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Diagnosing Plant Problems: Plant Diseases
National Plant Diagnostic Network First Detector Training Williamson, Riley, and Maloy. December 2006. Update, August 2008. NPDN Publication No. 0013
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Used by permission of the American Phytopathological Society
MODULE 4 – Diagnosing plant problems: Plant Diseases Adapted from “Plant Disease Diagnosis” by M. B. Riley, M. R. Williamson and O. Maloy. In: The Plant Health Instructor. DOI: /PHI-I Used by permission of the American Phytopathological Society
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Plant Problem Diagnosis
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Diagnosis –process used for the identification of problems. Tools: Plant sciences, especially plant pathology and entomology Arts of investigation and detective work
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Plant Problem Diagnosis: Preliminary Considerations
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis: Preliminary Considerations Know Normal Appearance of Plant Proper plant identification is critical. Scientific vs. common names Variety or cultivar Recognize healthy plant appearance. What’s normal for specific plant? Compare with healthy plants of same genus, species and cultivar.
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 1. Consider the Possible Agents: Abiotic – non-living agent Damage from chemicals, weather, mechanical Nutritional problems Biotic – living agent Pathogens - parasitic microorganisms that cause diseases Pests – insects or mammals feeding on or damaging plants
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 2. Consult literature resources for possible diseases and disorders. Indexes listing hosts and their pathogens Websites providing information Books with background info and host/pathogen lists Compendia for information on specific crops
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 3. Investigate symptom progression. Biotic disease – symptoms progress and nearby plants become infected. Abiotic disease – generally a lack of symptom progression. Does not spread. Exception – nutritional symptoms progress slowly.
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 4. Ask questions When was the problem noticed? Was the damage sudden or gradual? How old are affected plants? Percentage of plants affected? What is the degree of injury?
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 5. Observe patterns. Determine prevalence of problem Large area/All plants – generally abiotic Scattered, localized – generally biotic Check for distribution of symptoms Uniform – generally abiotic Random – generally biotic
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Observation of Patterns
MODULE 4 – Diagnosing plant problems: Plant Diseases Observation of Patterns Scattered symptom distribution Random pattern of damage Photo: R. Billings, Texas Forest Service, Loblolly pine killed by southern pine beetle
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Observation of Patterns
MODULE 4 – Diagnosing plant problems: Plant Diseases Observation of Patterns Symptoms distributed in a large area. Damage pattern is uniform. Used by permission of M. Williamson Cotton field with chemical damage
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 6. Review cultural practices. Proper planting technique Fertilizer and pesticide application Irrigation frequency 7. Review environmental conditions. Temperature extremes Drought or excess rain Soil type and conditions 8. Check host specificity – many hosts may mean an abiotic problem
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 9. Check for symptoms and signs. Symptoms - plant reactions or alterations of a plant’s appearance due to a disease or disorder Signs - actual pathogen, parts or by-products seen on a diseased host plant
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Symptoms Identify and classify symptoms Identify plant parts affected
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Identify and classify symptoms Underdevelopment Overdevelopment Necrosis or death Alteration of normal appearance Wilting Identify plant parts affected May need to identify source of symptoms within plant system
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Symptoms Underdevelopment Stunting of plants, leaves
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Underdevelopment Stunting of plants, leaves Shortened internodes Inadequate chlorophyll production Caused by many types of pathogens Photo: J. Dunez, Chrysanthemum stunt viroid infection of chrysanthemum showing stunting and earlier blooming of affected plants; healthy plants in middle.
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Symptoms - Overdevelopment
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms - Overdevelopment Photo: Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Photo: Joseph O'Brien, USDA Forest Service, Gall on stem of eastern redcedar caused by Agrobacterium tumefaciens. Oak leaf blister, caused by Taphrina caerulescens. Overgrowth of leaf tissue causes thickening and distortion. Galls form on all plant parts, caused by many pathogens.
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Symptoms Over-development - more biotic causes
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Over-development - more biotic causes Root knot nematodes - root galls Callus formation around cankers Tissue proliferation -some downy mildews and phytoplasmas Some insects and mites also cause galls.
