PATHOBIOLOGY OF XANTHOMONAS ORYZAE Pv. ORYZAE, THE CAUSAL ORGANISM OF BACTERIAL BLIGHT OF RICE

Abstract

A series of experiments were conducted to study the pathobiology of Xanthomonas oryzae pV. oryzae (Xoo) the causal organism of bacterial blight of rice. Studies were carried out in field based assessment of Bacterial Leaf Blight (BLB) incidence and severity in all rice growing zones of Pakistan, biochemical characterization and molecular confirmation of KP isolates, determination of best method of inoculation, KP races using International Rice Bacterial Blight (IRBB) lines, exploitation of rice cultivars and land races for resistance to BLB. Survival mechanism and source of primary inoculum of Xoo and comparison of the effect of planting dates on BLB incidence and yield perfonnance of selected rice cultivars in multi location trails were also undertaken. In KP, disease incidence ranged between 35-80%, whereas severity was between 3 to 7% during cropping season of 2005-2007. In Punjab, Sindh and Balochistan, incidence ranged between 37-75%, 12-47% and 12-22%, respectively. While severity ranged between 3-6%,0.7-3% and 0.3-1 %, respectively, in these areas. One hundred and twenty five Xoo isolates were collected from twelve districts of different agro-ecological zones of KP for characterization through a number of biochemical tests. Some of the isolates did deviate from the nonnal pattern and showed over lapping results. Therefore, identity of all the candidate Xoo isolates was verified through Polymerase Chain Reaction (PCR) with specific primers. To determine the best method of inoculation of Xoo, a bacterial suspension containing 10-8 colony forming units (cfu) was used for inoculating six commercial rice cultivars invitro and in-vivo. Three methods of inoculation i.e. clipping, pinprick and brush were evaluated. Clipping method resulted in much more lesion length than either the pinprick or brush method. Eighty isolates obtained during 2005 to 2007 from various rice growing zones of PK, were grouped using IRBB lines into six races by scheme adopted by Habgood (1970) and modified by Limpert and Muller (1994) nomenclature procedure. These races were ix distributed at almost all locations of PK. However, Race-l and Race-6 showed its dominance at all locations and were found the most prevailing races of PK. Race-3 was confined only to seven locations of rice growing areas of PK. Race-6 represented three isolates of Peshawar district alone. A total of thirty four rice cultivars were evaluated against KP races for resistance. Seventeen showed resistance or moderate resistance to race-I, while 18, 13, 14, 15 and 09 exhibited resistance or moderate resistance to race-2, race-3, race-4, race-5 and race-6, respectively. Varieties, Bas-2000, Shaheen Basmati, Malhar-346, Khushboo-95, Bas2008 and Jajai-77 proved to be resistant or moderately resistant to all the six races under study. Race-6 was the most virulent amongst all the KP races as 15 cultivars exhibited susceptible reaction. Conversely, race-l was found to be the least virulent in terms of response to artificial inoculation. For post-harvest survival of Xoo, 50 samples of field water, seven most common weeds of rice, plant residues and seed were assayed for recovery of the pathogen. Field water, plant residues and seed samples yielded 25%, 37% and 31 % of the causal bacterium, respectively. On the other hand, recovery of Xoo from all tested weeds was 7%. Both Cyperus difforrnis and Dactyloctenium aegyptiurn, collected from Charsadda, tested positive for isolation of pathogen. Similarly, successful recovery of Xoo was recorded from Echinochloa crus-galli and Leersia hexandra, both were collected from Peshawar. Additionally, Cyperus rotundus sampled from Swat also tested positive for recovery of the causal bacterium. Two years (2007 and 2008) data regarding the effect of planting dates on BLB revealed that sites and varieties significantly affected bacterial leaf blight, tillers planrl, grains panicle -\, 1000 grains weight, panicle length (em) and grain yield at both locations. A similar trend of high BLB was recorded for variety Fakhre Malakand followed by variety Basmati-385 during 2007 and 2008. Interaction of sowing dates x varieties indicated increased BLB Dilrosh-97 and Swat-l till 15th June but further delay did not increase disease incidence.