Algae, Fungi, Lichens and Plant Pathology BBO3T05

Syllabus topics (official RTMNU)

1. History of Phycology
2. Algology in India: M.O.P. Iyengar, Y. Bhardwaja, T.V. Desikachary, N.D. Kamath.
3. Habit and habitat of Algae: 1) Hydrophytes – Benthophytes, Thermophytes, Planktophytes, Halophytes, Epiphytes. 2) Edaphophytes, Aerophytes – Epihalophytes, Lithophytes
4. Range of vegetative structure of algae: Motile, Palmelloid, Coccoid, Filamentous, Siphonous, Advanced

History of Phycology

Phycology (Greek phykos = seaweed, logos = study) or Algology (Latin alga = seaweed) is the branch of botany that deals with the study of algae. Algae are simple, autotrophic, chlorophyll-bearing thalloid plants with no true root, stem or leaves, and with non-jacketed (unicellular) sex organs.

• Ancient period: Seaweeds were used as food and medicine in China and Japan from very early times. Greek and Roman writers (Theophrastus, Dioscorides) mentioned marine "algae" (fucus).
• Linnaeus (1753): used the word Algae in Species Plantarum, but his group also included liverworts and lichens.
• A. L. de Jussieu (1789): restricted the term Algae more or less to the present-day sense.
• Leeuwenhoek (1674): first observed microscopic freshwater algae with his simple microscope.
• W. H. Harvey (1836): first classified algae on the basis of pigments into Chlorospermeae (green), Melanospermeae (brown) and Rhodospermeae (red). Pigmentation is still the main basis of algal classification.
• Kützing (1843–1849), Thuret and Bornet (19th century): studied structure and sexual reproduction of many algae; Thuret observed fertilisation in Fucus.
• Pascher (1914, 1931): grouped algae into phyla based on pigments and flagella.
• F. E. Fritsch (1935, 1945): wrote the classic two-volume book The Structure and Reproduction of the Algae and classified algae into 11 classes (Chlorophyceae, Xanthophyceae, Chrysophyceae, Bacillariophyceae, Cryptophyceae, Dinophyceae, Chloromonadineae, Euglenineae, Phaeophyceae, Rhodophyceae, Myxophyceae). Fritsch's system is followed in most Indian B.Sc courses.
• G. M. Smith (1950, 1955) and Papenfuss (1955) gave classifications using divisions; later workers (Round, Lee, Bold & Wynne) used electron microscopy and molecular data.

Algology in India: M.O.P. Iyengar, Y. Bhardwaja, T.V. Desikachary, N.D. Kamath.

Habit and habitat of Algae: 1) Hydrophytes – Benthophytes, Thermophytes, Planktophytes, Halophytes, Epiphytes. 2) Edaphophytes, Aerophytes – Epihalophytes, Lithophytes

Algae are cosmopolitan. Most are aquatic, but many live on soil, rocks, bark, snow and even inside other organisms.

1) Hydrophytes (aquatic algae) – freshwater or marine.

• Planktophytes (phytoplankton): free-floating or swimming in the water column, e.g. Volvox, Chlamydomonas, diatoms, Microcystis. Excess growth forms water blooms.
• Benthophytes (benthic algae): attached to the bottom of water bodies (rocks, mud), e.g. Chara, Cladophora, most seaweeds like Sargassum (attached species), Laminaria.
• Thermophytes: grow in hot springs at high temperature (up to about 70–85 °C), mostly blue-green algae, e.g. Oscillatoria, Phormidium, Mastigocladus.
• Halophytes: grow in highly saline water (salt lakes, salt pans), e.g. Dunaliella salina, Enteromorpha.
• Epiphytes: grow attached on other aquatic plants or larger algae without harming them, e.g. Oedogonium, Coleochaete, Chaetophora. (Related: epizoic on animals – Cladophora on snail shells; endophytic/symbiotic – Anabaena azollae in Azolla, Nostoc in Anthoceros and cycad coralloid roots; parasitic – Cephaleuros virescens causing red rust of tea.)

