Nematology- ICAR JRF/SRF 2024

• Biology of Phytoplasmas: Statement (I) is correct because Candidatus Phytoplasma represents a genus of specialized, wall-less obligate prokaryotes belonging to the class Mollicutes. They lack a peptidoglycan cell wall and are bounded solely by a triple-layered unit membrane.
• Tissue Tropism: Statement (II) is correct because phytoplasmas are strictly confined to the phloem sieve tubes of infected host plants, where they feed on rich sugars and amino acids, and are vector-transmitted between plants by phloem-feeding sap-sucking insects (leafhoppers, planthoppers, and psyllids).
• Conclusion: Both Statement (I) and Statement (II) are correct (Option 1).

• The Taxonomic Rank 'Pathovar': In botanical and phytopathological nomenclature, the term pathovar (pv.) is defined by the International Code of Nomenclature of Bacteria (ICNB) specifically for plant pathogenic bacteria.
• Definition: A pathovar is an infrasubspecific taxon denoting a bacterial strain or set of strains with the same or similar characteristics, differentiated based on distinctive pathogenicity or host range on specific plant species or cultivars (e.g., Xanthomonas campestris pv. campestris, Pseudomonas syringae pv. phaseolicola).
• Other Groups: For fungi, similar intraspecific variants are termed forma specialis (f. sp.) or races; for nematodes, races or biotypes are used.
• Conclusion: Pathovar is used in bacterial taxonomy (Option 4).

Correct option: (4) Bacteria

• Etiology of Black Mould of Onion: Black mould of onion bulbs in field and storage is caused by the deuteromycetous/ascomycetous fungus Aspergillus niger.
• Symptoms and Characteristics: The fungus produces dense black, powdery crusts of conidia between the dry outer scales and on inner fleshy scales of the onion neck and bulb, causing softening and secondary bacterial rots.
• Other Aspergillus Species:
• Aspergillus flavus: Produces aflatoxins on groundnuts, maize, and stored grains.
• Aspergillus fumigatus: Thermotolerant human opportunistic pathogen.
• Aspergillus nidulans: Model genetic fungus.
• Conclusion: Aspergillus niger is the causal agent of black mould of onion (Option 1).

Correct option: (1) Aspergillus niger