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Home > Plant Diseases > Parasitic and Non Parasitic Diseases
Parasitic and Non Parasitic Diseases in Plants
Non Parasitic Diseases :
Parasitic and Non Parasitic DiseasesThis type of plant disorder is often called " functional disorder" or " physiological" disturbance. The term is, however, intended to include all those plant troubles which are not the result of infection by parasitic fungi, bacteria or viruses. It covers for example lasting ill health due water logged soil; excessive lime in the soil; shortage of an essential food element ; excess humidity; fume and spray injury and even short term damage by lightning, frost, hailstones and draught.

Parasitic Diseases
1. Fungus Diseases are those caused by parasitic fungi, and with them are grouped the very similar bacterial diseases.

Fungii - FungiiParasitic fungi are mostly microscopic. They invade higher plants and grow in their tissues (cells), which they kill and then absorb the contents for food. They penetrate and grow in the plant cells by means of fine fungi threads, and spread from plant to plant by means of spores ( the equivalent of seeds in higher plants). These spores are formed at the end of the special threads often inside special fruit bodies, and they are produced in enormous numbers. When released they are carried by wind currents or water to healthy plants where they alight, germinate, grow into the tissue and thus spread the disease again. Most of the fungus parasites overwinter on the plant or in the soil by forming a type of thick walled spore, or some other structure which is resistant to adverse weather.

These fungus parasites may be roughly divided into two types :

- includes powdery mildews ( common on many plants such as roses, delphiniums, michaelmass daisies and marigolds) - produces an obvious and superficial whitish growth on the surface of the leaves stems and petals. This growth is made of fungal threads and spores which cover the leaf surface and feed by sending down a kind of sucker into the surface cells (epidermis) to absorb nourishment.

- In the second type the parasite grows down deeply into the internal tissues sending up threads to produce spores at the surface. The first type is easy to check but unfortunately most fungus diseases belong to the second.

Bacteria
Bacteria DiseasesBacteria which attack plants are much smaller than parasitic fungi, but infect in a similar manner by living in and killing the tissues. They cannot, however, form resting spores. They are able to persist by remaining in plant debris or in dormant cells in the tissue of seeds, corms , bulbs etc.- a method often used by some fungal parasites as well, despite the ability to form resting spores.

Viruses
Viruses DiseasesWith virus diseases, the exact identification of the parasite is difficult. Viruses are so small that they cannot be seen through the ordinary microscopes used to detect and study fungus and bacterial parasites. They can be seen only by means of modern electronic microscopes but even so they are something of a mystery. Undoubtedly, plants suffering from viral disease have some form of infectious agent in their sap, but in many cases its exact nature has not been identified. It is, however, known to be very smalland to multiply within the plant`s cells, so that it is usually distributed throughout all the tissues, with the exception of the seed.

Plants, unlike animals, do not produce viruses to fight anti bodies to fight viruses, although, in some cases, they are able to resist to certain extent. Thus, more than one virus can exist in a plant at the same time. For some plants, though not many, virus attack means sudden death, but usually infected plants become more crippled and degenerate with the passing of each season.

Seeds of infected plants are usually free from virus, so, by saving seed, clean stock can be obtained again. This method is, however, suitable only in the case of fairly short -lived plants.

Special care has to be taken, for example, to exclude the risk of virus infection from good stocks of fruit trees, which are all propogated vegetatively as increase by seed is not feasible.

Symptoms of Virus Infection
The symptoms of virus infection are very varied. Common signs are stunted growth and mottled patterns on the leaves. Other signs are ring- like markings on the leaves; curling or distortion of leaves and shoots; " breaking" of flowers; abnormal production of shoots and many other abnormalities. Infection usually results in all the cells of the plants being invaded by the virus, although shoots already developed are not usually much altered. New shoots and leaves, however, begin to show abnormal symptoms as they begin to grow.

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