Etiology, incidence and management of "leaf yellowing syndrome" of rice (Oryza sativa L.) in Mvomero district

Masters Thesis

Introduction Rice (Oryza sativa L.) is a source of food and income to smallholder farmers in the Mvomero district. Recently, however, rice farmers in the Mvomero district have been experiencing a new leaf yellowing syndrome of unknown etiology, which is associated with leaf yellowing, affected plant showing stunting reduced tillering and low yield. This study was undertaken with the aim to establish the causal agent(s) of the leaf yellowing syndrome and then develop sustainable measures for its management. Specifically, the study sought to (i) determine the level of farmers’ awareness of and management practices for the leaf yellowing syndrome (ii) identify the main causes of leaf yellowing syndrome of organic produced rice in Mvomero District and (iii) develop sustainable management strategies for the leaf yellowing syndrome. The second, third and fourth chapters in the dissertation comprise manuscripts in the form of publishable papers which cover the first, second and third specific objectives. Methods With respect to specific objective 1, a field survey was conducted between October – November, 2018 in rice growing areas of Kimambila and Mingo villages in Mvomero District. The aim was to determine the level of famers’ awareness of and management practices for rice yellowing syndrome. A face-to-face interview using a semi-structured questionnaire was conducted to gather primary information from 120 randomly selected farmers. Data were analyzed to determine the distribution of respondents per variable using the Statistical Package for Social Sciences (SPSS) program version 16. A Chi-square tests and regression analysis were computed at (P ≤ 0.05) level of significance, to analyze relationships between variables. Objective 2; this part of the study was undertaken to identify the cause (s) of the yellowing syndrome in Mvomero district. Rice seeds, leaves and soil samples were collected from the villages under study for assessment. Seed health and seed quality tests were determined using blotter method and agar plate method. Isolation of fungi and biochemical tests of the fungal pathogens were conducted from randomly selected symptomatic rice leaves. The soil samples were analysed for physical and chemical properties following recommended procedures. Pertaining to objective 3; a study was conducted at Mingo village in Mvomero district, Morogoro region, Tanzania in 2019 aiming at improving rice performance through enhanced soil fertility using organic fertilizers. A split–split plot experiment in a randomized complete block design was laid out with four replications and two factors. The main factor comprised of four rice varieties that included (i) Mbawambili (ii) Mwangaza (iii) Supa and (iv) Saro while the sub factor consisted of (i) Cow dung manure at the rate of 5t ha-1, (ii) Urea 46% N at a rate of 80 kg N ha-1 (iii) Compost at the rate of 5t ha-1 and (iv) No fertilizer (control). Rice was sown in 2m x 1m plots each with five rice rows at 20cm inter–and intra–row spacing. Data on weather, leaf yellowing incidence, crop growth, yield components and grain yield were collected. Data were subjected to the analysis of variance (ANOVA) using Genstat 16th edition software (VSN International). Means separation test was conducted using Turkey’s Honest Significance Difference Test using P≤0.05 as the level of significance. A correlation analysis was calculated using Pearson product moment correlation coefficient (r) using the Microsoft office-Excel programme to explore the relationship between growth and yield components of rice. Findings Yellowing and stunting of rice were reported as predominant problems by 100% of interviewed farmers. Lack of knowledge on rice leaf yellowing syndrome was reported by all (100%) respondents as the main factor limiting the management of the yellowing syndrome. Bipolaris oryzae and Fusarium moniliforme pathogens in tested seeds were found to be predominant in all tested rice varieties. Soil pH was found suitable for rice production; Nitrogen and Potassium were very low than the suggested standards and therefore deemed deficient. Nitrogen deficiency in both sites was the major causes of yellowing syndrome reported. Disease index results indicated that all the fertilized rice varieties did not develop yellowing and stunting syndrome. However, high incidence of the yellowing syndrome and plant stunting were observed in the unfertilized control plots implying that N deficiency is the main contributing factor to the leaf yellowing and stunting syndrome. There were significant (p<0.01) effect in respect of varieties and fertilizer types used on growth parameters and yields components. Further, regression and correlation analyses showed positive correlation between yield and other growth parameters. Conclusions and recommendations Generally, lack of knowledge on the cause of rice leaf yellowing syndrome was the main factor limiting the effective management of the syndrome in Mvomero district. Several seed borne pathogenic fungi were found associated with rice seed samples from Mingo and Kimambila villages and therefore indicate the possibility of disease episode when such infected seeds are used to raise the next rice crop. Nitrogen deficiency was confirmed to be the major cause of yellowing syndrome. Measures should be taken to increase the availability of macronutrient in the soil especially N and P in order to increase rice productivity. Uses of organic manure together with other sustainable agronomic practice such as fallowing, intercropping, and planting leguminous crops in the field alongside rice varieties will help adjust soil fertility and spar rice yield. Mbawambili which is a commonly used local rice variety does not fair favourably against Supa, Mwangaza and Saro with regard to yield and yellowing syndrome. Therefore a breeding program to incorporate genes for higher yield and disease resistance in the variety should be undertaken in order to increase its disease resistance and yield potential.