The Mechanisms by Which Pathogens Cause Disease

Plant diseases cause a huge loose of fruits, vegetables and crops since long time ago, historically diseases causes huge loose that leads to the dead of millions of peoples on earth. During early peoples, not know the causes of diseases, they were considering the cause of diseases some mysterious agents, much myth related to the causes of diseases.	As science advance the mysterious agents of diseases discovered, peoples come to know the real causes and agents. Scientist observed fungi, bacteria and microorganisms were the causes of different diseases, but another big question arise after the discovery of casual agents, “How fungi and microorganisms cause diseases?”	Previously it became noticeable that fungi and other microorganisms were the causes rather than the results of plant disease, efforts began to understand the mechanisms by which microorganisms cause disease.
	In 1886, deBary, working with the Sclerotinia rot disease of carrots other vegetables, noticed that host cells were killed in advance of the invading fungal hyphae and a juice from rotted tissue released that could able break down healthy host tissue, when juice from rotted tissue boiled and treat healthy tissues, had no effect on healthy tissue. DeBary concluded that the pathogen produces enzymes and toxins that degrade and kill plant cells from which the fungus can then obtain its nutrients.
After deBary, many attempted to show that most plant diseases, particularly vascular wilts and leaf spots, were caused by toxins secreted by the pathogens, but those claims could not be confirmed. A 1925 suggestion that the bacterium Pseudomonas tabaci, the cause of the wildfire disease of tobacco, produces a toxin that is responsible for the bacteria-free chlorotic zone (“halo”) surrounding the bacteria-containing necrotic leaf spots was confirmed in 1934.		The wildfire toxin was the first toxin to be isolated in pure form in the early 1950s. In 1947, a species of the fungus Helminthosporium (Bipolaris), which attacked and caused blight only on oats of the variety Victoria and its derivatives, was shown to produce a toxin named victorin. This toxin could induce the symptoms of the disease only on the varieties susceptible to the fungus.
	Many other bacterial and fungal toxins were subsequently detected and identified. The toxins exhibited several distinctive mechanisms of action, each affecting specific sites on mitochondria, chloroplasts, plasma membranes, specific enzymes, or specific cells such as guard cells. In addition, several detailed biochemical studies were carried out to elucidate the mechanisms by which toxins affect or kill plant cells or by which cells of resistant plants avoid or inactivate them.