2) Terrestrial algae

• Edaphophytes (soil algae): on or in damp soil. Saprophytic/surface forms like Botrydium, Vaucheria, Fritschiella; subterranean forms like Nostoc, Anabaena (nitrogen fixers of paddy fields).
• Aerophytes (subaerial algae): exposed to air, getting moisture from humidity, e.g. Trentepohlia on bark, Desmococcus (Pleurococcus) on walls and tree trunks.
• Epihalophytes: aerial algae growing on the surface of saline soils and salt-encrusted surfaces, mostly blue-green forms. (Some books use the term epiphloeophytes here – algae on tree bark, e.g. Trentepohlia.)
• Lithophytes: grow on rocks and stones – epilithic (on surface) e.g. Gloeocapsa, Scytonema; endolithic (in crevices) e.g. some Chroococcus.
• Cryophytes (extra point): on snow and ice, e.g. Chlamydomonas nivalis producing "red snow".

Range of vegetative structure of algae: Motile, Palmelloid, Coccoid, Filamentous, Siphonous, Advanced

The algal thallus ranges from a single cell to large, tissue-like seaweeds many metres long (Fritsch).

1. Motile (flagellate) forms: cells bear flagella. Unicellular – Chlamydomonas; colonial (coenobium = colony with fixed number of cells) – Gonium, Pandorina, Eudorina, Volvox.
2. Palmelloid forms: non-motile cells irregularly embedded in a common mucilage, but each cell independent; can become motile again, e.g. Tetraspora, palmella stage of Chlamydomonas.
3. Coccoid forms: non-motile unicells or colonies without vegetative division, e.g. Chlorella, Chlorococcum; coenobia like Hydrodictyon (water net) and Pediastrum.
4. Filamentous forms: cells divide in one plane forming chains.
   • Unbranched – Ulothrix, Spirogyra, Oedogonium
   • Branched – Cladophora
   • Heterotrichous (prostrate + erect system, most advanced filament) – Stigeoclonium, Fritschiella, Draparnaldiopsis
5. Siphonous forms: large tubular, coenocytic (multinucleate, aseptate) thallus, e.g. Vaucheria, Botrydium, Caulerpa, Codium.
6. Advanced forms:
   • Parenchymatous – cells divide in many planes: Ulva (sheet), Enteromorpha (tube)
   • Pseudoparenchymatous – compacted interwoven filaments: Batrachospermum, Polysiphonia
   • Highly differentiated – holdfast, stipe, leaf-like parts: Sargassum, Laminaria, Fucus; nodes and internodes in Chara.

5 likely exam questions

Long answer

1. Describe the various habitats of algae with suitable examples, covering hydrophytes and terrestrial (edaphic and aerial) forms.
2. Give an account of the range of vegetative structure (thallus organisation) in algae from motile unicells to advanced forms, with examples and labelled sketches.

Short answer / short notes

3. Write a note on the contribution of Prof. M.O.P. Iyengar to Indian phycology.
4. What are thermophytes and halophytes? Give one example of each.
5. Distinguish between palmelloid and coccoid forms of algae.

Syllabus topics (official RTMNU)

1. Range of pigmentation in Algae (Chlorophylls, Carotenoids, Phycobilins)
2. Morphology and Reproduction of: Volvox, Vaucheria, Sargassum, Batrachospermum

Range of pigmentation in Algae (Chlorophylls, Carotenoids, Phycobilins)

Pigments trap light energy for photosynthesis and give each algal class its colour. Their type and combination is the main basis of algal classification. Three groups are found:

1. Chlorophylls (green, fat-soluble, Mg-porphyrin):

• Chl a – present in all algae (primary photosynthetic pigment).
• Chl b – Chlorophyceae and Euglenophyceae.
• Chl c – Phaeophyceae, Bacillariophyceae, Dinophyceae, Chrysophyceae.
• Chl d – some Rhodophyceae.
• Chl e – reported in some Xanthophyceae (e.g. Tribonema, zoospores of Vaucheria).

