Microorganisms – Viruses, Prokaryotes, Algae and Fungi BBO1T01

Syllabus topics (official RTMNU)

1. Introduction to microorganisms.
2. Classification of microorganisms (Outline) – Carl Woese's Domain system (1990).
3. Brief account of Bacteria – Archaebacteria, Mycoplasma, Actinomycetes
4. Viruses – General account, structure & multiplication of T4 Phage (Lytic, Lysogenic)
5. Bacteria: General characteristics, cell structure and economic importance.
6. Cyanobacteria: Nostoc (Morphology, Reproduction and importance in Agriculture)

Introduction to microorganisms

Microorganisms (microbes) are living organisms too small to be seen clearly with the naked eye (usually less than 0.1 mm); they are studied with a microscope. The branch of science dealing with them is Microbiology.

• Groups: viruses (acellular), bacteria and archaea (prokaryotes), cyanobacteria, microscopic algae, fungi (yeasts, moulds) and protozoa.
• Pioneers: Antonie van Leeuwenhoek (1676) first saw bacteria ("animalcules"); Louis Pasteur disproved spontaneous generation and explained fermentation; Robert Koch gave Koch's postulates (germ theory); D. Ivanowsky (1892) discovered the tobacco mosaic virus.
• Habitat: everywhere – soil, water, air, hot springs, inside plants and animals.
• Importance: decomposition and nutrient cycling, nitrogen fixation, fermentation (curd, bread, alcohol), antibiotics, vaccines, biotechnology; some cause diseases of plants, animals and humans.

Classification of microorganisms – Carl Woese's Domain system (1990)

Carl Woese, Kandler and Wheelis (1990) compared the sequence of 16S ribosomal RNA (rRNA) of organisms and proposed a level above kingdom called Domain. All life was placed in three domains:

1. Domain Bacteria (Eubacteria): true bacteria and cyanobacteria; cell wall with peptidoglycan; ester-linked membrane lipids.
2. Domain Archaea: methanogens, halophiles, thermoacidophiles; no peptidoglycan; ether-linked branched lipids; live in extreme habitats.
3. Domain Eukarya: organisms with a true nucleus – protists, fungi, plants and animals.

Brief account of Bacteria – Archaebacteria, Mycoplasma, Actinomycetes

Archaebacteria: ancient prokaryotes living in extreme conditions. Three groups: (a) Methanogens – anaerobic, produce methane in marshes and in the rumen of cattle (e.g. Methanobacterium), used in biogas plants; (b) Halophiles – live in very salty water (e.g. Halobacterium, has purple pigment bacteriorhodopsin); (c) Thermoacidophiles – hot acidic springs (e.g. Thermoplasma, Sulfolobus).

Mycoplasma (PPLO – pleuropneumonia-like organisms): smallest free-living cells (0.1–0.3 µm). They have no cell wall, so they are pleomorphic (no fixed shape) and resistant to penicillin. Bounded by a 3-layered plasma membrane; contain DNA, RNA and 70S ribosomes. They cause diseases such as little leaf of brinjal (phytoplasma), witches' broom and pleuropneumonia of cattle.

Actinomycetes: Gram-positive, filamentous, branched bacteria forming a mycelium-like structure (once called "ray fungi"). Mostly soil dwellers; give soil its earthy smell (geosmin). Reproduce by conidia. Streptomyces produces antibiotics like streptomycin, tetracycline, chloramphenicol; Frankia fixes nitrogen in root nodules of non-legumes (e.g. Casuarina).

Viruses – General account, structure & multiplication of T4 Phage (Lytic, Lysogenic)

Viruses are ultramicroscopic, acellular, obligate intracellular parasites made of nucleic acid (DNA or RNA, never both) enclosed in a protein coat (capsid). Outside the host they are inert particles (virions); inside a living cell they multiply. Hence they are called "link between living and non-living".

• Size 20–300 nm; seen only under electron microscope; can be crystallised (TMV – W.M. Stanley, 1935).
• Types by host: plant viruses (mostly RNA, e.g. TMV), animal viruses (DNA or RNA), bacteriophages (mostly DNA).
• Shapes: helical (TMV), polyhedral/icosahedral (adenovirus), complex (T4 phage).

