Rice leaf blast (Pyricularia oryzae) cavara pathogen distribution, cultivar resistance and yield loss in Zanzibar

Thesis 2019

Rice leaf blast is a rice disease caused by fungus Pyricularia oryzae Cavara. The disease is widely spread in all rice ecological systems in Zanzibar viz; rainfed upland, rainfed lowland and irrigated. In Zanzibar rice is a staple food crop that serves 3.1 millions people but the crop is highly infected with rice leaf blast disease a major producton constraint. A comprehensive survey was conducted in six districts of Zanzibar islands covering three districts in Unguja viz: North A, Central and West and Pemba districts were Micheweni Chake chake and Wete. Survey was conducted during two cropping seasons 2014-2015 and 2015-2016. Purposive sampling technique was used to select districts that carries three agro-ecologies viz; rainfed upland, rainfed lowland and irrigated which were considered as treatments. Two islands (Unguja and Pemba) were considered as blocks. Overall objective of this study was to establish rice genotypes with effective rice blast resistant genes and environmental conditions for increased rice productivity in Zanzibar and confirm current losses caused by (Pyricularia oryzae) pathogen. During survey, incidence, severity, prsevalence and control measures taken by farmers were recorded for the six districts within the islands. Results showed that the disease incidence and severity of rice leaf blast disease varied considerably across the surveyed districts. The highest rice blast incidence (68.57%) and severity (20.70%) were recorded in Unguja Island while in Pemba island incidence and severity were registered (46.85%) and (18.53%) respectively. Low disease rate were recoded within majority of farmers in Pemba using cultural practices as a disease reductiona measure. It was found that cultural practices in combination with fungicides were more effective in rice leaf blast disease control. Chake chake district registered lowest rice leaf blast incidence (35.96% and severity (16.58%), while the highest diseaseiii was record in North A and Central but did not differ significantly (P≤0.05). On the three agro-ecologies irrigated agro-ecology registered lowest rice leaf blast disease rate while highest disease rate was recorded in the rainfed upland egro-ecology. Disease incidence records were higher in 2015-2016 than 2014-2015 cropping season. Study was conducted in the laboratory of mycology in the Department of Crop Science and Horticulture involving isolation of pathogen from rice leaf samples showing rice leaf blast disease (Pyricularia orayzae) that was used for identification and molecular characterization of the pathogen. Three types of medium were used for culturing of Pyricularia oryzae and compared mycelia growth rates and other culture characteristics. Petri dish inoculated with Pricularia oryzae were laid in randomized complete randomized design and replicated three times. Data were collected at 4, 8 and 12days. Results showed that media differ signifycantly (p≤ 0.001) in influencing growth of fungi. Similarly, medium interacted significantly (p≤ 0.001) with days after inoculation. Oatmeal Agar Medium was suitable for mycelia growth followed by Potato dextrse agar medium while patato carrot agar was the lowest. For molecular analysis six strains were identified and characterized viz ZNZ/2017/CENTRAL, ZNZ/2017/CHK_2, ZNZ/2017/MICH, ZNZ/2017/NORTH_ A2, ZNZ/2017/NORTH_ A8 and ZNZ/2017/CHK_1. Using 19 rice blast differential genotypes and ten traditional varieties their seeds were sown in the screen house in completely randomized design and inoculated with the spores suspension of Pyricularia oryzae to evaluate the effect of rice blast disease. Disease symptoms appeared after two weeks after inoculation and data on disease severity were collected at the interval of ten days for sixty days. During the experimental duration temperature in the screen house ranged from 21.5 to 38.4 o C with the average of 27.4 o C. Relative humidity ranged from 31.626 to 100 with the average of 71.11. Results showediv that 13 rice blast differential genotypes were identified as resistant viz; IRBL11-Zh (Pi11(t), IRBL12-M (Pi12(t)), IRBLa-C (Pia), IRBLb-IT13[CO] (Pib), IRBLkh-K3 (Pik- h), IRBLkp-K60 (Pik-p), IRBLkp-K60[CO] (Pik-p), IRBLta-Me[CO] (Pita), IRBLta2-Pi (vbfchfdj), IRBLta2-Re[CO] (Pita-2), IRBLz5-Ca (Piz5), IRBLzt-IR56[CO] (Piz-t) and Moroberekan (Pi5(t), Pi7). Most of traditional varieties were susceptible to the disease. The lowest total disease severity and total area under disease progress curve were 8.63 % and 271.5 % respectively were recorded in the differential variety Moroberekan. The highest total disease severity and total area under development progress curve recorded on CO39 was (95.05%). Rice yield loss assessment was conducted in Zanzibar at two locations viz; Pemba Island and Unguja island during two rce cropping seasons of 2016 – 2018 using t-test. There were two sets of plots, one set its plots were protected by spraying with fungicide to prevent rice leaf blast disease and another set its plots were not protected. Results reported Pyricularia oryzae cause higher rice grain yield loss in Unguja Island at 70.670 % and in Pemba Island was 56.671 %. Unguja island had higher grain yield loss due to high disease severity than in Pemba island. Application of fungicide is an alternative measure of rice blast resistant variety in controlling rice leaf blast disease. From the results, in correlation coefficient we concluded positive correlated from two relationship between protected plots and unprotected plot that the increases of severity the increase yield loss and vice versa.