Whether on trees, shrubs, or other plants, plant diseases require three things to develop:
Presence
of a disease causing agent called a pathogen
Susceptibility to that particular pathogen
Environmental factors
which can affect both the plant and the
pathogen.
These factors above are subject to a number of variations, depending on the particular plant and pathogen involved. For example, genetic inconsistency cause plants to respond differently to disease, just as it controls height variations, autumn colour, and susceptibility to insect attack. Soil and weather conditions also have different effects on different plants.
Diseases can be classified into two broad categories:
Those caused by infectious
or living agents
Those caused by non-infectious,
or non-living agents.
Examples of infectious agents include fungi, viruses, and bacteria.
Non-infectious diseases can be caused by such factors as nutrient deficiencies, temperature extremes, vandalism, pollutants and fluctuations in moisture.
Factors of non-infectious diseases can often produce symptoms similar to those caused by infectious diseases, therefore it is essential to distinguish between the two in order to give proper treatment.