Concepts of Plant Disease
Plant diseases were considered to be a curse and punishment to the people by god for wrongs and since they had committed (religious belief and suspertition).
The greak philosopher Theophrastus (300, B.C) was the first to study and write about diseases of trees, cereals and legumes. He wrote a book named “Enquiry into Plants”. In this book he mentioned his experience about plant diseases. His experience was not based on experimentation. He being unable to explain diseases. He beloved that god controlled the weather that brought diseases. Plant diseases were a manifestation of the worth of God. It is due to religious belief, occulation, superstitions or it is the effect of star moon and bad wind. E.g. Romans actually created a special rust God called Robigo for rust diseases of grain crops. They offered sacrifice of red dogs and sheeps.
This was continued for almost 2000 years after Theophrastus. After invention compound microscope in the mid 1600 scientist enable to see many microorganisms associated with diseased plants and they come to believe that the mildews, rust, and other symptoms observed on plants and microorganisms found on diseased plant. Plant parts were the natural product of diseases than the cause and effect of disease.
Louis Pasteur (1860 -63) provided irrefusable evidence that microorganisms arises only from pre-existing microorganisms and fermentation is a biological phenomenon not just a chemical one. It is accepted that a plant is healthy or normal when it can carry out its physiological functions to the best of its genetic potentials.
1. Cell division, differentiation and development.
2. Absorption of water and nutrients.
3. Translocation of nutrients and water synthesis.
5. Translocation and Metabolism or store the photosynthetic products.
6. Production of seed or reproductive organ or survival and multiplication.
Whenever the ability of the cells of a plant or plant part to carryout one or more of these essential functions is interfered either by a pathogenic microorganism or adverse environmental factor. The activities of the cells are altered or inhibited, the cells die or plant become diseased.
Pathogen may Cause Disease in Plant by:
i) Weakening the host by continuously absorbing food from the host cells for their own use.
ii) Killing or disturbing metabolism of host cells through toxins enzymes or growth regulating substances, they secrete.
iii) Blocking the transportation of food, mineral, nutrients and water through the conductive tissues.
iv) Consuming the contents of the host cells upon contact.