Monitoring, Charactelization and management of Xanthomonas oryzae pv. Oryzae, the cause of bacterial leaf blight (BLB) disease of rice in Punjab, Pakistan

Abstract

The rice crop is affected from more than 76 diseases, among which Bacterial leaf blight (BLB), incited by Xanthomonas oryzae pv. oryzae (Xoo) is incurring significant yield losses in Asia, including Pakistan. In the present study, 19 major rice-growing districts of Punjab were surveyed in 2009, 2010, 2012 and 2013 and infected rice leaf samples were collected for the Xoo isolation. The incidence of BLB disease was recorded maximum in major Basmati rice growing districts i.e., Hafizabad, Gujranwala and Sheikhupura followed by Mandi Bahauddin during the four years. The maximum disease severity was observed in Hafizabad, Sheikhupura, Gujranwala and Sialkot followed by Nankana. The 300 Xoo isolates was confirmed through colony morphology, biochemical tests and pathogenicity on IR-24 (a highly susceptible rice line from IRRI, Philippines). The virulent pattern of 300 Xoo isolates was assessed on 6 rice IRRI-differentials for BLB disease. The present study is the first report to characterize a large population of Xoo isolates into pathotypes. The virulence level was assessed by using the principal component and cluster analysis. The 300 isolates were assorted by 29 pathotypes (Pt1-29). The highly virulent Pt-1 constituting 39 Xoo strains, widespread in 12 districts, poses the serious threat to Basmati rice and IRRI-differentials. The resistant potential of 26 International Rice Bacterial Blight (IRBB) lines (10 Near Isogenic lines (NILs) and 16 pyramiding lines) against 29 prevailing Xoo pathotypes, 54 mutant lines in M6-M7 generations, 72 in M3-M4 generations of the mutated rice population of Basmati Super induced by the gamma radiation (15kr, 20kr & 25kr) along with parent Basmati Super and 5 approved commercial Basmati varieties were evaluated under artificial inoculation field conditions. None of the NILs or gene pyramids provided complete protection against all the pathotypes. However, Xa21 alone expressed resistance against 93% Xoo pathotypes followed by xa13, Xa7, Xa10 and Xa14 conferring resistance to 79, 72, 72 and 72% pathotypes respectively. Among gene pyramids, IRBB-57 (Xa4, xa5, Xa21) exhibited incompatible reaction against 93% Xoo pathotypes followed by IRBB-64 showing resistance against 86% Xoo pathotypes including pathotype-1, IRBB-53 (xa5, xa13) and IRBB-54 (xa5, Xa21) were significantly more resistant against 82% Xoo pathotypes respectively. The rest of the Xa genes either alone or in pyramid was moderately susceptible to susceptible. Among the 54 mutant lines in M6-M7 generations, 12 mutants were resistant and 35 were moderately resistant but in M3-M4 generations, only 1 mutant was found resistant and 35 were moderately resistant while rest along with Basmati varieties was found moderately susceptible to susceptible. Thus, the resistant genotypes could be considered a potential source for disease resistance against the BLB and could be used further in the crossing programme for development of BLB resistant rice variety. Among the management strategies of this menace, Induction of Systemic Resistance (ISR) has been introduced as a harmonizing approach or a potential alternative for crop fortification. The effect of plant defense activators was studied in inducing the resistance in rice against BLB disease for the two years. In this connection, six resistance inducing chemicals (Salicylic acid, Bion, K2HPO4, Ascorbic Acid, 5% w/v plant extracts of Neem and Mango were applied exogenously on two rice commercial varieties, The SA was the most effective in this concern and reduced the disease 42.37% and 24.44% in Basmati Super and Basmati 515, respectively. The Bion reduced the leaf lesion length, 40.78% and 23.40%, in each variety. The present study indicates that Salicylic acid and Bion can be used in the field as a means of protection against BLB disease as well as to decrease the used of expensive and toxic chemicals/antibiotics.