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Most plant diseases are caused by fungi, bacteria, and viruses. Although the term disease is usually used only for the destruction of live plants, the action of dry rot and the rotting of harvested crops in storage or transport is similar to the rots of growing plants; both are caused by bacteria and fungi.
What causes plant disease? Most people would answer this question by saying that plant pathogens, such as fungi or bacteria, cause plant disease. But this is only part of the answer. Plant pathologists talk about a combination of three crucial factors that must be present to have plant disease.or example, to have Septoria leaf spot on tomato plants, we need a tomato plant that is able to get this disease. Secondly, a pathogen must be present--some organism that causes disease, such as the Septoria fungus that attacks tomato leaves.