Concept of Disease in Plants

Because it is not known whether plants feel pain or dis-comfort and because, in any case, plants do not speak or otherwise communicate with us, it is difficult to pin-point exactly when a plant is diseased. It is accepted that a plant is healthy, or normal, when it can carry out its physiological functions to the best of its genetic poten-tial. The meristematic (cambium) cells of a healthy plant divide and differentiate as needed, and different types  of specialized cells absorb water and nutrients from  the soil; translocate these to all plant parts; carry on photosynthesis, translocate, metabolize, or store the photosynthetic products; and produce seed or other reproductive organs for survival and multiplication.

When the ability of the cells of a plant or plant part to carry out one or more of these essential functions is interfered with by either a pathogenic organism or an adverse environmental factor, the activities of the cells are disrupted, altered, or inhibited, the cells malfunction or die, and the plant becomes diseased. At first, the affliction is localized to one or a few cells and is invisi-ble. Soon, however, thereaction becomes more wide-spread and affected plant parts develop changes visible to the naked eye. These visible changes are the symp-toms of the disease. The visible or otherwise measura-ble adverse changes in a plant, produced in reaction to infection by an organism or to an unfavorable environ-mental factor, are a measure of the amount of disease  in the plant. Disease in plants, then, can be defined as the series of invisible and visible responses of plant cells and tissues to a pathogenic organism or environmental factor that result in adverse changes in the form, func-tion, or integrity of the plant and may lead to partial impairment or death of plant parts or of the entire plant.

The kinds of cells and tissues that become affected determine the type of physiological function that will bedisrupted first. For example, infection of roots may cause roots to rot and make them unable to absorb water and nutrients from the soil; infection of xylem vessels, as happens in vascular wilts and in some cankers, interferes with the translocation of water and minerals to the crown of the plant; infection of the foliage, as happens in leaf spots, blights, rusts, mildews, mosaics, and so on, interferes with photosynthesis; in-fection of phloem cells in the veins of leaves and in the bark of stems and shoots, as happens in cankers and  in diseases caused by viruses, mollicutes, and protozoa, interferes with the downward translocation of photo-synthetic products; and infection of flowers and fruits interferes with reproduction. Although infected cells in most diseases are weakened or die, in some diseases, e.g., in crown gall, infected cells are induced to divide much faster (hyperplasia) or to enlarge a great deal  more (hypertrophy) than normal cells and to produce abnormal amorphous overgrowths (tumors) or abnor-mal organs. Pathogenic microorganisms, i.e., the transmissible biotic (=living) agents that can cause disease and are generally referred to as pathogens, usually cause disease  in plants by disturbing the metabolism of plant cells through enzymes, toxins, growth regulators, and other substances they secrete and by absorbing foodstuffs from the host cells for their own use. Some pathogens may also cause disease by growing and multiplying in the xylem or phloem vessels of plants, thereby blocking the upward transportation of water or the downward movement of sugars, respectively, through these tissues. Environmental factors cause disease in plants when abiotic factors, such as temperature, moisture, mineral nutrients, and pollutants, occur at levels above or below a certain range tolerated by the plants.