Rice is a globally important crop and highly vulnerable to rice blast disease (RBD). We studied the spatial distribution of RBD by considering the 2-year exploratory data from 120 sampling sites over varied rice ecosystems of Karnataka, India. Point pattern and surface interpolation analyses were performed to identify the spatial distribution of RBD. The spatial clusters of RBD were generated by spatial autocorrelation and Ripley’s K function. Further, inverse distance weighting (IDW), ordinary kriging (OK), and indicator kriging (IK) approaches were utilized to generate spatial maps by predicting the values at unvisited locations using neighboring observations. Hierarchical cluster analysis using the average linkage method identified two main clusters of RBD severity. From the Local Moran’s I, most of the districts were clustered together (at I > 0), except the coastal and interior districts (at I < 0). Positive spatial dependency was observed in the Coastal, Hilly, Bhadra, and Upper Krishna Project ecosystems (p > 0.05), while Tungabhadra and Kaveri ecosystem districts were clustered together at p < 0.05. From the kriging, Hilly ecosystem, middle and southern parts of Karnataka were found vulnerable to RBD. This is the first intensive study in India on understanding the spatial distribution of RBD using geostatistical approaches, and the findings from this study help in setting up ecosystem-specific management strategies against RBD.

Aims:To investigate the genetic diversity, population structure and mating-type distribution among the eco-distinct isolates of Magnaporthe oryzae from Karnataka, India. Methods and Results:A set of 38 isolates of M. oryzae associated with leaf blast disease of rice were collected from different rice ecosystems of Karnataka, India, and analysed for their diversity at actin, β-tubulin, calmodulin, translation elongation factor 1α (TEF-1α), and internal transcribed spacer (ITS) genes/region. The isolates were grouped into two clusters based on the multilocus sequence diversity, the majority being in cluster-IA (n = 37), and only one isolate formed cluster-IB. Population structure was analysed using 123 SNP data to understand the genetic relationship.Based on K = 2 and ancestry threshold of >70%, blast strains were classified into two subgroups (SG1 and SG2) whereas, based on K = 4 and ancestry threshold of >70%, blast strains were classified into four subgroups (SG1, SG2, SG3 and SG4).We have identified 13 haplotype groups where haplotype group 2 was predominant (n = 20) in the population. The Tajima's and Fu's Fs neutrality tests exhibited many rare alleles. Further, the mating-type analysis was also performed using MAT1 genespecific primers to find the potentiality of sexual reproduction in different ecosystems. The majority of the isolates (54.5%) had MAT1-2 idiomorph, whereas 45.5% of the isolates possessed MAT1-1 idiomorph.Conclusions: The present study found the genetically homogenous population of M. oryzae by multilocus sequence analysis. Both mating types, MAT1-1 and MAT1-2, were found within the M. oryzae population of Karnataka. Significance and impact of study:The study on the population structure and sexual mating behaviour of M. oryzae is important in developing region-specific blastresistant rice cultivars. This is the first report of MAT1 idiomorphs distribution in the M. oryzae population in any Southern state of India.

Rice is the most widely consumed cereal staple food for a significant part of the world, particularly in Asia. The Karnataka state of India is one of the highest rice producers, and it has a varied rice ecosystem from irrigated plains to rainfed hilly areas. The rice blast occurs at different severity in these ecosystems causing significant losses each year. The roving survey was carried out in the 120 villages of 18 districts distributed under five irrigated and two rainfed ecosystems of Karnataka during Kharif -2019. Within the irrigated ecosystems, the highest PDI was observed in the Kavery (50.85), followed by Varada (45.89), Bhadra (45.82), Tungabhadra (11.13), and Upper Krishna (10.58) command areas. In a rainfed ecosystem, the highest PDI was observed in the hilly ecosystem (53.38) and the least in the coastal ecosystem (3.73). Within 18 districts, the lowest PDI was observed in the Gadag district (1.68) of the Thungabhadra ecosystem, and the highest was observed in the Chikkamagalur district (81.60) of the hilly ecosystem. The disease was severe in the rainfed hilly ecosystem, followed by an irrigated and rainfed coastal ecosystem. This information is helpful in formulating the management strategies of rice blast in different rice ecosystems of Karnataka.

Single spore isolation from a diseased sample is an essential step in obtaining a pure culture of a fungal pathogen. Rice blast disease caused by Magnaporthe oryzae is an inferior saprophytic competitor, and therefore, many fast-growing fungal or bacterial contaminants are predominant during its isolation. For isolation of M. oryzae, several methods are being followed; however, they are complex and often lead to contamination. In the present study, we have standardized an efficient method for rapid isolation of M. oryzae from the blast disease infected rice-leaf using single spore isolation by spore-drop technique. Following the spore-drop technique, pure culture for an isolate of M. oryzae was obtained quickly with the least contamination (4%), whereas the conventional spore-dilution and leaf-press method recorded 26.12 and 45.50% contamination, respectively. The spore-drop approach has yielded the single spore isolates in the shortest time (10 days) and can be used for regular rice blast pathogen isolation. This method can also be used for other sporulating pathogens successfully.