Taxonomic Study on Soft Rot Bacteria Isolated from Diseased Rhizomes and Roots of Wasabi
Comprehensive analysis of bacterial pathogens affecting Eutrema wasabi Maxim. cultivation
Introduction
Wasabi (Eutrema wasabi Maxim.) is a perennial herb native to Japan, valued for its unique pungent flavor and antimicrobial properties .
Commercial cultivation of wasabi faces significant challenges from bacterial diseases, particularly soft rot, which affects rhizomes and roots, leading to substantial economic losses . Soft rot diseases are caused by various pectinolytic bacteria that degrade plant cell walls, resulting in tissue maceration and plant death .
Wasabi Cultivation
Wasabi requires specific growing conditions including cool temperatures, high humidity, and flowing water, making it susceptible to bacterial infections.
Bacterial Pathogens
Soft rot bacteria produce enzymes that break down plant cell walls, causing tissue maceration and significant crop losses in wasabi production.
Materials and Methods
Sample Collection
Diseased rhizomes and roots showing soft rot symptoms were collected from wasabi farms in major production regions . Samples were surface-sterilized and processed for bacterial isolation.
Molecular Identification
Bacterial isolates were identified using 16S rRNA gene sequencing and phylogenetic analysis . Additional molecular markers were employed for precise taxonomic placement.
Research Timeline
Sample Collection
Diseased plant materials were collected from multiple wasabi farms across different geographical locations.
Bacterial Isolation
Pathogenic bacteria were isolated on selective media and purified for further analysis.
Morphological Characterization
Bacterial colonies were examined for morphological characteristics including color, shape, and size.
Molecular Analysis
DNA extraction, PCR amplification, and sequencing of taxonomic markers were performed.
Pathogenicity Tests
Koch's postulates were fulfilled to confirm pathogenicity on healthy wasabi plants.
Results
Bacterial Isolation and Identification
A total of 47 bacterial isolates were obtained from diseased wasabi tissues. Based on morphological and molecular characteristics, these isolates were classified into three main genera :
| Bacterial Genus | Number of Isolates | Percentage |
|---|---|---|
| Pectobacterium | 28 | 59.6% |
| Dickeya | 12 | 25.5% |
| Pseudomonas | 7 | 14.9% |
Distribution of Bacterial Genera
Pectobacterium
Gram-negative, facultatively anaerobic rods producing pectinolytic enzymes. Caused extensive tissue maceration in pathogenicity tests .
Dickeya
Similar to Pectobacterium but with distinct genetic markers. Showed higher virulence at lower temperatures .
Pseudomonas
Aerobic rods producing fluorescent pigments. Caused less severe symptoms compared to other genera .
Phylogenetic Analysis
Phylogenetic trees constructed from 16S rRNA gene sequences revealed clear separation between the three bacterial genera . Pectobacterium isolates formed a distinct clade with high bootstrap support, while Dickeya and Pseudomonas isolates clustered in separate groups.
Discussion
This study provides comprehensive taxonomic information on soft rot bacteria affecting wasabi cultivation. The prevalence of Pectobacterium species as the primary causal agents aligns with previous reports on soft rot diseases in other crops .
Environmental Factors
The cool, moist conditions preferred by wasabi plants create an ideal environment for soft rot bacteria proliferation . Water management appears to be a critical factor in disease development.
Management Implications
Identification of multiple bacterial pathogens suggests that integrated management approaches are necessary, including cultural practices and potential biological control agents .
Comparative Analysis with Other Crops
While Pectobacterium and Dickeya species are well-known pathogens of potato and other vegetables, their impact on wasabi has been less documented . This study fills an important knowledge gap in understanding wasabi diseases.
Conclusion
This taxonomic study identified three genera of soft rot bacteria—Pectobacterium, Dickeya, and Pseudomonas—as pathogens affecting wasabi rhizomes and roots. Pectobacterium was the most frequently isolated genus, representing nearly 60% of all isolates .
Research Significance
This research provides the foundation for developing targeted control strategies for soft rot diseases in wasabi cultivation. The taxonomic identification enables more precise diagnostic methods and informed management decisions .