Structure of T4 bacteriophage (attacks E. coli; tadpole-like, about 200 nm long):

• Head: elongated hexagonal (prolate icosahedron) protein capsid containing linear double-stranded DNA.
• Collar: a thin disc between head and tail.
• Tail: a hollow core tube surrounded by a contractile sheath.
• Base plate: hexagonal plate at the tail end bearing 6 short tail pins (spikes) and 6 long tail fibres used for attachment.

Lytic cycle (virulent phage – host is destroyed):

1. Adsorption: tail fibres attach to receptor sites on the bacterial wall.
2. Penetration: lysozyme at the tail tip makes a hole; the sheath contracts and DNA is injected; the empty protein coat (ghost) remains outside.
3. Synthesis (eclipse phase): phage DNA takes over host machinery; host DNA is degraded; phage DNA and proteins are made.
4. Assembly (maturation): heads, tails and DNA assemble into new phages.
5. Lysis/release: the enzyme lysozyme bursts the cell, releasing about 100–200 new phages in about 20–30 minutes.

Lysogenic cycle (temperate phage, classic example lambda phage; T4 itself is strictly lytic): phage DNA integrates into the bacterial chromosome as a prophage and replicates along with it; the host is not killed and is called lysogenic. Under stress (UV light, chemicals) the prophage separates (induction) and enters the lytic cycle.

Bacteria: General characteristics, cell structure and economic importance

General characteristics: unicellular prokaryotes (0.5–5 µm); no nuclear membrane; 70S ribosomes; no membrane-bound organelles; cell wall of peptidoglycan; reproduce mainly by binary fission; genetic recombination by conjugation, transformation and transduction. Shapes: coccus (spherical), bacillus (rod), spirillum (spiral), vibrio (comma). Nutrition: autotrophic (photo/chemosynthetic) or heterotrophic (saprophytic, parasitic, symbiotic). Gram staining divides them into Gram-positive (thick peptidoglycan, violet) and Gram-negative (thin wall with outer membrane, pink).

Cell structure (from outside to inside):

• Capsule/slime layer: gummy layer of polysaccharide; protection and virulence.
• Flagella: locomotion; arrangement monotrichous, lophotrichous, amphitrichous, peritrichous.
• Pili/fimbriae: attachment; sex pili in conjugation.
• Cell wall: rigid, peptidoglycan (murein).
• Plasma membrane: lipid–protein; infoldings called mesosomes help in respiration and cell division.
• Cytoplasm: 70S ribosomes, food granules (glycogen, volutin).
• Nucleoid: single circular naked DNA; extra small circular DNA = plasmids.

Economic importance – Useful: nitrogen fixation (Rhizobium, Azotobacter); decay and recycling; curd (Lactobacillus); vinegar (Acetobacter); retting of fibres; antibiotics (Bacillus – bacitracin); insulin through genetic engineering (E. coli); biofertilizer and biopesticide (Bacillus thuringiensis). Harmful: human diseases (cholera, typhoid, TB), plant diseases (citrus canker, black rot), food spoilage, denitrification lowering soil fertility.

Cyanobacteria: Nostoc (Morphology, Reproduction and importance in Agriculture)

Systematic position: Domain Bacteria; Cyanobacteria; Order Nostocales; Family Nostocaceae; Genus Nostoc.

Morphology: found in moist soil, ponds and on wet rocks; also symbiotic in Anthoceros, Cycas coralloid roots and lichens. The thallus is a gelatinous colony (ball) containing many unbranched, bead-like trichomes; a trichome together with its mucilage sheath is a filament. Cells are round to barrel-shaped, prokaryotic, with peripheral pigmented chromoplasm (chlorophyll a, phycocyanin, phycoerythrin) and central colourless centroplasm. At intervals there are thick-walled, pale heterocysts with polar nodules; they are sites of nitrogen fixation (enzyme nitrogenase).

Reproduction (only vegetative and asexual; no sexual reproduction):

• Fragmentation of the colony.
• Hormogonia: trichome breaks at heterocysts into short pieces which grow into new filaments.
• Akinetes: thick-walled resting spores formed in unfavourable conditions; germinate later.
• Heterocysts may occasionally germinate into new filaments.

