(a) Define microbial biopesticides. What are their types? With suitable examples, evaluate the potential of microbial…

(a) Microbial biopesticides: types and potential in crop protection

Microbial biopesticides are formulations of living microorganisms, or their products, used to suppress insect pests, plant pathogens and weeds.

Types and examples:

• Bacterial: Bacillus thuringiensis var. kurstaki (Cry endotoxins against lepidopteran larvae such as Helicoverpa armigera); B. sphaericus against mosquito larvae; Pseudomonas fluorescens against fungal and bacterial diseases.
• Fungal: Beauveria bassiana and Metarhizium anisopliae against many insects; Trichoderma viride and T. harzianum against soil-borne Rhizoctonia, Fusarium and Pythium.
• Viral: nucleopolyhedroviruses such as HaNPV against Helicoverpa and SlNPV against Spodoptera.
• Nematodes (entomopathogenic): Steinernema and Heterorhabditis, which carry symbiotic bacteria that kill soil-dwelling larvae.

Potential: They are highly host-specific and safe for pollinators, predators and parasitoids; leave no toxic residues; degrade quickly; and with multiple modes of action slow the development of resistance. They fit integrated pest management, e.g. HaNPV in cotton and pulses, Trichoderma seed treatment for root rots, and Bt in vegetables. Limits are slow action, sensitivity to UV, heat and desiccation, short shelf life, narrow host range and quality variation in commercial products.

(b) Viroids: general features and difference from viruses

Viroids are the smallest known infectious agents of plants, discovered by T. O. Diener (1971) in potato spindle tuber disease.

General features:

• A single-stranded, covalently closed circular RNA of about 246–401 nucleotides, extensively base-paired into a rod-like structure.
• No protein coat and no envelope; they do not code for any protein.
• Replication depends on host enzymes, by a rolling-circle mechanism: in the nucleus by host RNA polymerase II (family Pospiviroidae), or in chloroplasts with self-cleaving hammerhead ribozymes (Avsunviroidae).

Differences from viruses: A virus has a DNA or RNA genome of much larger size inside a protein capsid (sometimes with an envelope) and codes for its own proteins, such as coat and replicase proteins. A viroid is naked RNA with no coding capacity.

Diseases: potato spindle tuber (PSTVd), with stunting and elongated, spindle-shaped tubers; and cadang-cadang of coconut (CCCVd), with yellow leaf spotting, non-bearing and death of the palm.

(c) Natural and phylogenetic systems of classification

Natural system: groups plants by overall natural resemblance in morphology, without evolutionary reasoning. Example: Bentham and Hooker, Genera Plantarum (1862–83).

• Merits: practical and easy to use for identification; the basis of many herbaria and floras.
• Demerits: no evolutionary basis; places gymnosperms between dicots and monocots; the group Monochlamydeae is artificial.

Phylogenetic system: arranges taxa according to evolutionary descent and relationships. Examples: Engler and Prantl, Hutchinson, Takhtajan, Cronquist, and the molecular APG IV (2016).

• Merits: reflects evolutionary history and monophyly; helps predict chemical and genetic characters; using DNA data (rbcL, matK) makes it increasingly objective.
• Demerits: the fossil record is incomplete, so many classical schemes were speculative; frequent revisions with new data; less suited to quick field identification.