Question

The smallest organisms which cause diseases in plants are-
A) Viruses
B) Fungi
C) Bacteria
D) Mycoplasma

Answer 1

Hint:Diseases in plants occur quite frequently and are mostly caused by pathogens. These pathogens can spread from one plant to another causing harmful diseases. Pathogens include bacteria, fungi, viruses, mycoplasma, protozoans, nematodes.

Complete answer:
Pathogens can cause diseases by the following methods-
Degradation of cell walls- some pathogens secrete specific enzymes that causes degradation of the cell wall following which they can enter into the plant and cause disease.
Release of toxins- many pathogens produce toxins which can be host specific or non -specific that can cause damage to the plant.
Production of effector proteins- these proteins are produced by many pathogens and can either be released in the extracellular environment or directly inside the plant. These proteins then interfere with the normal functioning of the plants.
Production of spores- many pathogens produce infectious spores that cause infection in plants.
Few of the organisms that can act as pathogens are- Fungi, mainly Ascomycetes and Basidiomycetes are known to cause several plant diseases like wilt diseases, rice blast, soybean rust, canker rot ; Bacteria such as Erwinia, Agrobacterium are well known plant pathogens ; Virus, plant viruses contain mostly RNA genome such as Tobacco mosaic virus, Cowpea virus; Protozoa like Phytomonas are also known to cause plant disease ; Mycoplasma are also known to cause plant diseases like white leaf disease ,yellow disease. Among them the smallest organism is Mycoplasma.

So the correct option is (D) Mycoplasma.

Note:Plant diseases are quite common and can cause economic impacts when economically important plants are involved. A knowledge about the plant pathogens will help us to treat these diseases in a better way and save these economically important plants from getting destroyed.