These are very small unicellular organisms. They are said to be ubiquitous because they can be found everywhere even in the remotest places where other plants and animals cannot live.
The attack of bacteria on plants is manifested as rottening of plant parts accompanied with very bad smell. Though most bacteria are destructive as they cause disease*, some of them are useful man since they help in the following ways:
(a) Decomposition of compost
(b) Production of drinks, e.g. beer
(c) Making of butter and cheese
(d) Fixation of atmospheric nitrogen in root nodules of leguminous plants, thereby helping in increasing soil fertility.
There are three types of bacteria based on their shapes. They are (spherical), bacilli (rod shaped) and spirilla (Spiral shaped).
These are plant species but they do not possess chlorophyll stems roots and leaves. The body is made up of myceliium which of tiny delicate branches called hyphae. The fruiting bodies originate from some of these hyphae. The sporangia many tiny spores through which the fungus is dispersed-. fungi have no chlorophyll, they cannot manufacture their own they depend on their host for their food, thereby causing use hi the host plant.
These are minute organisms that can only be seen under the tron microscope. Plants suffering from this disease show:
(a) Coloured spots and bands on the leavesu
(b) Twisted and wrinkled leaves
(c) Stunted growth of the entire plant.
Sucking insects e.g. mealy bugs, aphids and white flies can limit virus disease from one infected plant to a healthy one.
Almost all virus diseases have no effective cure. It is advisable to prevent and control the disease as the case may be.
These are soil-borne organisms which usually attack the roots of plants. They are capable of causing considerable damage to the plant. Affected plants show decline in productivity and the roots become distorted with enlarged growths.
Soil sterilization may help to control the spread of nematodes.
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