ABSTRACT
Plants are attacked by different groups of pathogens individually or sometimes by more than one pathogen-producing complex and more severe disease. The type of external symptoms can, in most cases, indicate the nature of the pathogen responsible for the disease. Fungi, bacteria, viruses, and phytoplasmas cause distinct types of symptoms in most host-pathogen interactions. However, there are some diseases which show similarity in symptoms, though they are induced by different groups of pathogens. Further careful examination under microscope or by other methods may be necessary to establish the nature of the pathogen(s). Internal symptoms, such as histological and cytological changes caused by viruses, have diagnostic value, facilitating the identification of viruses in certain host-virus combinations.