2. Carotenoids (yellow/orange, fat-soluble; accessory and protective pigments):

• Carotenes (hydrocarbons): β-carotene in almost all algae; α-carotene in some green and red algae.
• Xanthophylls (oxygen-containing): lutein (green, red), fucoxanthin (brown colour of Phaeophyceae and diatoms), peridinin (dinoflagellates), diadinoxanthin, violaxanthin; myxoxanthin and myxoxanthophyll in Cyanophyceae.

3. Phycobilins (biliproteins) – water-soluble, protein-bound, open-chain tetrapyrroles located in phycobilisomes on thylakoids:

• Phycocyanin (blue) – mainly Cyanophyceae (c-phycocyanin); also red algae (r-phycocyanin).
• Phycoerythrin (red) – mainly Rhodophyceae (r-phycoerythrin); some blue-greens.
• Allophycocyanin – in both.

Phycoerythrin absorbs green-blue light that penetrates deep water, so red algae can live at great depths. Change of pigment ratio with light colour in blue-green algae is called complementary chromatic adaptation (Gaidukov phenomenon).

Morphology and Reproduction of: Volvox, Vaucheria, Sargassum, Batrachospermum

A. Volvox (Chlorophyceae, Volvocales) – freshwater, free-floating, motile colonial alga of ponds in rainy season.

• Thallus: hollow spherical coenobium of 500–60,000 biflagellate cells arranged in a single peripheral layer in mucilage; centre filled with watery mucilage.
• Each cell is Chlamydomonas-like: two equal whiplash flagella, cup-shaped chloroplast with pyrenoid, eyespot, 2–3 contractile vacuoles. Cells joined by cytoplasmic strands (e.g. V. globator).
• Colony shows polarity; anterior cells have larger eyespots; reproductive cells (gonidia) are in the posterior half.
• Asexual reproduction: a gonidium enlarges, loses flagella and divides repeatedly to form a curved plate (plakea) and then a hollow daughter colony with flagellar ends facing inward; it turns inside-out by inversion, so flagella face outward. Daughter colonies are released when the parent ruptures.
• Sexual reproduction: oogamous; monoecious (V. globator, protandrous) or dioecious (V. aureus). Androgonidium forms a plate/bundle of 16–512 spindle-shaped biflagellate antherozoids. Oogonium is flask-shaped with one non-motile egg. Antherozoids enter the oogonium and fertilise the egg.
• Zygote (oospore) develops a thick spiny wall and orange haematochrome; it undergoes meiosis on germination, three nuclei degenerate, one biflagellate zoospore forms a new colony. Life cycle haplontic.

B. Vaucheria (Xanthophyceae, Heterosiphonales; older books Chlorophyceae, Siphonales) – "water felt", on damp soil and in fresh water.

• Thallus: siphonous coenocyte – long, sparingly branched aseptate tubes; septa form only at reproductive organs or injury. Colourless branched rhizoids anchor it.
• Wall of cellulose; thin peripheral cytoplasm with many nuclei and numerous discoid chloroplasts without pyrenoids; large central vacuole; food stored as oil.
• Asexual: (i) compound zoospore (synzoospore) in a club-shaped terminal zoosporangium – large, multinucleate, with many pairs of flagella; (ii) aplanospores (V. geminata); (iii) akinetes/hypnospores in dry conditions; a branched mass of akinetes is the Gongrosira stage.
• Sexual: oogamous, mostly monoecious and protandrous. Antheridium – curved, hook-like, cut off by septum, produces many biflagellate antherozoids (flagella unequal, laterally inserted – one tinsel, one whiplash). Oogonium – sessile, rounded, beaked, with one uninucleate egg; a receptive spot at the beak.
• Zygote becomes thick-walled oospore; germinates after rest by meiosis into a new filament. Standard view: haplontic life cycle.

