Syamala Swayamvaran Veena

Rapid and sensitive detection of Phytophthora colocasiae responsible for the taro leaf blight using conventional and real‐time PCR assay

Conventional and real-time PCR assays were developed for sensitive and specific detection of Phytophthora colocasiae, an oomycete pathogen that causes leaf blight and corm rot of taro. A set of three primer pairs was designed from regions of the RAS-related protein (Ypt1), G protein alpha-subunit (GPA1) and phospho-ribosylanthranilate isomerase (TRP1) genes. In conventional PCR, the lower limit of detection was 50 pg DNA, whereas in real-time PCR, the detection limit was 12.5 fg for the primer based on Ypt1 gene. The cycle threshold values were linearly correlated with the concentration of the target DNA (range of R(2) = 0.911-0.999). All the primer sets were successful in detecting P. colocasie from naturally infected leaves and tubers of taro. Phytophthora colocasiae was detected from artificially infested samples after 18 and 15 h of postinoculation in conventional and real-time PCR assay, respectively. The developed PCR assay proved to be a robust and reliable technique to detect P. colocasiae in taro planting material and for assessing the distribution of pathogen within fields, thus aid in mitigating taro leaf blight.

Evaluation of fungicides on Indian isolates of Phytophthora colocasiae causing leaf blight of taro

Leaf blight caused by Phytophthora colocasiae is the most destructive disease affecting taro (Colocasia esculenta) worldwide including India. Fungicides (primarily metalaxyl) remain as an important strategy to manage taro leaf blight in India over decades. It is important to monitor isolate sensitivity to identify build-up of fungicide resistance and thereby modify fungicide usage strategies. P. colocasiae isolates representing four different geographical regions of India were evaluated for their sensitivity to metalaxyl and three other commercially available fungicides viz. Samarth, Biofight and Akoton by poisoned media technique. All the isolates tested were sensitive to metalaxyl, nevertheless there is an increase in the effective concentration compared to the previous reports. Among the other fungicides, Samarth was found to be superior in completely inhibiting mycelial growth at 0.05% followed by Biofight at 1%. Metalaxyl and Akoton® shared a common inhibitory concentration at 2%. The most effective fungicide determined by the in vitro method was evaluated in vivo for studying the pattern of inhibition before and after the disease development in detached taro leaf. The results of the study revealed that build-up on metalaxyl resistance in P. colocasiae is in its course and caution should be taken while administering against taro leaf blight. Fungicide Samarth could be used as an alternative to metalaxyl for management of taro leaf blight.

Morphological, pathological and molecular characterization of Phytophthora colocasiae responsible for taro leaf blight disease in India

The oomycetous fungus Phytophthora colocasiae that causes taro leaf blight is one of the most devastating diseases of taro and is widely distributed in India. A combination of morphological (colony morphology, mating type, pathogenicity, metalaxyl sensitivity) and molecular techniques (rDNA ITS sequencing and Start codon targeted polymorphism, ScoT analysis) was used to characterize 50 isolates of P. colocasiae obtained from different locations in India. Considerable differences in morphological parameters were observed. ScoT analysis revealed high polymorphism among the isolates. This study confirms that isolates of P. colocasiae are highly dynamic in nature and a considerable degree of diversity exists among them. A detailed knowledge of the morphological and molecular characters of P. colocasiae will help in developing suitable control strategies against the taro leaf blight disease.

Genetic and Phenotypic characterization of Phytophthora colocasiae inTaro Growing Areas of India

Phenotypic and molecular methods were used for characterizing 40 Phytophthora colocasiae isolates obtained from Andhra Pradesh, Assam, Kerala, and Odisha regions of India over a period of five years. Phenotypic parameters such as virulence, colony morphology and mating type varied among isolates collected from different regions over the years. No correlation was observed between phenotypic parameters of the isolates and their geographical origins. Considerable inter and intra specific variation were detected by random amplified microsatellites (RAMS) analysis with 100% polymorphism among the isolates. Dendrogram constructed based on RAMS data using the unweighted pair group method with arithmetic mean (UPGMA) grouped the P. colocasiae isolates into two major clusters. No relationship was obtained between RAMS groups of the isolates and phenotypic characters/geographical origin. Population genetic analysis showed that P. colocasiae isolates were highly diverse among different regions. Analysis of molecular variance (AMOVA) showed that most of the genetic variability in P. colocasiae was confined to within a population (93.21%). These results indicate that P. colocasiae populations in India are highly diverse and care should be taken in developing disease management programmes or in breeding resistant cultivars.

PCR-based approach for mining of resistant gene analogues in taro (Colocasia esculenta)

Primers based on the conserved motifs were used to isolate nucleotide-binding sites (NBS) type sequences in taro (Colocasia esculenta). Cloning and sequencing identified three taro NBS-type sequences called resistance gene analogues (RGAs) that depicted similarity to other cloned RGA sequences. The deduced amino acid sequences of the RGAs detected the presence of conserved domains, viz. P-loop, categorising them with the NBS–leucine-rich repeat class gene family. Phylogenetic characterisation of the taro RGAs along with RGAs of other plant species grouped them with the non-toll interleukin receptor subclasses of the NBS sequences. The isolation and characterisation of taro RGAs have been reported for the first time in this study. This will provide a starting point towards characterisation of candidate resistance genes in taro and can act as a reference guide for future studies.