Root-knot disease, in which galls appear on the roots of vegetable crops, is caused by
- A.a nematode
- B.a fungus
- C.a virus
- D.an excess of nitrogen
Correct answer
Explanation
The correct answer is A, a nematode. Root-knot is caused by microscopic soil roundworms of the genus Meloidogyne, which enter the young root, settle in it and make the surrounding tissue swell into knots or galls; the plant then cannot draw water properly and wilts in the afternoon heat even when the soil is moist. Option B is wrong because fungal root diseases such as damping off and root rot kill the tissue instead of making it swell. Option C is wrong because viral symptoms appear on the shoot as mosaic, curling and stunting, not as galls on the root; the bacterial crown gall, by contrast, forms a tumour at the collar of the plant and is caused by Agrobacterium tumefaciens. Option D is wrong because excess nitrogen produces lush, dark, weak growth that is prone to lodging and disease, but no galls. Rotation with a non-host crop is the usual control.
Read the full article: Plant Diseases and Their Causes: Notes for Exams
Practice Questions
View allLate blight of potato is caused by which organism?
- A.A virus
- B.Phytophthora infestans
- C.Xanthomonas citri
- D.Meloidogyne incognita
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The epidemic of the 1840s destroyed the potato crop of Ireland and brought on the famine, and the study of that outbreak by Anton de Bary established plant pathology as a science.
Read the full explanation →Rust of wheat is caused by a fungus of which genus?
- A.Puccinia
- B.Ustilago
- C.Fusarium
- D.Claviceps
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The rusts of wheat, named black, brown and yellow after the colour of the pustules they raise on the stem and leaves, are caused by species of Puccinia, and resistance to them was one of the traits selected in the dwarf wheats of the Green Revolution.
Read the full explanation →Red rot, one of the most serious diseases of sugarcane, is caused by
- A.Colletotrichum falcatum
- B.Ralstonia solanacearum
- C.a phytoplasma
- D.a nematode
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The correct answer is A, Colletotrichum falcatum, a fungus that rots the internal tissue of the cane, turning it red with white patches and giving it a sour smell, and which spreads mainly through infected setts used for planting.
Read the full explanation →Citrus canker in plants is caused by
- A.a fungus
- B.a bacterium
- C.a virus
- D.a deficiency of boron
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Citrus canker is caused by a species of Xanthomonas, which enters through wounds, insect punctures and stomata and raises raised corky brown lesions, often with a yellow halo, on leaves, twigs and fruit; it spreads in splashing rain, on tools and on infected nursery plants.
Read the full explanation →Khaira disease of rice is caused by the deficiency of which nutrient?
- A.Zinc
- B.Boron
- C.Molybdenum
- D.Copper
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In khaira disease the leaves of young rice plants develop rusty brown spots that spread until the crop looks scorched, tillering is poor and growth is stunted; an application of zinc sulphate to the soil or as a foliar spray corrects it, which confirms that the cause is a deficiency…
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