Named Plant Diseases

OCR specification 4.1.1 (b) requires you to know a defined list of plant diseases, together with the type of pathogen responsible and the effects on the host plant. These diseases matter both scientifically and economically: crop losses due to communicable disease reduce global food production by an estimated 10–30% each year. In this lesson we look at the four named OCR plant diseases — ring rot, tobacco mosaic virus, potato late blight, and black sigatoka — and see how different pathogens attack different plant tissues.

Key Definitions:

• Blight — any plant disease that causes rapid withering and death of tissues, often caused by fungi or oomycetes.
• Mosaic — a patterning of light and dark patches on leaves, typical of viral infections.
• Necrosis — localised death of plant tissue.
• Lesion — an area of damaged tissue caused by disease.

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Ring Rot — Clavibacter michiganensis

Ring rot is a serious bacterial disease of potatoes, tomatoes and aubergines. It is caused by the Gram-positive bacterium Clavibacter michiganensis subsp. sepedonicus.

How it causes damage

• The bacterium invades the vascular tissue (xylem) of the plant, where it multiplies and releases exopolysaccharide slime.
• Xylem vessels become blocked, preventing water transport to the leaves.
• Leaves wilt and the potato tubers develop a characteristic ring of necrotic tissue just inside the skin — hence the name.

Economic impact

• In the EU, ring rot is a "notifiable disease": a single infection leads to destruction of the entire crop and a ban on growing potatoes on that land for two years.
• No treatment exists once a plant is infected. Control is by certified disease-free seed potatoes and strict hygiene.

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Tobacco Mosaic Virus (TMV)

Tobacco mosaic virus is a viral disease affecting tobacco, tomatoes, peppers and over 150 other plant species. It was the first virus ever discovered, in 1892 by Dmitri Ivanovsky.

Structure

• A rod-shaped virion, about 300 nm long.
• Single-stranded RNA enclosed in a helical capsid of 2130 identical protein subunits.

How it causes damage

• TMV enters through wounds in the leaf surface (e.g., insect damage, human handling, or rubbing between plants).
• Inside cells, viral RNA hijacks the host's ribosomes to produce viral proteins and new RNA.
• Infected cells produce chlorotic (yellow) patches in a characteristic mosaic pattern, because chloroplasts are damaged and chlorophyll production falls.
• Growth is stunted, yields drop, and flowers and fruit may be distorted.

Control

• No cure once infected. Control is by resistant varieties, strict hygiene (disinfection of tools and hands between plants), and removal of infected plants.

Exam Tip: TMV is famously stable — it can survive for decades in dry plant debris and can be transmitted on clothes or tools. Examiners love to ask about this.

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Potato Late Blight — Phytophthora infestans

Late blight is caused by Phytophthora infestans, an oomycete (water mould). Despite being traditionally grouped with fungi, oomycetes are more closely related to diatoms and brown algae — OCR accepts it as a protoctistan or oomycete for this specification.