Abstract
Blast caused by Pyricularia grisea (Cooke) Sacc is one of the major diseases of rice in Bangladesh. To study the epidemiology of blast disease, genetic variability of P. grisea and PCR-based screening for blast resistant genes three experiments were conducted at BRRI during 2010-2013. Incidence and severity of blast disease of rice was recorded in ten agro- ecological zones (AEZ1, AEZ2, AEZ9, AEZ11, AEZ12, AEZ13, AEZ19, AEZ20, AEZ23 and AEZ28) of Bangladesh during boro (irrigated) and transplanted aman (rainfed) seasons. Disease incidence and severity was higher in irrigated ecosystem (boro) (21.19%) than in rainfed ecosystem (aman) (11.98%) regardless of locations (AEZs). It was as high as 68.7% in Jhalak hybrid rice followed by BRRI dhan47 (58.2%), BRRI dhan29 (39.8%), BRRI dhan28 (20.3%) during boro and in BRRI dhan34 (59.8%) during aman. Yield loss was estimated from survey data on neck blast infection. Maximum yield loss was noted in AEZ9 for both seasons. Percent yield loss was higher in all the locations for boro season (irrigated ecosystem) compared to aman season (rainfed ecosystem). In the crop sequencel (CS-1= Crop cycle with one rice followed by fallow/other crops) disease incidence was 16.7% and in crop sequence2 (CS-2= Crop cycle with two rice followed by fallow/other crops) it was 31.9%. Most popularly adopted boro rice was BRRI dhan28 (29.6%) followed by BRRI dhan29 (25.9%) and aman rice was BRRI dhan34 (22.9%). A total of 139 isolates representing 8 AEZS (AEZ1, AEZ2, AEZ9, AEZ11, AEZ13, AEZ19, AEZ23 and AEZ28) of Bangladesh were characterized for their pathogenecities using 26 differential varieties (DVs) targeting 23 resistant genes namely, Pish., Pib, Pit, Pia, Pii, Pi3, Pi5, Pik-s, Pik-m, Pil(t), Pik-h, Pik, Pik-p, Pi7(t), Pi9, Piz, Piz- 5(pi-2(t)), Piz-t, Pita-2, Pi12(t), Pita= Pi4(t), Pi19 and Pi20 including a susceptible check LTH. Isolates clarified on MLS through pathogenecity test indicated that pi9, pish, pita and pita2 were the major genes responsible for blast resistance in Bangladesh. Among them Pi9, Pishi showed resistance frequencies of 90% and above while Pita and Pita-2 showed 80-87% against all blast isolates. These blast isolates were categorized into 112 races based on the reaction patterns against DVs. Twenty five isolates were found suitable with high differentiating ability of 23 resistant genes and they had good sporulation ability. The monogenic lines as DVs and the preliminary selected 25 blast isolates could be used as the first differential system in Bangladesh, which can characterize the resistance of rice cultivars. All the five pathotypes viz. U, i, k, z and ta were identified in the isolates. Each pathotype was farther separated into 13, 8, 46, 13 and 37 groups respectively for U, i, k, z and ta. Of them seven types, U63, 10, i7, k177, z00, z04 and ta403 were basically dominant and commonly found in all eight AEZS. The computations from distance matrix gave hierarchical clustering among 139 blast isolates and grouped them into five distinct clusters. The inter-cluster distance was maximum between cluster I and cluster V (D2 = 5.51). Minimum inter-cluster distance was observed between cluster III and cluster IV (2.32) which indicated that the isolates of these clusters were genetically closer. On the other hand, these two clusters maintained maximum distance from cluster V. One hundred forty four plant materials were classified into 10 cultivar groups (CG) based on their reaction patterns to four distinct blast isolates, H-11-64, H-1-8, H-1-1 and H-11-67. The reaction patterns of plant materials to these four standard blast isolates indicated presence of Pish, Pi9, Pita-2 and Pita genes and their combination in the genetic background of tested plant materials. Gene specific primer 195R-1/195F-1, Pita440 (YL153)/Pita440 (YL154), OSM89 and AOL45/AOL48 were used to identify Pi9, Pita, Pita-2 and Pishi genes, respectively through PCR-based assay. Plant materials, H13, H23, H25, H35, H47, H49, H136, H138 harbored all three genes, pita, pita2 and pish in their genetic background. The pi9 gene together with Pita-2 was detected in local rice cultivar H100 and H129 which was confirmed by DNA analysis in PCR assay. These materials could be used in gene pyramiding in promising high yielding cultivar for durable blast resistance.