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Monday, October 19, 2009

How do plants and trees get infected?

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The outer surfaces of trees like our skin remain exposed to the external environment which contains everything living as well as non-living. These areas remain exposed all the climatic factors and energies and human activities. The sad thing is that a tree can not ask us anything in its defense. Nor can it do anything to take revenge. It means a tree is like a fool who can not understand his benefit or loss, and even if it understands, it can do nothing. Humans identified this weakness of trees in the beginning of civilization. Hence his first fearless attack was oriented towards trees. He is still going on attacking plants and trees as plants and trees cannot take revenge. This is one angle of my thought. But let me proceed in this angle only.

Besides humans, other living beings like termites, ants, fungi etc. too understand the weakness of plants and trees. Fungi and bacteria being microscopic in their forms in the initial stages, invade our skin and plant- skin also, as neither we nor plants can see them developing even in the superficial parts of our/their bodies. But as the development proceeds our nervous system reports and we start scratching and searching for some poison to kill the invader. Plants cannot do this. Though many plants have specific chemicals in their skin that oppose the invasion of foreign things, but invaders remain equipped with more effective weapons and get success in establishing in the infection. Here, one thing is important to be considered. And that is the perfect defense of our skin and also of the skin of a plant or tree. The basic difference between our skin and the skin of plants and trees are that our skin remains elastic where as their skin is not. And probably, this is the reason why plant skin gets ruptured when it experiences the secondary growth. In very general and not too scientific terms the broken skin of trees and plants act as very good habitats for both the parasitic and saprophytic microorganisms.Sometimes the tree skin is ruptures by rubbing of deer or other animals in the forest. Such rubbed areas remain exposed to parasitic infections. This is the reason why some good forest men fence most of the trunks of young plants. Many different types of plants, animals, plant associations etc. have also been reported to colonize a sick tree trunk.Mosses, lichens, algae, and even ferns have also been reported to colonize tree trunks, but only after they have been infected repeatedly in succession

Parasitic microorganisms invade living tissues only, whereas saprophytic microorganisms colonize on dead tissues. After a long time is passed after the rupture of a plant skin it dies and swells. The saprophytic fungi colonize there and start decomposing the dead material of the skin- now called as cork. They excrete certain substances; call them enzymes that dissolve the cellulose and lignin of the cork and release gases into the atmosphere. Thus a day comes when the inner skin is exposed. The inner skin is now invaded by some parasitic microorganisms. Thus succession goes on and on, and the tree or the plant becomes sick. If we don’t care and don’t attend it with useful substances- say fungicides, antibiotics etc. whatever you may call, the plant or the tree is sure to proceed gradually towards death. This story is retold by tree-trunks in the following picture.

1 - Fungi on the dead bark

2 - Microorganisms developing their colonies on the bark of a tree trunk

3 -Bracket fungi on tree-trunk

(credit- Global Forest Watch, Canada)

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