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Symptoms Over-development Abiotic causes Oedema
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Over-development Abiotic causes Oedema Fasciation can be physiological or due to biotic causes. Some tree burls Used by permission of M. Willliamson Oedema of camellia, a physiological disorder
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Symptoms and Signs Tissue Necrosis Fungal leaf spots
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms and Signs Tissue Necrosis Fungal leaf spots Usually round, not vein-limited Elongated on narrow leaves or stems May have alternating zones of light and dark tissue Sporulation or mycelia may be evident Photo: Paul Bertrand, University of Georgia, Target spot of tobacco, caused by Thanatephorus cucumeris.
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Symptoms and Signs Tissue Necrosis Bacterial leaf spots
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms and Signs Tissue Necrosis Bacterial leaf spots Often dark and water-soaked Often vein-limited, giving angular shape Bacterial “flow” observed under microscope Photo: Volcani Center Archives, Agricultural Research Organization, Bacterial leaf spot of sweet pepper caused by Xanthomonas campestris pv. vesicatoria
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Symptoms and Signs Tissue Necrosis Fruit rots Firm or soft and watery
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms and Signs Tissue Necrosis Fruit rots Firm or soft and watery Colors vary Fungal sporulation may be present Fungal and bacterial causes Photo: Clemson University - USDA Cooperative Extension Slide Series, , Brown rot of peach, caused by Monilinia fruticola
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Symptoms Tissue Necrosis Cankers = localized necrotic lesions
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Tissue Necrosis Cankers = localized necrotic lesions Sunken or swollen or both Mainly caused by fungi and bacteria Mechanical injury can cause Photo: Joseph O'Brien, USDA Forest Service, Sugar maple canker caused by Nectria spp.
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Symptoms Tissue Necrosis Blight = rapid death or dieback.
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Tissue Necrosis Blight = rapid death or dieback. Also from coalescing leaf spots, e.g. early blight of tomato Mainly fungal and bacterial causes Photo: Robert L. Anderson, USDA Forest Service, Fire blight caused by Erwinia amylovora
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Symptoms Tissue Necrosis Dieback - many causes
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Tissue Necrosis Dieback - many causes Girdling cankers. Follow dieback to detect Dieback pathogens w/out cankers Root problems Mechanical, chemical or cold damage Photo: Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Pitch canker of Virginia pine, caused by Fusarium subglutinans
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Symptoms Tissue Necrosis Root rots Root lesions
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Tissue Necrosis Root rots Root lesions Darkening and softening of roots Sloughing off of outer tissues Yellowing of foliage and stunting of plants Fungal and bacterial causes Used by permission of M. Williamson Root rot of boxwood, caused by Pythium sp.
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Symptoms Tissue Necrosis Damping off
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Tissue Necrosis Damping off Rapid death of seedlings and cuttings Stem infected at soil line, seedling topples Spreads rapidly under crowded conditions Mainly fungal agents Photo: Clemson University - USDA Cooperative Extension Slide Series, Damping off of tobacco, caused by Rhizoctonia spp. and Pythium spp.
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Symptoms Wilting Biotic factors Root, crown or stem rots
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Wilting Biotic factors Root, crown or stem rots Vascular wilts Root crown or stem damage from insects or animals Mainly fungal and bacterial causes Used by permission of M.Williamson Stem wilt of Exacum from INSV infection
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Symptoms Wilting Abiotic factors Damage from weather extremes
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Wilting Abiotic factors Damage from weather extremes Dry or flooded soil Mechanical damage to roots, crown or stem Used by permission of M. Williamson Cold damage on azalea stem. Shoots wilt above damage.
