Named Plant Diseases

Spec Mapping — OCR H420 Module 4.1.1 — Communicable diseases, content statement on named plant diseases caused by viruses, bacteria, fungi and oomycetes, with their causative pathogens and effects on the host plant (refer to the official OCR H420 specification document for exact wording). This lesson covers the four named OCR plant diseases — ring rot, tobacco mosaic, potato late blight and black sigatoka — and frames each by the tissue attacked and the cellular mechanism of damage.

Plant pathology matters because plants matter: they fix the atmospheric CO₂ that becomes our food and our timber, and they support every terrestrial food web. Communicable plant disease destroys an estimated 10–30 % of the global harvest each year, a fraction that for some staple crops in some seasons climbs much higher. The science is not new — the Irish Potato Famine of 1845–1849 catastrophically demonstrated what a single pathogen can do to a monoculture-dependent society, with approximately one million deaths and another million emigrants — and it remains urgent: black sigatoka now threatens the world's banana supply, and Phytophthora species continue to evolve resistance to fungicides. OCR specification 4.1.1 requires you to know the four named diseases, the pathogen responsible, the type of pathogen (bacterium / virus / fungus / oomycete), the tissue attacked and the symptom pattern, and to recognise these in unseen contexts.

Key Definitions:

• Blight — a rapid plant disease characterised by widespread withering, necrosis and death of tissues, typically caused by fungi or oomycetes.
• Mosaic — a patterning of light and dark patches on leaves caused by uneven chlorophyll loss; characteristic of many viral infections.
• Necrosis — localised death of plant tissue, usually visible as a dark brown patch.
• Lesion — a region of damaged tissue caused by disease, often with a distinctive shape (concentric rings, halos, streaks).
• Chlorosis — yellowing of plant tissue due to loss of chlorophyll, often a sign of viral disease or nutrient deficiency.
• Notifiable disease — a plant or animal disease that, by law, must be reported to government authorities on suspicion of infection (e.g. ring rot in the UK and EU).

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A Visual Map of the Four Named Diseases

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Ring Rot — Clavibacter michiganensis (Bacterium)

Ring rot is a serious bacterial disease of potatoes (Solanum tuberosum), tomatoes and aubergines. It is caused by the gram-positive coryneform bacterium Clavibacter michiganensis subsp. sepedonicus — a rod-shaped, non-spore-forming actinobacterium in the same broad family as Corynebacterium diphtheriae and the streptomycetes.

Pathogen structure

Clavibacter is a club-shaped (the name means "club-bearing") gram-positive rod, 0.5–0.7 µm by 0.7–3 µm, with a thick peptidoglycan wall and no flagellum. It is an obligate aerobe and grows slowly on standard culture media; clinical identification relies on PCR for the sepedonicus-specific 16S rRNA region rather than waiting for visible colonies.

How it causes damage

• Bacteria enter through wounds in roots, stolons or stems — usually contaminated cutting knives during seed-potato preparation are the proximate source.
• Inside the plant, Clavibacter colonises the xylem vessels of the vascular bundle, multiplying to dense populations and secreting exopolysaccharide slime.
• The slime and the bacterial cells physically block the xylem lumen, preventing the transpiration stream from delivering water to the leaves.
• Leaves wilt — the classical sign of xylem occlusion (note: this is the same hydraulic logic as the cohesion–tension theory of Module 3.1.3).
• In the tuber, the disease produces the characteristic ring of necrotic tissue just inside the skin — the vascular ring of the tuber turns brown and cheesy, eventually disintegrating.

Economic impact and control

In the UK and EU ring rot is a notifiable disease: any suspected case must be reported to the relevant plant-health authority, and confirmation triggers destruction of the entire crop and a multi-year ban on growing host crops on the affected land. There is no curative treatment: control depends on certified disease-free seed potatoes, strict hygiene (disinfection of cutting knives, machinery, boots), and avoidance of cross-contamination during sorting and transport. Resistant cultivars have proved hard to breed because Clavibacter lives in the protected xylem environment and the plant has limited internal defences.

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Tobacco Mosaic Virus (TMV) — Virus

Tobacco mosaic virus is the prototype of the Tobamovirus genus and was the first virus ever discovered: in 1892 the Russian botanist Dmitri Ivanovsky showed that the sap of diseased tobacco plants remained infectious even after passage through a porcelain filter fine enough to remove all known bacteria. In 1898 Martinus Beijerinck named the infectious agent a contagium vivum fluidum — the first explicit recognition of viruses as a new class of entity. Wendell Stanley crystallised TMV in 1935, demonstrating its molecular nature and earning a Nobel Prize.

