Plant Disease
Common vegetable diseases. An expert from the University of Nebraska informs us about vegetable diseases and proper management practices to control them.
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
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
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
Protozoa are primitive forms of microscopic animals. A few species have been associated with some plant diseases.
Control
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 (see INSECT, BENEFICIAL). Recently, research has yielded some transgenic plants with increased tolerance to pathogens. Transgenic plants are produced by inserting specific isolated genes into plants (see GENETICS).
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.
Author ALAIN ASSELIN
Suggested Reading
G.N. Agrios, Plant Pathology (1980, rev 1988).
Links to Other Sites
Davidsonia
The website for "Davidsonia," a journal that provides original, review, discussion or summary work that is of interest to the botanical and botanical garden communities at large. Offers full text articles online. From the University of British Columbia.
Canadian Food Inspection Agency
See the latest news about food saftey issues in Canada from the Canadian Food Inspection Agency.
Eat Your History
A series of stories about the amazing histories of local food delicacies. From The Tyee website.
Glossary: Biotechnology
This glossary was developed to help you understand the terms used in the field of biotechnology. From the website for Health Canada.
House Plants: Diseases
Brief descriptions of common diseases of house plants. A Government of Alberta website.
Plant Health and Pest Management
A well illustrated information source about plant diseases, insect pests, weeds, vertebrate pests, integrated pest management, pesticide management, and invasive pests. A Government of British Columbia website.
Pest Diagnostic Clinic
The website for the Pest Diagnostic Clinic at the University of Guelph. Check out the "Fact Sheets Archive" for information on specific plant diseases.
Plant Pests
A guide to plant pests in Canada. From the Canadian Food Inspection Agency.
Canadian Agri-Food Policy Institute
Check out this website for information and reports about current issues impacting on the productivity and competitiveness of Canada's agri-food sector.
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