Importance in agriculture: fixes atmospheric nitrogen and increases soil fertility, especially in rice (paddy) fields; used as biofertilizer (blue-green algal biofertilizer, with Anabaena); adds organic matter, improves soil structure and water-holding capacity; helps reclaim alkaline (usar) soils; secretes growth-promoting substances.

5 likely exam questions

Long answer

1. Describe the structure of T4 bacteriophage and explain its lytic and lysogenic modes of multiplication with labelled diagrams.
2. Describe the general characteristics and cell structure of bacteria with a labelled diagram. Add a note on their economic importance.

Short answer / short notes

3. Explain Carl Woese's three-domain system of classification.
4. Write a short note on Mycoplasma.
5. Describe the morphology of Nostoc and its importance in agriculture.

Syllabus topics (official RTMNU)

1. General characteristics and Economic importance of Algae.
2. Classification of algae: Lee (2008) up to phylum with examples
3. Morphology and reproduction of the following: Oedogonium, Chara and Ectocarpus

General characteristics and Economic importance of Algae

Algae are simple, chlorophyll-bearing, autotrophic, mostly aquatic thalloid organisms without true roots, stems, leaves or vascular tissue. The study of algae is Phycology (Algology).

• Habitat: fresh water, sea water, moist soil, tree bark; some in snow (Chlamydomonas nivalis), hot springs, or as symbionts (lichens).
• Thallus range: unicellular (Chlamydomonas), colonial (Volvox), filamentous (Oedogonium), heterotrichous (Ectocarpus), siphonous (Vaucheria) to large parenchymatous kelps.
• Pigments: chlorophyll a in all; accessory chlorophyll b, c or d, carotenoids and phycobilins vary by group.
• Reserve food: starch (green algae), floridean starch (red), laminarin and mannitol (brown).
• Sex organs are unicellular and not covered by a jacket of sterile cells (exception: Chara); no embryo is formed after fertilization.
• Reproduction: vegetative (fragmentation), asexual (zoospores, aplanospores, akinetes) and sexual (isogamy, anisogamy, oogamy).

Economic importance:

Harmful: water blooms make water toxic and smelly (Microcystis); red tides by dinoflagellates kill fish; Cephaleuros causes red rust of tea.

Classification of algae: Lee (2008) up to phylum with examples

R.E. Lee (Phycology, 4th edition, 2008) classified algae mainly on the basis of evolution of chloroplast by endosymbiosis – the number of membranes around the chloroplast and whether chloroplast endoplasmic reticulum (CER) is present.

Morphology and reproduction of Oedogonium

Position: Chlorophyta – Chlorophyceae – Oedogoniales – Oedogonium. Freshwater, attached to stones/plants when young by a holdfast, later free-floating.

Morphology: unbranched filament of cylindrical cells; each cell has one nucleus, a reticulate chloroplast with many pyrenoids, and a large vacuole. Some cells show transverse rings at the upper end called cap cells, formed by its peculiar cell division.

Asexual reproduction: by fragmentation, akinetes, and a single large zoospore per cell, which bears a crown (ring) of many flagella (stephanokont); it settles and forms a new filament.

Sexual reproduction: highly oogamous. Oogonium is a swollen cell with one egg and a receptive spot; antheridia are short disc-like cells producing 2 multiflagellate antherozoids each.

• Macrandrous species: male filaments of normal size. They may be homothallic or heterothallic.
• Nannandrous species: special androsporangia produce androspores which settle on or near the oogonium and grow into small dwarf males (nannandria) bearing antheridia.

After fertilization the oospore (zygote, 2n) develops a thick wall and rests; it undergoes meiosis to form 4 haploid zoospores (meiozoospores), each giving a new filament. Life cycle is haplontic.

Morphology and reproduction of Chara

Position: Chlorophyta – Charophyceae – Charales – Chara (stonewort, as it is encrusted with calcium carbonate). Grows submerged in fresh water.

