Citation: 2006, Iraqi Agriculture (Extension), 2006, No.1:56-60.
Since all plants regardless their economic important are subject to be sick, we have to know why and how the plant became sick at any stage of their development. As the plants grown their roots are surrounded by
Soil and their vegetative growth (stems, leaves, flowers and fruits are surrounded by the air. Unfortunately both of them (soil and the atmosphere) contain plenty of phytopathogenic microorganisms. Plant to sick, three very important parameters must interact equally. These parameters are susceptible plant, pathogen with high pathogenicity and suitable environmental conditions (Temperature, Humidity, light etc.). This kind of disease is called infectious diseases because their causal agents are alive (Biotic agents) like fungi, bacteria, nematodes, Viruses, Phyroid, phytoplasma, recketsia, alge, and the parasitic plants such as cuscuta and Orobanch. Other kind of disease caused by abiotic factors such as salinity, drought, air pollutants, temperature (high or low), and the chemicals. This group of diseases called noninfectious diseases.
Key Words: Plant Diseases, Disease Pyramid, Infectious and noninfectious Plant Diseases, Biotic and abiotic diseases agents.