Pathogen structure

• A rod-shaped virion, ~300 nm long and 18 nm in diameter.
• Single-stranded (+)-sense RNA genome (~6,400 nucleotides) enclosed in a helical capsid built from 2,130 identical 17.5 kDa coat-protein subunits, arranged in a right-handed helix.
• No envelope — TMV is naked, which contributes to its extraordinary stability.

Host range and stability

TMV infects tobacco (Nicotiana tabacum), tomatoes, peppers and over 150 other plant species across diverse families. The naked rod-shaped virion is remarkably resistant: TMV survives for decades in dry plant debris and on contaminated tools and clothing, making it a recurring nuisance for plant breeders and tobacco growers.

How it causes damage

• TMV enters through micro-wounds caused by insect feeding, mechanical handling, or simple rubbing of one leaf against another.
• Once inside a cell, the (+)-sense RNA genome is directly translated by host ribosomes to produce the viral RNA-dependent RNA polymerase, which then replicates the genome and produces sub-genomic mRNAs for the coat protein and movement protein.
• The viral movement protein enlarges plasmodesmata, allowing virions to spread cell-to-cell.
• Infected cells produce chlorotic (yellow) patches in the characteristic mosaic pattern, because chloroplast division is disrupted and chlorophyll synthesis falls.
• Growth is stunted, yields drop and flowers and fruit may be distorted.

Control

There is no cure once a plant is infected. Control measures are entirely about prevention: resistant cultivars (modern tobacco and tomato breeding has introduced the N resistance gene from N. glutinosa, which triggers a hypersensitive response), strict hygiene (disinfection of tools and hands between plants), and removal and destruction of infected plants. Tobacco smokers have historically been excluded from greenhouse work because cigarette tobacco can carry viable TMV.

Exam Tip: TMV's extreme stability is a favourite examiner point. Saying "TMV survives for decades in dry debris" plus "transmitted on tools and clothing" picks up easy marks.

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Potato Late Blight — Phytophthora infestans (Oomycete)

Late blight is caused by Phytophthora infestans, the agent of the Irish Potato Famine of 1845–1849 and still the world's most destructive potato disease. Phytophthora (literally "plant-destroyer") is an oomycete — historically classed with fungi because of its hyphal growth and absorptive heterotrophy, but molecularly a member of the Stramenopiles, more closely related to diatoms and brown algae than to true fungi. OCR accepts the description "fungus-like protoctist" or "oomycete".

Pathogen structure

• Filamentous hyphae lacking cross-walls (coenocytic) — distinct from the septate hyphae of true fungi.
• Cellulose cell walls (true fungi have chitin walls).
• Asexual reproduction via lemon-shaped sporangia at the tips of branched sporangiophores; in cool wet conditions sporangia release motile bi-flagellate zoospores that swim through films of water.
• Sexual reproduction via oospores between mating types A1 and A2 (the A2 mating type re-entered Europe in the 1980s, generating much more aggressive sexual recombinants).

How it causes damage

• Sporangia and zoospores land on potato leaves and germinate in films of water.
• The germ tube produces an appressorium that mechanically penetrates the leaf cuticle, and hyphae grow into the mesophyll.
• Specialised haustoria push into living host cells, absorbing nutrients without immediately killing them — Phytophthora is a hemibiotroph: biotrophic at first, necrotrophic later.
• Hyphae secrete cellulases, pectinases and proteases that digest plant cell walls.
• Infected leaves develop dark brown-black necrotic lesions surrounded by pale halos.
• Sporangia burst out of stomata; spores are spread short distances by rain splash and longer distances on the wind, and can travel hundreds of metres in cool wet weather.
• The pathogen reaches the tubers by sporangia washed into the soil; infected tubers rot in the field and continue to rot in storage, producing the foul smell that helped scouts in 1845 detect outbreaks before symptoms appeared above ground.

Historical significance

Phytophthora infestans arrived in Ireland in 1845 from continental Europe (probably via the same trade routes that brought it from its Mexican-Andean centre of origin). The Irish reliance on a single susceptible potato variety (the "Lumper") meant that when blight reduced yields by ~50 % in 1845 and devastated the crop in 1846, famine was inevitable. Approximately 1 million people died and ~1 million emigrated (largely to the United States and Canada), a demographic collapse from which Ireland did not recover for over a century.

Control

• Resistant cultivars carrying R genes from wild Solanum species (sustained breeding effort — Phytophthora readily evolves around single R-gene resistance, so multi-R-gene "pyramided" varieties are now the goal).
• Fungicide sprays — mancozeb, chlorothalonil, metalaxyl, fluazinam (resistance to metalaxyl is widespread).
• Crop rotation and careful disposal of infected material; volunteer (re-growing) potatoes from previous crops act as overwintering reservoirs and must be eliminated.
• Forecast systems (e.g. the Smith Period in the UK) predict high-risk weather windows and trigger preventive spraying.

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Black Sigatoka — Mycosphaerella fijiensis (Fungus)