Plant disease can decrease the economic, aesthetic and biological value of all kinds of plants. Plant pathology (phytopathology) is the study of the nature, causes, prevention and socioeconomic aspects of plant diseases. Plant diseases are recognized by symptoms such as necrosis (death of cells or tissues), chlorosis (yellowing), wilting (shoot and leaf drooping), rot, dwarfing, tumefaction (formation of gall or localized swelling), bronzing or damping-off (plant toppling), etc. Plant diseases are separated into nonparasitic (noninfectious, nontransmissible) and parasitic (infectious) diseases.
Nonparasitic diseases are caused by improper environmental conditions such as deficiencies and excesses of nutrients, biological toxicants, adverse soil and weather conditions and pollutants. Deficiencies of mineral nutrients (nitrogen, phosphorus, potassium, boron, calcium, copper, iron, magnesium, manganese, molybdenum, sulphur and zinc) can induce some diseases in all kinds of crops. Diseases caused by pollutants can also be found. Air pollutants from combustion include sulphur dioxide and fluorides; those from photochemical reactions include complex nitrates and ozone. In addition, some toxic chemicals occur naturally.
Most plant diseases are caused by parasitic FUNGI, bacteria, mycoplasma, spiroplasma, viruses, viroids, nematodes and protozoa. In addition, some plants (eg, dodders, MISTLETOES) can parasitize other green plants.
Fungi are microscopic or macroscopic threadlike organisms which lack the photosynthetic pigment chlorophyll, and which bear reproductive structures (usually spores). Thousands of fungi cause approximately 100 000 diseases in green plants including rusts, smuts, powdery mildews and ergot of cereals; blights of potatoes and tomatoes; scab of apples; heart rots of trees; downy mildew of tobacco and damping-off of seedlings, etc.
Bacteria, Mycoplasma and Spiroplasma
Bacteria, mycoplasma and spiroplasma are simple cells which lack chlorophyll and which usually reproduce by cell division. Mycoplasma can be considered as simple forms of bacteria which lack cell walls. Spiroplasma are mycoplasmalike cells with a spiral structure. In nature, mycoplasma and spiroplasma are essentially dependent upon leafhoppers for their dispersal. In some cases, bacteria can be disseminated by insects but may also be dispersed by splashing rain, wind, contact, etc. A few hundred species of bacteria attack plants.
Viruses and Viroids
Viruses and viroids represent the simplest form of parasitic entities. Viruses are made up of proteins and nucleic acids; viroids, of unprotected ribonucleic acids. They are considered molecular parasites, using host components for the replication (ie, multiplication) of their infectious nucleic acids. A few hundred plant viruses cause diseases known as tobacco, cucumber or tomato mosaics, potato leafroll, raspberry ringspot, tulip flower breaking, barley yellow dwarf, etc. Several viroids cause diseases such as potato spindle tuber, cucumber pale fruit, hop and chrysanthemum stunt, etc. Viroids and some viruses are transmitted by contact. Many viruses are disseminated in nature by arthropod vectors (eg, APHIDS, leafhoppers, THRIPS, white flies, mealy bugs, MITES); some are also transmitted by nematodes and soil-borne fungi.
Nematodes (eg, eelworms) are nonsegmented INVERTEBRATE animals. Most plant-parasitic nematodes cause root galls, rots and lesions and can severely retard root growth. Some nematodes feed on plants with their stylets (spears). Nematodes produce eggs and larvae which undergo several molts before becoming plant-pathogenic adults. Nematodes are also troublesome because they can act as very efficient vectors of 2 groups of plant viruses.
Protozoa are primitive forms of microscopic animals. A few species have been associated with some plant diseases.
Because of the economic losses (billions of dollars worldwide each year) resulting from plant diseases, control measures are commonly used. Exclusion is prevention of the entry of a pathogen into an area by plant quarantines, certification programs, voluntary or mandatory inspection and pathogen-free production of plant material. Eradication is accomplished by removal of the pathogen hosts, by crop rotation and by heat or chemical treatment of the soil harbouring the pathogen.
Protection methods depend primarily on chemical PESTICIDES such as fungicides, bactericides, nematicides, fumigants and insecticides (against insect vectors). However, some plant pathogens (eg, viruses and viroids) cannot be chemically suppressed because these agents multiply so intimately with the plant cells. Some cultural practices (eg, early and shallow seeding, fertilization) can also protect the plants against the disease-causing agents or conditions. Genetic improvement is the best overall control method, when stable resistance or tolerance genes are easily found and incorporated into the plant's hereditary material. Many agronomically important crops have resistance or tolerance genes against several fungal and viral diseases. Biological control measures and integrated pest-management approaches represent promising avenues in the search for effective and safe control of plant diseases. These methods involve using natural predators against the disease-causing organism (seeINSECT, BENEFICIAL). Recently, research has yielded some transgenic plants with increased tolerance to pathogens. Transgenic plants are produced by inserting specific isolated genes into plants (seeGENETICS).
Research in Canada
Because of the importance of FORESTRY operations and the AGRICULTURE AND FOOD system to Canada's economy, control of plant diseases has been a major focus of research (see AGRICULTURAL RESEARCH AND DEVELOPMENT). Forest stands and extensive hectarages seeded to a single crop, because they are both monoculture systems, are particularly vulnerable to damage from disease pathogens. The cost of such losses is difficult to estimate, however, studies by Agriculture and Agri-Food Canada have shown that it is in the millions of dollars.
Research into the eradication or control of plant diseases takes place in federal and provincial government laboratories, university faculties or colleges of agriculture and forestry and some private companies. Because many horticultural plants begin as transplants from foreign nurseries, the possibility of introducing disease-causing organisms in these plants presents special problems.