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Symptoms Alteration of normal appearance Mosaic
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Alteration of normal appearance Mosaic Irregular patches of discolored tissue. Often with distortion Viruses mainly Photo: David B. Langston, University of Georgia, Mosaic symptoms on cucumber due to a viral infection
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Symptoms Alteration of normal appearance Ringspot
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Alteration of normal appearance Ringspot On leaves and fruits. Distinct ring shaped lesions, often in concentric zones Viruses Photo: Dr. Backhaus, Biologische Bundesanstalt für Land- und Forstwirtschaft, Tomato spotted wilt virus (TSWV) on geranium leaf
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Symptoms Alteration of normal appearance Abiotic causes
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Alteration of normal appearance Abiotic causes Chemical damage Nutritional deficiencies Used by permission of M. Williamson Cotton damage by the herbicide 2,4-D
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Symptoms Alteration of normal appearance Yellowing
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Alteration of normal appearance Yellowing Often nutritional, cultural or environmental Root malfunction – biotic or abiotic Nematode infestation “Yellows” phytoplasma diseases Used by permission of M. Williamson Burford hollies yellowing due to root malfunction. Soil is poorly drained.
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Symptoms Be aware of symptom variability.
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms Be aware of symptom variability. May have more than one problem. More than one pathogen may be involved. Pathogens have varying levels of virulence. Environmental conditions can affect symptom expression. Host genetics and physiology can effect symptom expression.
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Symptom variability due to multiple pathogens
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptom variability due to multiple pathogens Peach seedling on left infected with both Prune dwarf virus (PDV) and Prunus necrotic ringspot virus (PNRSV) Seedling in middle infected with PDV alone Seedling on right infected with PNRSV alone Used by permission of S. Scott Peach seedlings infected by single or multiple viruses show symptom variability.
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Signs Fungal signs Evidence of pathogen on tissue.
MODULE 4 – Diagnosing plant problems: Plant Diseases Signs Fungal signs Evidence of pathogen on tissue. Spores, mycelia or fruiting bodies. Use hand lens or knife for field detection. Photo: USDA Forest Service Archives, USDA Forest Service, White mycelia of Armillaria sp. on tree trunk affected by Armillaria root and stem rot.
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Signs Fungal signs Fruiting bodies
MODULE 4 – Diagnosing plant problems: Plant Diseases Signs Fungal signs Fruiting bodies Shape of fruiting body aids in fungal identification. Tissue location may help differentiate between species. Photo: Central Science Laboratory, Harpenden Archives, British Crown, White rust of chrysanthemum, fruiting bodies of Puccinia horiana
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MODULE 4 – Diagnosing plant problems: Plant Diseases
Signs Fungal signs Powdery mildew fungi form mycelia and spores on tissue surface. Powdery material rubs off. Leaves often distorted, discolored Dark, round fruiting bodies form in fall. Used by permission of M. Williamson Oidium sp. causing powdery mildew on Viburnum suspensum.
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Symptoms and Signs Downy mildews often produce angular leaf spots.
MODULE 4 – Diagnosing plant problems: Plant Diseases Symptoms and Signs Used by permission of M. Williamson Used by permission of M. Williamson Downy mildews often produce angular leaf spots. Downy mildews develop sporulation on leaf undersides only. Downy mildew of Buddleia caused by Peronospora harotii. Make all permission lines size 12. Used by permission of M. Williamson Downy mildew of veronica, caused by Peronospora sordida
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MODULE 4 – Diagnosing plant problems: Plant Diseases
Signs Bacterial signs “Streaming” from freshly cut stem in water. “Stringing” from cut stems pushed together, then pulled apart. Bacteria “flow” observed from tissue mount on compound microscope. Bacterial ooze can be observed on-site with some diseases. Used by permission of M. Williamson Bacterial streaming from tomato infected by Ralstonia solanacearum.
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Plant Problem Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Problem Diagnosis Steps 12. Laboratory Examination and Testing Dissecting microscope Compound microscope Electron microscope Moist chamber incubation Culturing Additional tests for biotic agents Tests for abiotic agents
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Laboratory Examination Examination using dissecting microscope
MODULE 4 – Diagnosing plant problems: Plant Diseases Laboratory Examination Examination using dissecting microscope Observe sporulation that’s invisible to naked eye. Closer view of structures seen by naked eye Nematologist can ID nematodes to genus. Used by permission of M. Williamson
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MODULE 4 – Diagnosing plant problems: Plant Diseases
Laboratory Examination Examination using microscope of higher magnification ID fungi to genus or to species. View bacterial flow; observe morphology at highest magnification. View virus inclusion bodies. Using electron microscope, view viruses, phytoplasmas Used by permission of M. Williamson Rust spores, Puccinia sp.
