ 0.05) on rice seeds treated with B. subtilis. Therefore, the use of microbial agents has the potential for effective management of rice blast disease. Field experiments were conducted twice in three locations to determine the effect of rice blast disease on grain yield of upland rice genotypes. A randomized complete block design experiment (RCBD) was used in a paired block with P. oryzae naturally inoculated and fungicide sprayed blocks. Results showed that the effect of rice blast disease on grain yield of upland rice genotypes depended on disease pressure, which differed across rice genotypes, locations and rice growing seasons. In general, the disease caused 26.6 to 52.1% grain yield reduction in the two rice growing seasons. Improved rice genotypes such as NERICA 7 and WAB 450 were also found susceptible to rice blast disease in the study areas. In general, this study, gives highlights of the incidence and severity of rice blast disease, its management using bio - agents (T. asperellum and B. subtilis) and the effects of the disease on grain yield of selected upland rice genotypes grown under rain-fed conditions in Morogoro and Tanga regions. Such information is important in designing rice blast disease management options.

International Centre of Insect Physiology and Ecology (ICIPE) Grain IPM for East Africa project, Cooperative Agreement No. AIDOAA- L-15-0000.