Phytopathology is most simply translated as the study of disorders in plants. This includes sickness caused by pathogens (disease-causing bacteria, viruses, and fungi). However, plant-eating insects like locusts, or other complex organisms which harm the plant by eating it, are not included in this discipline. It does include non-organic plant disorders, such as drought, nutrient deficiency, wildfires, and pollution.

Phytopathology has an incredibly long history, as farmers going back into antiquity have had problems with their crops dying to disease.

Some of the most common plant diseases are caused by fungi, which infest the tissue of plants and steal nutrients from them, slowly crippling them and potentially killing them. Bacteria also infest plants, but many of them do not harm the plant at all. Most of them eat dead plant matter slowly as it degrades away. As few as one hundred species of bacteria, mostly native to tropical regions, can harm plants, causing rot or tumors on living plants.

Other plant pathogens include viruses, phytoplasmas (which are similar to human-affecting bacteria), molds, and even other plants. Mistletoe, commonly known as a Christmas decoration, is a parasitic plant. Its seeds are deposited in other trees by birds, and the mistletoe grows into the tree, stealing water and nutrients from its host. It can grow to dominate and even kill its host tree.

The most critical application of this study is protecting the food supply. Nematodes, or tiny, worm-like creatures which infect food crops and cause as much as $300 million of damage annually, in Europe alone. The worst-case scenario in this field is a famine caused by crop failure. Famines due to crop failure have a long history. One of the most famous recent examples was the Irish Potato Famine which took place during the mid-1800s. During the time, over one third of the population of Ireland was dependent on potatoes for survival, as they were the cheapest food available. In about 1845, the disease Phytophthora infestans, commonly known as the blight, reached the island, and devastated as much as half of the potato crops, and as much as three-fourths of the subsequent crop in 1846. This led to the deaths of over a million people and forced many others to emigrate to other countries.

Scientists have developed several methods of containing or preventing pathological outbreaks among plants. The most common approach, used worldwide, is the application of pesticides. This has grown controversial as people are rightly afraid to ingest the pesticide chemical accidentally. Just as controversial-if not more-is the genetic engineer of plants. Scientists can "breed" plants to have desirable traits, such as disease resistance, similarly to how dogs have been bred over the centuries into the diverse breeds we know today. Another method is crop rotation, which involves alternating planting different kinds of crops in the same field in different seasons, which starves some plant-specific parasites. Preventing a scenario similar to the Potato Famine in the modern day is incredibly important.

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