ABSTRACT
Characteristics of Microbial Pathogens Crop plants suffer due to several pathogenic (biotic) and physiogenic (abiotic) diseases. Microbial pathogens responsible for most plant diseases may be fungi, bacteria, phytoplasmas, viruses, and viroids. Physiogenic diseases are due to nutrient deficiencies, toxicity due to excess of minerals, lack or excess of soil moisture, extremities of temperature, light, and oxygen, variations in soil pH, air pollution, etc. Most microbial pathogens induce characteristic external symptoms (observable with the naked eye) on susceptible host plant species, whereas some viruses cause no observable symptom of infection in certain plant species known as carriers. This type of infection is termed latent infection. Symptoms of disease may be seen on certain plant parts such as leaves, stem, flowers or fruits and the pathogen may be confined to such tissues or organs showing symptoms. This kind of infection is called local infection. Diseases such as leaf spots, blights, powdery mildews, and rusts are due to local infection by fungal pathogens. Some pathogens may be able to spread from the point of entry into the plant to different organs in which symptoms are induced. Fungal pathogens inducing wilts and smuts, viruses, and phytoplasmas cause systemic infection in their respective host plants. In addition to the external symptoms (observed on the plants), some viruses cause characteristic internal symptoms of diagnostic value, such as formation of intracellular inclusions and phloem degeneration (Narayanasamy, 1997, 2001). The symptoms may indicate the nature of the pathogen to some extent.
