Plant Pathology

Plant pathology is the study of diseases, injuries, or other factors that affect the welfare of plants. It is mostly an applied science, meaning that it is studied with a specific, practical purpose in mind, usually of a commercial or economic nature. Also called phytopathology, it is studied by plant pathologists who try to understand and control the many factors that may affect a plant's health and productivity.

All species of plants are subject to disease that may be caused by infectious agents, poor environmental conditions, or the effects of parasites or predators. Unlike animals, there is sometimes no clear distinction between a healthy and an unhealthy plant, and plant pathologists generally describe a plant as diseased when it is regularly disturbed or badly affected by something outside itself. The actual cause might be a living, disease-carrying organism (called a pathogen) or unfavorable environmental conditions. If either factor results in a plant's biochemical or physiological (functions) systems being disturbed, the plant is considered diseased. As with animals, a plant's disease often shows itself in what are called symptoms, and it is therefore important to know how a certain plant looks and behaves in its normal, healthy state in order to recognize any symptoms of disease. Thus, plant pathologists must have knowledge of a plant's normal growth habits as well as the range of variability in the species—or what differences are normal.

CAUSES OF PLANT DISEASE

Plant pathologists divide plant diseases into categories. Infectious diseases include those caused by transmissible (capable of being spread) biological agents or pathogens such as bacteria, fungi, or viruses. Noninfectious disease (usually physiological or functional disorders in which something goes wrong with the way its major systems work) can be caused by environmental factors like nutrient deficiency, excess of minerals, a lack or an excess of moisture, soil temperature that is too high or low, a lack or an excess of light, a lack of oxygen, extreme soil acidity or alkalinity, and air pollution. Finally, noninfectious biological agents that harm plants by either eating them or living off them, called parasites, include such organisms as arthropods (like a mite, spider, or centipede), nematodes (roundworms), and other parasitic plants (like mistletoe that can only live off another plant). The most common causes of disease among plants grown commercially are pathogens (like bacteria), pests (like mites), and bad weather.

In treating plant diseases, the first thing a plant pathologist must do is to determine whether the diseases are caused by a pathogen or by a noninfectious factor in its environment. In many cases, a diseased plant shows obvious symptoms such as an unusual color change in its leaves or a loss of flowers, that immediately indicate its cause. A good example of disease symptoms are the bands of white on the foliage of pine trees caused by too much ozone in the air. Other times, identical symptoms can have very different causes.

Viruses. Among diseases classified as infectious diseases are those caused by a virus. A virus is an extremely small organism that cannot live or reproduce outside its host. A virus usually spreads easily and can severely damage or kill a plant. Viruses can be transmitted by insects feeding off a diseased plant or by a gardener's shears. Viral infections in plants are often called mosaics because of the blotchy appearance they cause. Viruses are extremely difficult to remove and it is often best to try to prevent infection from the start.

Bacteria. Bacteria (a group of one-celled organisms so small they can only be seen with a microscope) are essential to the life cycle on Earth and most are beneficial, but some attack and even destroy plants. Bacteria enter a plant through wounds or natural openings and can be spread by the wind. Like viral infections, bacterial diseases are often difficult to control. Bacteria cause a plant to wilt and even rot. For example, fire blight is a type of bacterial disease that makes an apple tree appear scorched.

Fungi. The greatest numbers of infectious plant diseases are caused by fungi (a group of many-celled organisms that live by absorbing food and are neither plant nor animal) that enter a plant through any openings, as do bacteria. Dutch elm disease is one of the best known fungal diseases. It killed millions of elms in the United States. Fungi cause diseases that are described by words like rust, smut, and mildew.

Parasites. Parasites also can be harmful to plants. For example, nematodes are parasitic worms too small to be seen with the naked eye that suck vital juices from a plant, causing major losses of fruit and vegetables. Other plants can be parasites as well. The mistletoe is a good example of a plant that invades the tissues of another and steals its nutrients. Other plants like strangleweed and witchweed stunt or kill their hosts by robbing them of water and food.

CONTROLLING PLANT DISEASES

Control of plant disease is usually a combination of several strategies, but it always begins by using healthy seeds. To combat actual pests, pathologists might use chemicals called toxicants that kill bacteria, fungi, or parasites. Seeds and soil can be treated chemically and leaves can also be sprayed. Without the widespread use of these chemicals, the successful commercial production of fruits and vegetables would probably not be possible. However, other less dangerous methods can be used, since strong chemicals can harm people as well as pests. Some of these strategies involve simply keeping anything infected away from other plants, properly rotating crops (not growing the same thing in the same location every season), pruning and burning diseased tissue, and ensuring that a plant's environment is the best it can be. Ideal disease control is best achieved by breeding plant varieties that can resist diseases on their own.

[See alsoBotany; Plant Anatomy; Plant Reproduction; Plants; Pollution ]