C. Sargassum (Phaeophyceae, Fucales) – marine brown alga, "gulf weed"; free-floating S. natans forms the Sargasso Sea; Indian coasts have S. wightii, S. tenerrimum (attached to rocks).

• Thallus: diploid sporophyte, bushy, differentiated into disc-like holdfast, cylindrical main axis, primary and secondary laterals, leaf-like laminae with midrib and cryptoblasts (sterile pits with hairs), stalked air bladders for floating, and receptacles bearing conceptacles.
• Anatomy: outer meristoderm (photosynthetic), cortex, central medulla; growth by a three-sided apical cell.
• Reproduction: vegetative by fragmentation (floating species); no asexual spores.
• Sexual: oogamous; sex organs in flask-shaped conceptacles (with ostiole and paraphyses). Antheridia on branched paraphyses; nucleus divides meiotically then mitotically → 64 pear-shaped laterally biflagellate antherozoids. Oogonium on a stalk cell; meiosis + mitosis → 8 nuclei, only one egg functional.
• Eggs come out of the ostiole but remain attached by a mucilage stalk; fertilisation occurs outside; zygote (2n) starts developing on the parent and then detaches to form a new plant.
• Meiosis at gamete formation (gametic meiosis) – life cycle diplontic; no alternation of generations.

D. Batrachospermum (Rhodophyceae, Nemalionales) – freshwater red alga of clear, cold, flowing streams; gelatinous, bluish-violet, "frog-spawn alga".

• Thallus: haploid gametophyte, uniaxial, beaded appearance. Main axis of a row of large cells; at each node arise whorls of short (limited) branches; basal cells give cortical filaments that cover the axis. Growth by dome-shaped apical cell; cells joined by pit connections.
• Asexual: by monospores (mainly in Chantransia stage).
• Sexual: oogamous, mostly monoecious; no motile gametes. Spermatangium at tips of short branches – one non-motile spermatium. Carpogonium – flask-shaped with a long receptive trichogyne.
• Spermatium carried by water to the trichogyne; male nucleus fuses with egg nucleus. Zygote divides (meiosis, as per standard textbooks) to produce gonimoblast filaments ending in carposporangia; this cluster is the carposporophyte. Each carposporangium releases one carpospore.
• Carpospore germinates into a small heterotrichous filament, the Chantransia stage (juvenile), which reproduces by monospores; an apical cell of its erect branch grows into the adult Batrachospermum. Standard B.Sc life cycle: haplobiontic with zygotic meiosis.

5 likely exam questions

Long answer

1. Describe the range of pigmentation in algae with special reference to chlorophylls, carotenoids and phycobilins. Add a note on their importance in classification.
2. Describe the thallus structure and sexual reproduction of Sargassum with labelled diagrams. Why is its life cycle called diplontic?

Short answer / short notes

3. Describe asexual reproduction in Volvox, explaining the process of inversion.
4. Write a note on the compound zoospore of Vaucheria.
5. What is the Chantransia stage of Batrachospermum?

Syllabus topics (official RTMNU)

1. Vegetative structure in fungi: Hyphae, Mycelium, Haustorium
2. Life Cycle of: Aspergillus, Puccinia and Cercospora

Vegetative structure in fungi: Hyphae, Mycelium, Haustorium

Fungi are eukaryotic, achlorophyllous, heterotrophic (saprophytic, parasitic or symbiotic) organisms with absorptive nutrition. Their cell wall is mostly of chitin (cellulose in Oomycetes) and reserve food is glycogen and oil. Except unicellular yeasts and some chytrids, the body is filamentous.

