Plant Diseases

processes that occur in plants under the influence of various factors—pathogens and unfavorable environmental conditions—and are manifested in the disturbance of functions (photosynthesis, respiration, synthesis of tissue and growth substances, and the flow of water and nutritive substances), and the structure of the organism, causing premature destruction of the plant or affecting some of its organs.

There is not yet a precise and comprehensive definition of plant diseases. In the early stages of the development of phytopathology, any deviation from the normal condition of a plant was considered plant disease. The inadequacy of this definition lay in the difficulty of distinguishing a normal (healthy plant) from an abnormal (diseased plant) condition. The determination of the presence of a pathological process in a plant organism made it possible to redefine plant disease in a new way and to conceive it not as a static condition but as a dynamic process that arises and develops as a result of interaction of the plant with its environment.

Plant diseases diminish yields and impair the quality of plant production. For instance, in years favorable to the spread of phytophthora infection of potatoes, the yield of tubers is decreased by 15 to 20 percent and, in some regions, by 50 percent or more.

More than 30,000 separate plant diseases are known. They are classified by symptoms or types (pathographic classification), by the plants affected (plant-growing classification), and by causes, or causative agents, of the disease (etiological classification). The last, according to which plant diseases are divided into noninfectious and infectious, plays a leading role.

Noninfectious plant diseases are caused mainly by abiotic factors in the environment: disruptions in the regime of mineral feeding, most often by a deficiency (rarely, a unilateral excess) of macroelements (nitrogen, phosphorus, potassium, and magnesium) or a deficiency of microelements, especially boron, zinc, iron, copper, and molybdenum; an unfavorable water regime (deficiency or excess of water in the soil, prolonged rains, or high relative humidity of the air), causing “bleeding” of plants, premature drying up, premature withering of plants, or leaves falling under conditions of water deficiency; or the effects of high or low temperatures on plants, abrupt changes in air and soil temperatures (freezing of shoots, frost cracks, chilling of heat-loving plants in greenhouses and hotbeds or during irrigation of the soil with cold water, and so forth). Causes of noninfectious plant diseases may be harmful impurities in the air and soil (blight and falling of leaves from the effects of sulfur dioxide gas, for example, in the vicinity of metallurgical and chemical plants); residual effects of certain herbicides carried into the soil; an unfavorable light regime, mainly a deficiency of light in greenhouses and hothouses (chlorosis and lodgment or dwarfing with a shortened day); ionizing radiation (alpha, beta, and gamma rays, X rays, and neutrons); or toxins excreted into the soil by certain fungi (species of Fusarium, Botrytis, and so forth) and some higher plants.

Classified as infectious plant diseases are viral, bacterial, and fungous plant diseases. Viral plant diseases include various species of mosaic diseases, yellows, and diseases of withering, dwarfing, proliferation, and pupation. Bacterial plant diseases, or bacterioses, include plant cancer, stem wilt of potatoes, various types of bacterial rot, and bacterial blight of fruit trees and tobacco. Fungous plant diseases, or mycophytoses, include various types of plant rusts, smuts, powdery mildew, fusarioses, rots, cytosporoses, and ascochytoses. Also included in the category of infectious plant diseases are the actinomycoses caused by ray fungi, or actinomycetes (for example, ordinary potato scab and clitellar sugar beet scab); algophytoses, caused by parasitic algae (for example, red rust of the tea bush); anthophytoses, caused by parasitic and semiparasitic flowering plants (for example, broomrape, dodder, mistletoe, Loranthus, yellow rattle, and eyebright); and helminthophytoses, caused by parasitic nematodes—gall, sugar beet, potato, strawberry, garlic, and others. The causes of infectious plant diseases may also be damage to plants by parasitic insects (entomophytoses). For example, grapevines are damaged by phyloxera, fruit trees by the aphid, and some plants by species of insects that form galls on leaves and branches, such as arachnids (mainly plant-eating ticks, among which the most widespread and harmful are the spider mite and the fruit mite). (The disease caused by arachnids is called arachnophytoses.)

The characteristics of the pathogenesis of plant diseases are determined first by the properties of the causative agent, the susceptibility of the plants, and the coincident environmental conditions. Several principal phases are distinguished in the pathogeneses of plant diseases: the preinfectious phase, infection, incubation period, and postincubation phase. During the preinfectious phase, spores and other agents of infection fall on leaves, flowers, fruits, and other organs with raindrops or dew. Under favorable conditions they are embedded in plant tissues through the stoma or other paths, or they first germinate and propagate on moist, dead organic substances found on living plants or in their immediate vicinity and then become embedded in living tissues. In the phase of infection, the causative agent invades cells from the intercellular interstices and infects the plant. In many infectious diseases, such as gray rot, there is another mechanism of infection. Infectious agents on dead plant parts in contact with living organs of the plant begin to germinate under favorable conditions and secrete toxins that penetrate the living cells of the plant, poison it, and kill or weaken it. Then hyphae grow in those cells. The incubation period is the period of the latent development of pathological processes in the plant from infection to the appearance of external symptoms. Its duration depends on the temperature and humidity of the air and the resistance or susceptibility to disease of the plant or its separate organs. The postincubation phase is characterized by the intensification of symptoms and growing intensity of infection. The causative agent multiplies internally or on the surface of the diseased plant; infectious elements are spread through the air by raindrops, insects, or other means and may cause massive affection of the plant. Plants have various defensive reactions. In response to the implantation of the causative agent, the activity of the plant’s oxidative enzymes is intensified, the quantity and activity of phytoncides are increased, cell walls become clogged and atrophy, and the infected cells fall out together with the causative agent. As a result of this process, some groups of cells around the primary focus of infection, and sometimes the whole plant, acquire increased resistance and become a sort of barrier against the spread of the causative agent in the plant. If the causative agent cannot overcome the resistance of the tissues, the disease is limited to a spot of chlorotic or atrophied tissue (necrosis).

In controlling plant disease, prophylactic measures are of decisive value. These include the creation of the best conditions for the growth and development of agricultural crops, development of resistant varieties, rational seed-growing, chemical treatment of seeds, and chemical treatments of vegetating plants such as spraying and dusting. The treatment of diseased plants is also of considerable value—for example, restoration of chlorotic trees, thermal disinfection of wheat and barley seeds infected with powdery mildew, heating of tubers and slips, and grafting material infected with certain viruses. In order to prevent the transference of causative agents of plant diseases from one country to another, quarantine measures are taken.