MSc

Grain discolouration is becoming a major problem of rice cultivation in recent
years in India. This disease is prevalent in major rice growing tracts of Kerala. Since
the disease directly affects the grains, it causes considerable yield loss. The extent of
loss mainly depends on variety and climatic conditions. In Kerala, no systematic
study has been conducted to identify the pathogens of grain discolouration which is
essential for the effective management of the disease. Hence, this study was
conducted to identify the pathogens causing grain discolouration of rice in Kerala and
also to evaluate different management strategies.
Purposive sampling survey was conducted in three rice growing districts of
Kerala viz., Palakkad, Thrissur and Malappuram, covering 14 locations to study the
occurrence of disease in farmers field. The incidence and severity of grain
discolouration varied with the locations. Incidence varied from 6.8 to 100 per cent
and severity varied from 21.47 to 95.49 per cent. Predominant rice varieties Uma and
Jyothi were severely affected by the disease in all the locations.
The common symptoms observed were brown to black discolouration of
varying degrees ranging from brown to black spots to discolouration of entire glumes.
The other symptoms were ash, cream and pink discolouration of glumes. The
symptoms like brown blotches with creamy centre on the glumes and brown
discolouration of glumes which later turned to cream colour from tip downwards
were also noticed. Discolouration and deformation of the kernels were also observed
in some cases. In severe infection chaffiness was observed.
On isolation of the mycoflora associated with the grain discolouration, 62
fungal isolates were obtained. Pathogenicity of these fungal isolates, obtained from
various locations was tested by artificial inoculation on susceptible variety Jyothi.
Among these, 47 isolates were found pathogenic.
The cultural and morphological characters studied revealed that these 47
isolates mainly comes under four genera viz., Curvularia, Bipolaris, Alternaria and
Fusarium. Representative virulent isolates of these four genera were identified from
Rajiv Gandhi Centre for Biotechnology (RGCB), Thiruvananthapuram as Curvularia
lunata, Bipolaris oryzae, Alternaria alternata, Alternaria padwickii and Fusarium
equisetii through molecular characterization. These pathogens were used for further
studies.
In vitro evaluation of biocontrol agents, Pseudomonas fluorescens and
Trichoderma viride against the pathogens of grain discolouration revealed that both
the biocontrol agents inhibited all the five pathogens significantly compared to
control. T. viride inhibited all the pathogens (60-85%) significantly higher than P.
fluorescens (12-16%).
Among the seven fungicides tested in vitro, tebuconazole 250 EC (0.15%) and
propiconazole 25 EC (0.1%) completely inhibited all the five pathogens compared to
trifloxystrobin 25% + tebuconazole 50% WG which inhibited the growth of four
pathogens completely and one pathogen by 87 per cent. The contact fungicide
mancozeb 75 WP and the combination fungicide mancozeb 63% WP + carbendazim
12% WP inhibited two pathogens completely. Considering the overall performance,
tebuconazole 250 EC and propiconazole 25 EC ranked first in their efficacy against
Curvularia lunata, Bipolaris oryzae, Alternaria alternata, Alternaria padwickii and
Fusarium equisetii.
The biocontrol agents and fungicides were evaluated in the field, against grain
discolouration of rice in kharif and rabi seasons of 2017-18 at RARS, Pattambi.
Prophylactic application of talc based formulation of P. fluorescens as seed treatment
(10g kg seed-1) + soil application (2.5 kg ha-1) + foliar spray (10g l-1) at booting or as
seed treatment + soil application + foliar spray at 50% flowering was effective for the
management of grain discolouration of rice and thereby improving the yield. The
application of T. viride as seed treatment (10g kg seed-1) + foliar spray (10g l-1) at
booting or as seed treatment + foliar spray at 50% flowering was also equally
effective for the management of grain discolouration of rice and improving the yield.
Among the fungicides tested in the field, tebuconazole 250 EC (0.15%) and
propiconazole 25 EC (0.1%) were significantly superior in reducing the disease and
increasing the yield followed by mancozeb 75 WP (0.3%), mancozeb 63% WP +
carbendazim 12% WP (0.2%) and carbendazim 50 WP .
The results of the present study revealed the wide spread occurrence of grain
discolouration of rice in major rice growing districts viz., Malappuram, Thrissur and
Palakkad of Kerala in all the predominant rice varieties. The major pathogens
associated with the disease were identified as Curvularia lunata, Bipolaris oryzae,
Alternaria alternata, Alternaria padwickii and Fusarium equisetii. The biocontrol
agents, Pseudomonas fluorescens and Trichoderma viride were effective for the
management of grain discolouration. The fungicides tebuconazole 250 EC (0.15%)
and propiconazole 25 EC (0.1%) followed by mancozeb 75 WP (0.3%), mancozeb
63% WP + carbendazim 12% WP (0.2%) and carbendazim 50 WP were effective for
the management of the disease.