1. Hypha (plural hyphae): a thin, tubular, branched filament that grows at the tip (apical growth).

• Aseptate (coenocytic) hyphae: no cross walls, many nuclei in common cytoplasm – lower fungi (Phycomycetes), e.g. Mucor, Rhizopus, Albugo.
• Septate hyphae: divided by cross walls (septa) into cells – Ascomycetes, Basidiomycetes, Deuteromycetes. Septa have a central pore for cytoplasmic continuity: simple pore with Woronin bodies in Ascomycetes; barrel-shaped dolipore with parenthesome in Basidiomycetes.
• Cells may be uninucleate, binucleate (dikaryotic, n+n) or multinucleate. Clamp connections in dikaryotic Basidiomycetes.

2. Mycelium: the mass or network of hyphae forming the fungal body (thallus).

• Monokaryotic (primary) and dikaryotic (secondary) mycelium; tertiary mycelium forms fruit bodies in Basidiomycetes.
• In parasites: ectophytic (on host surface, e.g. powdery mildews) or endophytic – intercellular (between host cells, e.g. Albugo, Puccinia) or intracellular.
• Plectenchyma – fungal tissue: prosenchyma (loosely interwoven, hyphae distinguishable) and pseudoparenchyma (compact, cells look isodiametric).
• Modifications: rhizoids (Rhizopus), rhizomorphs (root-like strands, Armillaria), sclerotia (hard resting bodies, Claviceps), stroma (cushion bearing fruit bodies), appressoria (flattened tips that attach to host surface before penetration).

3. Haustorium (plural haustoria): a specialised short branch of the hypha of obligate parasites that pierces the host cell wall and absorbs food from the living cell. It does not break the host plasma membrane; the membrane is invaginated around it (extrahaustorial membrane).

• Shapes: knob-like/button-like – Albugo; branched – Peronospora; globose with finger-like lobes – Erysiphe (in epidermal cells); simple to slightly lobed – Puccinia.
• Function: increases absorbing surface while keeping the host cell alive.

Life Cycle of: Aspergillus, Puccinia and Cercospora

A. Aspergillus (Ascomycotina, Plectomycetes, Eurotiales; sexual states Eurotium, Emericella) – common blue, green or black mould on bread, fruits, pickles and leather; A. niger (black mould).

• Mycelium: septate, branched, hyaline; cells multinucleate.
• Asexual reproduction (conidia): a thick-walled cell of mycelium, the foot cell, gives rise to an erect unbranched conidiophore. Its tip swells into a round vesicle bearing flask-shaped phialides (sterigmata) – in one series (uniseriate) or two series (metulae + phialides, biseriate). Phialides cut off conidia in basipetal chains (youngest at base). The whole head looks like a brush or "aspergillum" (holy-water sprinkler). Conidia spread by air and germinate into new mycelium.
• Sexual reproduction (only in some species, e.g. A. nidulans, A. glaucus): coiled ascogonium (with trichogyne) and antheridium develop; plasmogamy gives dikaryotic ascogenous hyphae; by crozier formation, karyogamy occurs in the ascus mother cell; meiosis followed by mitosis gives 8 ascospores per ascus.
• Asci are globose, scattered irregularly inside a closed fruit body – cleistothecium; ascus walls dissolve; ascospores (often pulley-wheel shaped) are released when the cleistothecium bursts.
• Species without a known sexual stage are kept in Deuteromycetes.
• Importance: citric acid and enzymes from A. niger; aflatoxin from A. flavus; aspergillosis of lungs.

B. Puccinia graminis var. tritici (Basidiomycotina, Uredinales) – causes black (stem) rust of wheat. It is an obligate parasite, heteroecious (two hosts: primary host wheat, alternate host barberry, Berberis) and macrocyclic (five spore stages).

• On wheat, mycelium is intercellular with haustoria; elongated rusty pustules on stems and leaves.
• Indian situation: barberry is not important in the plains. Prof. K. C. Mehta showed that urediniospores survive the summer on wheat in the hills (Nilgiris, Himalayas) and are blown to the plains to start fresh infection each year.

