The control of plant diseases by using microorganisms (bacteria, viruses) is called as biological control of plant diseases.
Some of the examples are: Lepidoptera
The use of Bt toxin used in the control of plant diseases caused by Lepidoptera (boll worms). Bt toxin is an crystalline proteinous toxin produced by the Gram negative bacteria Bacillus thuringiensis.
The fungus Trichoderma helps in the treatment of plant diseases. They are free-living fungi found in the root ecosystems. They serve as effective biocontrol agents of several plant pathogens.