Plant diseases are caused by pathogenic bacteria, viruses, fungi, etc. which are called pathogens. Control of plant diseases are possible by controlling pathogens and pests. When it is controlled by micro-organisms then it is called biological control and these microbes are called biological control agents. The controlling measures are-
(i) Control by Viruses – In this categories the viruses are used to control plant disease caused by pest. This is achieved by killing targeting pests. eg. Baculoviruses are pathogens that attack insects.
(ii) Control by Bacteria – In this categories the Bacillus thuringiensis is used to control caterpillars of butterfly.
(iii) Control by Fungi – In this category the freeliving fungi Tricoderma viridi is used in treatment of wound of fruit trees.