Plants can be infected a number of diseases. These are caused by micro-organisms (e.g. bacteria, fungi, viruses; and multi-cellular creatures such as nematodes).
Macroscopic creatures (e.g. mites and aphids) and water/nutrient problems (too little or too much) can also cause symptoms that can be confused with diseases.
Use this tag for all questions about diseases - especially diagnosis, management, and cures.