C. Cercospora (Deuteromycotina, Hyphomycetes, Moniliales; sexual state Mycosphaerella) – causes leaf spots, e.g. tikka disease of groundnut (C. arachidicola – early leaf spot; C. personata – late leaf spot).

• Mycelium: septate, branched, intercellular, hyaline to light brown; forms a dark stroma below the epidermis or in substomatal spaces.
• Conidiophores: arise in clusters (fascicles) from the stroma through stomata or epidermis; olive-brown, septate, unbranched, geniculate (zig-zag) with conidial scars.
• Conidia: long, slender, obclavate to cylindrical, hyaline to pale olive, multiseptate (3–12 septa), borne singly at the tips.
• Life cycle: conidia spread by wind, rain splash and insects → germinate on leaf, germ tube enters through stoma → intercellular mycelium kills cells → circular brown/black spots with yellow halo → new conidiophores and conidia → secondary infections. The fungus survives in infected crop debris, soil and self-sown plants (and rarely as perithecia of Mycosphaerella). Only asexual cycle is usual.
• Control: crop rotation, burning debris, seed treatment, sprays of carbendazim or mancozeb/Bordeaux mixture.

5 likely exam questions

Long answer

1. Describe the life cycle of Puccinia graminis tritici with the help of labelled diagrams of all spore stages.
2. Describe the vegetative structure of fungi with reference to hyphae, mycelium and haustoria.

Short answer / short notes

3. Describe the asexual reproduction of Aspergillus.
4. Write a note on the conidiophores and conidia of Cercospora.
5. What is a haustorium? Name its types with examples.

Syllabus topics (official RTMNU)

1. Components of Lichens: Phycobiont, mycobiont and cyanobiont
2. Occurrence: Corticolous, Lignicolous, Saxicolous, Terricolous, Marine, Freshwater
3. Nature of Association: Mutualism, Helotism and Parasitism
4. Classification of plant diseases: Endemic, Epidemic and Sporadic
5. Causal organism, symptoms, transmission and control measures of following plant diseases: Ring rot of potato, Powdery mildew of Teak, Yellow vein mosaic of Bhindi

Components of Lichens: Phycobiont, mycobiont and cyanobiont

A lichen is a composite, self-supporting organism formed by a stable association of a fungus and a photosynthetic partner (alga or cyanobacterium). The dual nature was explained by Schwendener (1867).

• Mycobiont (fungal partner): forms most of the thallus and the fruit bodies. Mostly Ascomycetes (ascolichens, e.g. Parmelia, Usnea); few Basidiomycetes (basidiolichens, e.g. Cora, Dictyonema); sterile forms in Deuteromycetes (e.g. Lepraria).
• Phycobiont (green algal partner): Trebouxia (commonest), Trentepohlia, Coccomyxa.
• Cyanobiont (cyanobacterial partner): Nostoc, Scytonema, Gloeocapsa; fixes nitrogen. In tripartite lichens (e.g. Peltigera aphthosa) the cyanobiont lies in small warts called cephalodia.

In a heteromerous thallus the layers are upper cortex, algal (photobiont) layer, medulla and lower cortex with rhizines; in a homoiomerous thallus algal cells are evenly scattered. External forms: crustose, foliose, fruticose.

Occurrence: Corticolous, Lignicolous, Saxicolous, Terricolous, Marine, Freshwater

Lichens are pioneers on bare rock (xerosere) and sensitive indicators of SO₂ air pollution.

Nature of Association: Mutualism, Helotism and Parasitism

• Mutualism (symbiosis): both partners benefit. The fungus absorbs water and minerals, protects the alga from drying and strong light; the alga/cyanobiont supplies carbohydrates (and fixed nitrogen). The term symbiosis was given by de Bary (1879).
• Helotism (master–slave relation): the fungus is the master and the alga the slave (helot); the alga is enclosed and its products are used mainly by the fungus, while the alga gets little benefit.
• Parasitism: the fungus is a parasite on the alga – haustoria penetrate algal cells in some lichens and some algal cells die. Ahmadjian called it controlled parasitism, since the alga survives and multiplies.

