Question

modes of infection and dissemination

Answer 1

The micro-organisms obtain their food either by breaking down dead plant and animal remains (saprophytes) or by attacking living plants and animals (parasites). In order to obtain nutrients, the parasitic organisms excrete enzymes or toxins and kill the cells of the tissues of host plant, as a result of which either the whole plant or a part of it is damaged or killed, or considerable disturbance takes place in its normal metabolic processes. Micro-organisms are reproduced and disseminated mainly by spores. Wind, water, seed, diseased plants, cuttings and tubers, animals, men and insects and soil help to disseminate diseases.

Viral diseases are spread as a result of direct contact, through seeds, diseased cuttings and grafts, and more commonly by insects and other vectors. Most of the fungi and bacteria erupt or ooze from the surface after the host tissues have been damaged, whereas, the viruses work altogether internally.