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