Question

Several microorganisms cause diseases in plants and reduce yield. Which of the following plant diseases is incorrectly matched with its causal organism?
A) Citrus canker- bacteria
B) Tobacco mosaic – virus
C) Rust of wheat – fungus
D) Yellow vein mosaic of bhendi - bacteria

Answer 1

Hint: Any agent that can cause a disease is known as a pathogen. These include viruses, bacteria, fungi, protozoans, and parasitic infections. When talking of plant pathology, we can also call these agents causal organisms.

Complete Answer:
- The bacterium Xanthomonas citri is responsible for citrus canker which affects plants like lemons, oranges, etc. In this disease, the leaves, small branches, and fruit are affected, so the unripe crop drops off the plant. It easily spread over long distances. The causative agent matches the disease. Option A is incorrect.
- Viruses of the genus Tobamovirus are responsible for the tobacco mosaic infection. This affects many plants of the family Solanaceae, especially tobacco. Tobacco mosaic virus is one of the best studied plant pathogens. Option B is incorrect.
- The fungus Puccinia triticina is responsible for causing leaf rust or brown rust in cereal crops. Plants suffering from this condition typically have small pustules on their leaves, which can easily be rubbed to spread the spores. The overall growth and yield is stunted. Option B is incorrect as the organism matches the disease.
- The bhindi or okra yellow vein mosaic is caused by the Begomovirus, not by a bacterium. The disease is characterised by chlorosis, mosaic green discolouration, stunted plant and fruit growth, and depending on when the plants were infected, they may even dry up. As the causal agent is not a bacterium, but a virus, option D is the correct answer.

Hence the correct answer is option D.

Note: Both living and non-living agents can cause plant diseases, like environmental factors which stress the plant. The plants appearance, physiological processes, and yield or harvest have to be affected for a condition to count as a disease.