Disease Triangle and Concept of Plant Disease

Disease is simply defined as the going away of the normal condition of health. In plants, disease is associated with a set of symptoms of the disease and sometimes there are associated signs of the disease. 
Many authors have tried to define disease in plants and they have defined it differently. But disease can be defined in a comprehensive manner as; 

Disease Triangle of Plant Pathology

"Disease is a malfunctioning and complex phenomenon which is caused by the interaction of host, parasite and environment."

In any kind of disease the result is the damage caused to the economic product of the plant and plant scientists are concerned to prevent this damage.

Host is the plant to which a particular pathogen attacks, where as pathogen is any disease causing organism. Environment is perhaps the most important factor which helps the pathogen to cause symptoms on the diseased plant. This particular relationship is called Disease Triangle.
 Some scientists are of the view that "time" is also an important factor in causing disease in plants.

HEALTHY Vs. DISEASED PLANT

A plant is considered healthy if it is carrying out its normal physiological functions such as, meristematic tissues are dividing normally adding to normal plant growth, similarly other specialized plant tissues like vascular bundles are translocating water, minerals and synthesized food from root hairs to the aerial plant parts and vice versa. 
But in case of disease, these normal functions are disrupted; there may be disruption in normal division of the meristematic tissues, blockage of the xylem vessels by the pathogen(s) etc. Most of the times, there are visible symptoms of the plant diseases. 

The normal processes are disrupted, altered, inhibited, cell may malfunction or die. At the beginning of the disease, these affliction is localized and later it may spread to other parts of plant (systemic infection) or it may remain at the site of primary infection (localized infection). 
Symptoms are visible manifestation of the response of the plants to the attack of the pathogen or the unfavorable environmental conditions. The ultimate result of all the processes occurring during disease inciting, there will be a change in form, function and integrity of the plants. In severe cases, the part of plant or entire plants may also die.