ABSTRACT
Organisms suffer from disease or disorders due to some abnormality in the functioning of their system. When the living organisms were divided into plant and animal kingdoms, fungi were included among the plants. Viroids are molecular parasite of higher plants composed of naked, single stranded, low molecular weight, circular RNA which utilizes only host component for its replication. The fungi that cause disease on plants are a diverse group, and because of their large numbers and diversity, only a broad, sketchy classification of some of the most important genera is presented here. A number of flowering plants are parasitic on economic plants and cause considerable damage. Reproduction of plant parastic nematodes is of three general types, varying with species. Bacteria multiply with astonishing rapidity and their significance as pathogens stems particularly from the fact that they can produce tremendous numbers of cell in a short period of time.