Morphology: plant body is erect, branched, with multicellular branched rhizoids; main axis differentiated into nodes (small cells) and long single-celled internodes. From nodes arise a whorl of branches of limited growth (primary laterals/"leaves") and branches of unlimited growth in axils. Internodes are often covered by cortical cells (corticated). Growth by a dome-shaped apical cell.

Reproduction: vegetative by amylum stars, bulbils and secondary protonema. No asexual spores. Sexual reproduction is oogamous with complex sex organs on branches of limited growth: the nucule (oogonium) lies above the globule (antheridium).

• Globule: spherical, red-orange; wall of 8 shield cells; inside, each shield cell bears a rod-like manubrium with a head cell (capitulum) giving rise to antheridial (spermatogenous) filaments; each cell forms one biflagellate antherozoid.
• Nucule: oval, with a single egg, surrounded by 5 spirally twisted tube cells ending in a coronula of 5 cells.

The zygote (oospore) develops a hard wall, rests, and undergoes meiosis; one haploid nucleus survives and germinates into a protonema on which the new plant arises. Life cycle haplontic.

Morphology and reproduction of Ectocarpus

Position: Heterokontophyta – Phaeophyceae – Ectocarpales – Ectocarpus. Marine brown alga, attached to rocks or other algae.

Morphology: thallus heterotrichous – a prostrate creeping system and an erect much-branched filamentous system with uniseriate branches tapering into hairs. Cells uninucleate, with ribbon-like or discoid brown chromatophores containing fucoxanthin; reserve food laminarin and mannitol. Growth is intercalary (trichothallic).

Reproduction: on the diploid sporophyte two kinds of sporangia are borne:

• Unilocular sporangia: single-chambered; nucleus divides by meiosis then mitosis to form 32–64 haploid zoospores which grow into haploid gametophytes.
• Plurilocular (neutral) sporangia: many-chambered; diploid zoospores formed by mitosis repeat the sporophyte.

The haploid gametophyte bears plurilocular gametangia producing pear-shaped biflagellate gametes with laterally inserted unequal flagella. Fusion is isogamous or physiologically anisogamous (in E. siliculosus the less active female gamete settles first and attracts males). Zygote forms the sporophyte. Gametophyte and sporophyte look alike, so the life cycle is diplohaplontic with isomorphic alternation of generations.

5 likely exam questions

Long answer

1. Describe the thallus structure and sexual reproduction of Oedogonium with special reference to nannandrous species. Add a labelled diagram.
2. Give the general characteristics and economic importance of algae, and outline the classification of algae proposed by Lee (2008) up to phylum with examples.

Short answer / short notes

3. Describe the structure of globule and nucule of Chara.
4. Distinguish between unilocular and plurilocular sporangia of Ectocarpus.
5. Write a note on isomorphic alternation of generations in Ectocarpus.

Syllabus topics (official RTMNU)

1. Fungi: General characteristics and Economic importance.
2. Classification outline: Alexopoulos and Mims, 1996
3. Life cycle of Albugo, Rhizopus and Agaricus.
4. Mycorrhiza: ectomycorrhiza and endomycorrhiza.

Fungi: General characteristics and Economic importance

Fungi are eukaryotic, achlorophyllous, heterotrophic, spore-producing organisms with absorptive nutrition. The study of fungi is Mycology.

• Thallus: unicellular (yeast) or filamentous mycelium made of thread-like hyphae; hyphae may be aseptate and coenocytic (Rhizopus, Albugo) or septate (Agaricus).
• Cell wall: mostly of chitin (cellulose in Oomycetes).
• Reserve food: glycogen and oil, never starch.
• Nutrition: saprophytic, parasitic (obligate or facultative) or symbiotic (lichens, mycorrhiza). Parasites absorb food through haustoria.
• Reproduction: vegetative (fragmentation, budding, sclerotia); asexual by spores (zoospores, sporangiospores, conidia); sexual by planogametic copulation, gametangial contact, gametangial copulation, spermatization or somatogamy.
• Sexual reproduction has three phases: plasmogamy, karyogamy and meiosis; in higher fungi a dikaryotic (n+n) phase lies between plasmogamy and karyogamy.

