Etiology, incidence and management of "leaf yellowing syndrome" of rice (Oryza sativa L.) in Mvomero district
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Date
2020
Authors
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Publisher
Sokoine University of Agriculture
Abstract
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.
Description
Masters Thesis
Keywords
Etiology, Leaf yellowing syndrome, Oryza sativa L., Rice management, Mvomero district