Plant diseases are generally caused by fungi, bacteria and viruses. A disease is a plant malfunction, not the causal organism.
Grey mould disease on tomato
Fungi are like very tiny plants, often microscopic, with no chlorophyll. To stay alive, they must feed off dead, or living, organic material and they cause most plant diseases. Bacteria consist of single cells, so small they are visible only with a microscope. There are not many major bacterial plant diseases. Viruses are even smaller than cells; they consist mainly of the genetic code for their own production. They cause many plant diseases.