Economic importance – Useful:

• Food: mushrooms (Agaricus bisporus, Pleurotus, morels); yeast in bread making.
• Industry: alcohol and beer by Saccharomyces (fermentation); citric acid by Aspergillus niger; cheese ripening by Penicillium roqueforti and P. camemberti; enzymes (amylase).
• Medicine: penicillin (Penicillium notatum, discovered by Fleming, 1928), griseofulvin, cyclosporin (immunosuppressant); ergot alkaloids from Claviceps.
• Agriculture: decomposers and soil fertility; mycorrhiza; biocontrol (Trichoderma); gibberellin from Gibberella fujikuroi.
• Research: Neurospora and yeast as genetic models.

Harmful: plant diseases (rusts, smuts, white rust, late blight of potato, wilt); spoilage of food, wood, leather and paper; mycotoxins such as aflatoxin (Aspergillus flavus); human diseases like ringworm, candidiasis, aspergillosis; poisonous mushrooms (Amanita).

Classification outline: Alexopoulos and Mims, 1996

C.J. Alexopoulos, C.W. Mims and M. Blackwell (Introductory Mycology, 4th edition, 1996) distributed organisms traditionally studied as fungi into three kingdoms:

Fungi with no known sexual stage are kept as an artificial group, Mitosporic fungi (Deuteromycetes / Fungi Imperfecti), e.g. Alternaria, Fusarium, Colletotrichum.

Life cycle of Albugo

Albugo candida (Oomycota) causes white rust of crucifers (mustard, radish). It is an obligate parasite.

• Mycelium: aseptate, coenocytic, intercellular, sending small knob-like haustoria into host cells.
• Asexual: hyphae form a mat below the epidermis bearing club-shaped sporangiophores which cut off chains of sporangia (basipetal, youngest at base) separated by gelatinous disjunctors. Pressure ruptures the epidermis giving white powdery pustules. In water and cool conditions a sporangium forms 4–12 biflagellate (heterokont) zoospores, which encyst and infect new hosts; in warm dry conditions it may germinate directly by a germ tube.
• Sexual (oogamous): oogonium (globular, multinucleate) is differentiated into outer periplasm and central ooplasm (oosphere); a single functional egg nucleus remains in A. candida. A club-shaped antheridium lies beside it (paragynous) and sends a fertilization tube; one male nucleus fuses with the egg nucleus. A dark coenocentrum is seen in the oosphere.
• The oospore develops a thick warty wall, rests, undergoes meiosis and many mitoses, and on germination releases zoospores inside a thin vesicle.

The mycelium is diploid and meiosis occurs in the gametangia, so the life cycle is regarded as diplontic (modern view).

Life cycle of Rhizopus

Rhizopus stolonifer (Zygomycota), the common black bread mould, is a saprophyte.

• Mycelium: haploid, aseptate, coenocytic, of three types of hyphae – horizontal stolons, root-like rhizoids at nodes, and erect sporangiophores.
• Asexual: the tip of each sporangiophore swells into a black sporangium with a central dome-shaped columella; the swollen sporangiophore base under it is the apophysis. Numerous non-motile sporangiospores (aplanospores) are dispersed by wind and germinate into new mycelium.
• Sexual (gametangial copulation): species are heterothallic (+ and − strains). Opposite hyphae form progametangia which touch; each divides into a terminal multinucleate gametangium and a suspensor. Gametangia fuse to form a thick-walled, black, warty zygospore where nuclei pair and fuse.
• Germination: after rest, meiosis occurs; the zygospore forms a promycelium with a germ sporangium containing + and − spores.

Only the zygote is diploid; the life cycle is haplontic.

Life cycle of Agaricus

Agaricus (Basidiomycota) – field mushroom (A. campestris) and cultivated button mushroom (A. bisporus); saprophytic on humus-rich soil.

