Pathogenic and genetic variability in Xanthomonas oryzae pv. oryzae (Ishiyama) Swings et al. and the management of bacterial blight disease
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Date
2013
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Department of Plant Pathology, College of Horticulture, Vellanikkara
Abstract
Bacterial blight of rice is a major threat in rice cultivation causing huge yield loss to the crop. Realizing the practical importance, an investigation entitled on “Pathogenic and genetic variability in Xanthomonas oryzae pv.oryzae (Ishiyama) Swings et al. and management of bacterial blight disease” was carried out during 2006-2011. A series of surveys conducted in 14 locations of three major rice growing districts (Alappuzha, Palakkad and Thrissur) of Kerala during September 2007 to find out the occurrence of bacterial blight disease. High incidence was recorded in Palakkad district. During, the survey, the characteristic symptom of bacterial blight as yellow lesions on the both the margins of the leaf leaving a green area in the centre of leaf were observed. The pathogen causing bacterial blight of rice was isolated from 14 locations of the said districts and their pathogenicity was established. Based on the cultural, morphological and biochemical characters, the pathogen was identified as Xanthomonas oryzae pv.oryzae (Xoo) (Ishiyama) Swings et al. The 14 isolates showed slight variation in their cultural, morphological and biochemical characters. Pathotype studies were conducted in net house using 14 isolates on 20 popular and commonly cultivated rice varieties, six near isogenic lines having different R genes/gene combination and two rice differentials with no genes. It could distinguish the virulence spectrum of the isolates into three groups/categories namely ‘highly virulent’, ‘moderately virulent’ and ‘weakly virulent’ isolates/strains. The study on the kresek symptom also confirmed the above finding. BOX and ERIC-PCR fingerprinting depicted the existence of high level of genetic variability among the pathogen population in the rice growing areas of Kerala. In vitro sensitivity of six bactericides against the four highly virulent isolates of Xoo revealed that tetracycline 250 ppm, tetracycline 100 ppm, tetracycline 50 ppm, streptocycline 250 ppm, streptocycline 200 ppm and Bactrinashak 250 ppm were effective against the pathogen. In vitro sensitivity of different organics and agrochemicals revealed that cow dung extract 2% + vermicompost extract 2%, cow dung extract 2% + P. fluorescens 2%, vermicompost extract 2% + P. fluorescens 2%, vermicompost extract 2 %, copper hydroxide 0.15% were found effective against the pathogen, in that order. The 110 bacterial isolates obtained from rice rhizosphere, rice endosphere and cowdung and vermicompost, screened against the Polpully virulent pathogen could yield six prominent bacterial isolates viz., RE-1, RR-26, RR-53, CB-39, VB-67 and VB-69 and were tentatively identified as Pseudomonas sp. Pot culture experiment, showed that tetracycline 250 ppm, tetracycline 100 ppm, streptocycline 250 ppm, Bactrinashak 250 ppm, tetracycline 50 ppm, bacteria from cow dung (CB-39), bacteria from vermicompost (VB-69), endosphere bacteria (RE-1), rhizosphere bacteria (RR-26), cow dung extract 2% + P. fluorescens 2%, vermicompost extract 2% + P. fluorescens 2% and cow dung extract 2% + vermicompost extract 2%, rhizosphere bacteria (RR-53) and copper hydroxide 0.15% were found best in managing the bacterial blight disease. Seven bacterial antagonists viz., RE-1, RR-26, RR-53, CB-39, VB-67 VB-69 and Pf1 when subjected to compatibility studies against Xoo under in vitro showed 17 combinations, showing synergistic effect in inhibiting the Xoo. In the compatibility study of seven antagonists with nine pesticides, 71 two way combinations were found synergestic effect against Xoo. The compatibility of 13 agrochemicals under in vitro against Xoo, revealed that 50 two way combinations showed synergistic action in inhibiting the pathogen. Three two way combinations showed compatible action in inhibiting the pathogen. In the compatibility of four fertilizers viz., urea, rajphos, muriate of potash and ammonium sulphate showed that five two way combinations proved synergistic action in inhibiting the pathogen. All the seven antagonistic bacteria and 17 agrochemicals showed the compatible reaction in inhibiting Xoo. The seven bacterial antagonists were subjected for various growth promoting characters viz., ‘P’ solubilization, NH3 and HCN production. The six isolates differed slightly for the above characters. All the isolates could produce siderophore and IAA. A few isolates could produce non volatile metabolites. The field study established the most practical finding that the tetracycline 50 ppm, tetracycline 100 ppm, bacterial consortium (RE-1+CB-39), bacterial consortium (CB-39 +VB-69), bacteria from cow dung (CB-39), endosphere bacteria (RE-1), bacteria from vermicompost (VB-69), streptocycline 250 ppm, Bactrinashak 250 ppm, cow dung extract 2% + KAU-(Pf1) 2%, rhizosphere bacteria (RR-26) and KAU-(Pf1) 2% were found promising in managing bacterial blight disease of rice. Thus, apart from bactericides, there was a variety of highly promising organic management possibilities to combat the disease.
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