Rice leaf blast (Pyricularia oryzae) cavara pathogen distribution, cultivar resistance and yield loss in Zanzibar
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Date
2019
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Sokoine University of Agriculture
Abstract
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.
Description
Thesis 2019
Keywords
Rice leaf blast, Pyricularia oryzae, Zanzibar, Characterization, Correlation, Inoculation, Conidia