• Primary mycelium: develops from a basidiospore; septate, uninucleate (n), short-lived.
• Secondary mycelium: formed by somatogamy (fusion of two compatible primary hyphae, plasmogamy without karyogamy); cells are dikaryotic (n+n); long-lived, perennial.
• Tertiary mycelium: organized dikaryotic hyphae forming the fruit body (basidiocarp). It starts as small "button" stages.
• Basidiocarp: a stalk (stipe) bearing a cap (pileus); a membrane (velum) joining cap margin to stipe breaks to leave a ring, the annulus. On the underside of the pileus are radiating gills (lamellae).
• Gill structure (V.S.): central trama of loose hyphae, then subhymenium, and outer hymenium of club-shaped basidia mixed with sterile paraphyses.
• Basidium: the two nuclei fuse (karyogamy, 2n), then meiosis forms 4 haploid nuclei which pass through sterigmata into 4 basidiospores (2 in A. bisporus). Spores are shot off and dispersed by wind.

Mycorrhiza: ectomycorrhiza and endomycorrhiza

Mycorrhiza (Frank, 1885) is a mutually beneficial symbiotic association between a fungus and the roots of higher plants. The fungus increases absorption of water and minerals, especially phosphorus, protects roots from pathogens and drought; the plant gives sugars to the fungus.

Types of endomycorrhiza: vesicular-arbuscular (VAM / AM), orchid mycorrhiza and ericoid mycorrhiza. VAM fungi are used as biofertilizers; ectomycorrhizal inoculation helps raise pine nurseries.

5 likely exam questions

Long answer

1. Describe the life cycle of Albugo candida with labelled diagrams.
2. Give the general characteristics of fungi and outline their classification according to Alexopoulos and Mims (1996). Add a note on their economic importance.

Short answer / short notes

3. Describe sexual reproduction in Rhizopus.
4. Describe the structure of the basidiocarp and V.S. of gill of Agaricus.
5. Distinguish between ectomycorrhiza and endomycorrhiza.

Syllabus topics (official RTMNU)

1. Lichens: General account, Types of lichens, Internal Structure, Reproduction and Economic importance.
2. Plant Pathology: Causal organism, Symptoms, transmission and control measures of Plant diseases – Citrus canker, Red Rot of Sugar cane, Little leaf of Brinjal and Leaf Curl of Papaya.

Lichens: General account

Lichens are composite organisms formed by a symbiotic association of an alga (or cyanobacterium) and a fungus. The algal partner is the phycobiont (photobiont), e.g. Trebouxia, Trentepohlia, Nostoc; the fungal partner is the mycobiont, mostly an Ascomycete (ascolichens), rarely a Basidiomycete (basidiolichens). The alga makes food by photosynthesis; the fungus gives shelter, absorbs water and minerals. The relationship is often called mutualism (some call it controlled parasitism, helotism). The study of lichens is Lichenology.

• Grow on bark (corticolous), rocks (saxicolous), soil (terricolous), leaves; tolerate extreme cold and dryness.
• Slow-growing; produce special lichen acids (usnic acid etc.).
• Very sensitive to air pollution, especially SO₂.

Types of lichens

On the basis of distribution of algal cells: homoiomerous (algae uniformly spread, e.g. Collema) and heteromerous (algae in a definite layer, e.g. Parmelia). Some books add leprose (powdery) and squamulose (scale-like) types.

Internal Structure

V.S. of a heteromerous foliose thallus (Parmelia) shows four layers:

1. Upper cortex: compactly interwoven fungal hyphae (pseudoparenchyma), protective.
2. Algal (gonidial) zone: algal cells entangled in loose hyphae; photosynthetic layer.
3. Medulla: loosely arranged thick-walled hyphae with large air spaces; storage and aeration.
4. Lower cortex: compact hyphae; gives out root-like rhizines for attachment.

Reproduction

• Vegetative: fragmentation of thallus; soredia – small powdery groups of a few algal cells wrapped by hyphae, formed in soralia; isidia – small coral-like outgrowths of the upper surface containing both partners, which break off.
• Asexual: by the fungal partner through pycnidiospores (conidia) formed in flask-shaped pycnidia.
• Sexual: only the fungus reproduces sexually. Ascolichens form ascocarps – cup-shaped apothecia (Parmelia) or flask-shaped perithecia; female organ carpogonium with trichogyne, male spermatia. Ascospores germinate and must meet a suitable alga to form a new lichen.