Classification of plant diseases: Endemic, Epidemic and Sporadic

• Endemic: constantly present year after year in a particular area, moderate to severe, e.g. wart disease of potato in Darjeeling hills.
• Epidemic (epiphytotic): appears suddenly over a large area, spreads rapidly and causes heavy losses, e.g. late blight of potato (Irish famine, 1845), brown spot of rice (Bengal famine, 1942–43).
• Sporadic: occurs at irregular intervals in scattered places, e.g. Fusarium wilt of cotton.
• Extra: Pandemic – worldwide, e.g. late blight of potato, wheat rust.

Causal organism, symptoms, transmission and control measures of following plant diseases: Ring rot of potato, Powdery mildew of Teak, Yellow vein mosaic of Bhindi

1. Ring rot of potato

• Causal organism: bacterium Clavibacter michiganensis subsp. sepedonicus (= Corynebacterium sepedonicum) – Gram-positive, non-motile, rod-shaped, aerobic.
• Symptoms: late in season lower leaves wilt, margins roll upward, turn pale yellow and die. In tubers, a cut shows a creamy-yellow to light-brown ring in the vascular region; on squeezing, a cheesy, odourless ooze comes out; later the vascular ring separates and secondary rots set in.
• Transmission: chiefly infected seed tubers; contaminated cutting knives, machinery, bags and stores; bacteria survive long in dried slime.
• Control: certified disease-free seed; plant whole tubers; disinfect knives and equipment; quarantine; crop rotation and removal of volunteer plants.

2. Powdery mildew of Teak (Tectona grandis)

• Causal organism: Uncinula tectonae (now Erysiphe tectonae), Ascomycetes, Erysiphales; obligate, ectophytic parasite with haustoria in epidermal cells.
• Symptoms: after the monsoon (Sept–Dec) white powdery patches appear, mainly on the lower leaf surface, with yellow patches above; leaves become chlorotic, curl and fall early; nurseries and young plantations suffer most. Later, minute black dots – cleistothecia with hooked (uncinate) appendages – appear.
• Transmission: wind-borne conidia (in chains, Oidium stage) cause secondary spread; cleistothecia on fallen leaves carry it to the next season.
• Control: collect and burn fallen leaves; dust sulphur or spray wettable sulphur (0.2%), dinocap or carbendazim in nurseries.

3. Yellow vein mosaic of Bhindi (okra, Abelmoschus esculentus)

• Causal organism: Bhendi yellow vein mosaic virus (BYVMV), a Begomovirus (family Geminiviridae) – twinned (geminate) particles with circular single-stranded DNA, associated with a betasatellite.
• Symptoms: vein clearing, then a network of yellow veins enclosing green islands; later whole leaf turns yellow or creamy; plants stunted; fruits few, small, pale yellow, hard and fibrous. Early infection may cause 50–90% loss.
• Transmission: by the whitefly Bemisia tabaci (persistent, circulative manner) and by grafting; not by seed or sap. Weed hosts keep the virus between crops.
• Control: resistant varieties (e.g. Parbhani Kranti, Arka Anamika); uproot and destroy infected plants early; control whitefly with yellow sticky traps, neem oil or imidacloprid; remove weed hosts; barrier crops like maize.

5 likely exam questions

Long answer

1. Describe the components of lichens and discuss the nature of association between the partners (mutualism, helotism and parasitism).
2. Give the causal organism, symptoms, mode of transmission and control measures of yellow vein mosaic of bhindi and ring rot of potato.

Short answer / short notes

3. Classify lichens on the basis of their occurrence (substratum) with one example each.
4. Distinguish between endemic, epidemic and sporadic plant diseases with examples.
5. Write a note on powdery mildew of teak.