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False smut disease (FSD) of rice incited by Ustilaginoidea virens is an emerging threat to paddy cultivation worldwide. We investigated the spatial distribution of FSD in different paddy ecosystems of South Indian states, viz., Andhra Pradesh, Karnataka, Tamil Nadu, and Telangana, by considering the exploratory data from 111 sampling sites. Point pattern and surface interpolation analyses were carried out to identify the spatial patterns of FSD across the studied areas. The spatial clusters of FSD were confirmed by employing spatial autocorrelation and Ripley’s K function. Further, ordinary kriging (OK), indicator kriging (IK), and inverse distance weighting (IDW) were used to create spatial maps by predicting the values at unvisited locations. The agglomerative hierarchical cluster analysis using the average linkage method identified four main clusters of FSD. From the Local Moran’s I statistic, most of the areas of Andhra Pradesh and Tamil Nadu were clustered together (at I > 0), except the coastal and interior districts of Karnataka (at I < 0). Spatial patterns of FSD severity were determined by semi-variogram experimental models, and the spherical model was the best fit. Results from the interpolation technique, the potential FSD hot spots/risk areas were majorly identified in Tamil Nadu and a few traditional rice-growing ecosystems of Northern Karnataka. This is the first intensive study that attempted to understand the spatial patterns of FSD using geostatistical approaches in India. The findings from this study would help in setting up ecosystem-specific management strategies to reduce the spread of FSD in India.

Aims: To investigate the diversity of eco‐distinct isolates of Magnaporthe oryzae for their morphological, virulence and molecular diversity and relative distribution of five Avr genes. Methods and Results: Fifty‐two M. oryzae isolates were collected from different rice ecosystems of southern India. A majority of them (n = 28) formed a circular colony on culture media. Based on the disease reaction on susceptible cultivar (cv. HR‐12), all 52 isolates were classified in to highly virulent (n = 28), moderately virulent (n = 11) and less‐virulent (13) types. Among the 52 isolates, 38 were selected for deducing internal transcribed spacer (ITS) sequence diversity. For deducing phylogeny, another set of 36 isolates from other parts of the world was included, which yielded two distinct phylogenetic clusters. We identified eight haplotype groups and 91 variable sites within the ITS sequences, and haplotype‐group‐2 (Hap_2) was predominant (n = 24). The Tajima's and Fu's Fs neutrality tests exhibited many rare alleles. Furthermore, PCR analysis for detecting the presence of five Avr genes in the different M. oryzae isolates using Avr gene‐specific primers in PCR revealed that Avr‐Piz‐t, Avr‐Pik, Avr‐Pia and Avr‐Pita were present in 73.68%, 73.68%, 63.16% and 47.37% of the isolates studied, respectively; whereas, Avr‐Pii was identified only in 13.16% of the isolates. Conclusions: Morpho‐molecular and virulence studies revealed the significant diversity among eco‐distinct isolates. PCR detection of Avr genes among the M. oryzae population revealed the presence of five Avr genes. Among them, Avr‐Piz‐t, Avr‐Pik and Avr‐Pia were more predominant. Significance and Impact of the Study: The study documented the morphological and genetic variability of eco‐distinct M. oryzae isolates. This is the first study demonstrating the distribution of the Avr genes among the eco‐distinct population of M. oryzae from southern India. The information generated will help plant breeders to select appropriate resistant gene/s combinations to develop blast disease‐resistant rice cultivars. [ABSTRACT FROM AUTHOR]

Sheath blight of rice is a global disease that significantly reduces rice yield. This study reports the antifungal activity of an active compound of essential oil, thymol, at different concentrations against Rhizoctonia solani (strain RS-Gvt). In vitro assay results indicated that thymol concentrations (0.5 mg mL -1 and 0.25 mg mL -1 ) completely inhibited (100%) the mycelial growth of RS-Gvt (p ≤ 0.01). Microscopic observations of thymol-treated mycelium of RS-Gvt at 0.0312 mg mL -1 and above concentrations, revealed a distorted mycelial morphology with deformed hyphae. Hyphae showed a bead-like appearance, reduction in size, and constriction of the hyphae at uneven points with increased hyphal density often entangling with each other. Further, an on-field experiment was conducted to study the field bio-efficacy of thymol for two consecutive Kharif seasons of 2022 and 2023 using a factorial RCBD design. The disease severity was measured as the percent disease index (PDI), and the results of two seasons were pooled. Pathogen (RS-Gvt) and thymol were inoculated in different combinations/methods as main treatments (M1-M3), and concentrations of thymol (0.0625-1.0 mg ML -1 ) as sub-treatments. The results indicated that all two factors significantly (P = 0.05) influenced the PDI and grain yield. The pooled data of two seasons indicated a significant difference between the main treatments (M1: RS-Gvt + thymol together; M2: thymol sprayed first followed by RS-Gvt; M3: RS-Gvt first followed by thymol spray) on PDI (53.39-59.67) and grain yield (4.16-4.75 t ha -1 ). M1 exhibited a lower PDI (53.39) and a higher grain yield (4.75 t ha -1 ) compared to M2 and M3, indicating a protective mode of action of thymol against sheath blight disease of rice. The sub-treatments have shown significant variation in PDI and grain yield. The PDI and grain yield ranged from 33.70 (at 1 mg mL -1 ) to 66.21 (at 0.0625 mg mL -1 ) and 4.18 (at 1 mg mL -1 ) to 5.26 (at 0.0625 mg mL -1 ) t ha -1 , respectively, among the thymol concentrations. This indicates that increasing concentrations of thymol have negatively influenced the PDI and positively impacted the yield. Therefore, the spray of 1 mg mL -1 of thymol at the potential disease-infection stage is most effective in controlling the sheath blight disease of rice. This study provides an alternative green bioactive compound for controlling the sheath blight disease, and thymol can be included in developing eco-friendly integrated disease management practices. © 2024 Society of Chemical Industry., (© 2024 Society of Chemical Industry.)