Economic importance

• Pioneers of succession on bare rocks; produce acids that weather rock and form soil.
• Pollution indicators – absent in SO₂-polluted cities.
• Food: reindeer moss (Cladonia rangiferina) for animals; Parmelia ("pathar phool / dagad phool") used as a spice in India.
• Dyes: litmus indicator from Roccella; orchil dyes.
• Medicine: usnic acid (antibiotic) from Usnea; Iceland moss (Cetraria) for cough.
• Perfumes and cosmetics: Evernia, Ramalina.
• Harmful: damage stone monuments and glass; some are poisonous.

Plant Pathology: Citrus canker, Red Rot of Sugar cane, Little leaf of Brinjal and Leaf Curl of Papaya

Plant pathology (phytopathology) is the study of plant diseases – their causes, symptoms, spread and control. For each disease, write: causal organism, symptoms, transmission, control.

1. Citrus canker

• Causal organism: bacterium Xanthomonas axonopodis pv. citri (older name X. campestris pv. citri); Gram-negative rod with one polar flagellum. Acid lime is very susceptible.
• Symptoms: small watery spots on leaves, twigs and fruits enlarge into raised, rough, corky brown lesions with a yellow halo; centre becomes crater-like; seen on both leaf surfaces; fruit quality falls; severe attack causes defoliation and dieback.
• Transmission: rain splash and wind-driven rain, infected nursery plants and budwood, wounds made by citrus leaf miner; survives in old lesions.
• Control: healthy nursery stock; prune and burn infected twigs before monsoon; spray Bordeaux mixture (1%) or copper oxychloride with streptomycin sulphate (streptocycline, about 100 ppm); control leaf miner; grow tolerant varieties.

2. Red rot of sugarcane

• Causal organism: fungus Colletotrichum falcatum (perfect stage Glomerella tucumanensis). Called the "cancer of sugarcane".
• Symptoms: third or fourth leaf from top turns yellow and dries; when the cane is split, internal tissue is red with white transverse patches; sour/alcoholic smell; pith shrinks and cane becomes hollow; reddish lesions with dark margins on midrib; black acervuli on rind.
• Transmission: primarily through infected setts; secondary spread by conidia through rain, irrigation water and wind; survives in crop debris and soil.
• Control: use healthy setts and resistant varieties; treat setts with hot water or moist hot air and fungicide (carbendazim); rogue and burn diseased clumps; crop rotation; avoid ratooning infected crops; good drainage.

3. Little leaf of brinjal

• Causal organism: phytoplasma (mycoplasma-like organism, MLO) – wall-less prokaryote living in phloem.
• Symptoms: leaves become very small, narrow, soft, smooth and yellowish; petioles short; many axillary buds grow giving a bushy appearance; internodes short; flowers become green leaf-like (phyllody) and sterile; little or no fruiting.
• Transmission: by the leafhopper Hishimonus phycitis; also by grafting; not by sap or seed.
• Control: uproot and destroy infected plants; spray systemic insecticides (dimethoate, imidacloprid) to kill vectors; dip seedling roots in tetracycline solution; remove weed hosts (e.g. Datura); grow tolerant varieties.

4. Leaf curl of papaya

• Causal organism: Papaya leaf curl virus (PaLCuV), a begomovirus (Geminiviridae) with single-stranded DNA in twin particles.
• Symptoms: severe downward and inward curling and crinkling of leaves; veins thickened and dark green; leaves leathery and brittle with enations on lower surface; petioles twisted; plant stunted; few or no fruits.
• Transmission: by whitefly Bemisia tabaci; also by grafting; not by sap or seed.
• Control: remove and burn infected plants early; spray insecticides against whitefly; raise seedlings under insect-proof nets; remove weed and alternate hosts (tomato, tobacco) near orchards; use yellow sticky traps.

5 likely exam questions

Long answer

1. Give a general account of lichens. Describe their types, internal structure of a heteromerous thallus and modes of reproduction.
2. Describe the causal organism, symptoms, transmission and control measures of red rot of sugarcane and citrus canker.

Short answer / short notes

3. Write a note on the economic importance of lichens.
4. Describe the symptoms and control of little leaf of brinjal.
5. Write a short note on leaf curl of papaya.