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Laboratory tests Moist Incubation Goal – to induce sporulation
MODULE 4 – Diagnosing plant problems: Plant Diseases Laboratory tests Moist Incubation Goal – to induce sporulation Important for obligate fungal pathogens Avoid overly moist conditions. Can surface sterilize Used by permission of M. Williamson Moist chamber
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Laboratory tests Fungal isolation Surface sterilize.
MODULE 4 – Diagnosing plant problems: Plant Diseases Laboratory tests Fungal isolation Surface sterilize. Use margin of diseased area. Nutritive agar media, can be selective Diagnostician can often ID to genus from appearance of culture Used by permission of M. Williamson Fungal culture.
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Isolate identification
MODULE 4 – Diagnosing plant problems: Plant Diseases Isolate identification Fungal identification – Other methods Examination and comparison with drawings in literature Utilizing keys in literature Literature sources for host diseases and disorders Serological, molecular, other tests
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Laboratory tests Bacterial isolations Grind tissue in sterile water.
MODULE 4 – Diagnosing plant problems: Plant Diseases Laboratory tests Bacterial isolations Grind tissue in sterile water. Streak suspension on bacterial medium. Can use selective media Transfer single, isolated colony Cultures of Ralstonia solanacearum Used by permission of K. Ong
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Isolate identification
MODULE 4 – Diagnosing plant problems: Plant Diseases Isolate identification Bacterial identification Biochemical tests Carbohydrate utilization (BIOLOG) Fatty acid methyl ester analysis (FAME) Molecular methods Polymerase chain reaction (PCR) Serological tests, e.g. ELISA
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Identification of other pathogens
MODULE 4 – Diagnosing plant problems: Plant Diseases Identification of other pathogens Non-culturable pathogens Viruses, viroids Phytoplasmas Fastidious bacteria
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Identification of non-culturable pathogens
MODULE 4 – Diagnosing plant problems: Plant Diseases Identification of non-culturable pathogens Methods Serological tests, e.g. ELISA Electron microscopy Staining for virus-induced inclusion bodies Molecular tests, e.g. PCR Enzyme activity testing Host range testing
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Identification of other pathogens
MODULE 4 – Diagnosing plant problems: Plant Diseases Identification of other pathogens ELISA tests Multi-well test for multiple samples Individual ELISA kit Used by permission of M. Williamson
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Identification of Nematodes
MODULE 4 – Diagnosing plant problems: Plant Diseases Identification of Nematodes Extraction from Soil and Plant Material Soil extraction using centrifugal flotation Baerman funnel techniques for soil and tissue Mist chamber extraction for nematodes in root or stem tissue Direct examination of nematodes in tissue.
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Laboratory tests Diagnostic tests for abiotic causes Soil, water
MODULE 4 – Diagnosing plant problems: Plant Diseases Laboratory tests Diagnostic tests for abiotic causes Soil, water pH testing Nutrient analysis Soluble salts analysis Analysis for chemicals Plant tissue tests
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Plant Disease Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Disease Diagnosis Steps 13) Investigate pathogenicity. Is identified organism a known pathogen - of host? - of hosts in same genus? Consult literature to determine If not a host, Kochs Postulates may be next step A lengthy process, rarely used in routine diagnoses Problem arises when organism cannot be cultured
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Koch’s Postulates Robert Koch – German physician in late 1800s
MODULE 4 – Diagnosing plant problems: Plant Diseases Koch’s Postulates Robert Koch – German physician in late 1800s Proved that a bacterium caused anthrax in cattle Developed steps proving the bacterium was causal Same steps used to prove pathogenicity for plant disease organisms
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MODULE 4 – Diagnosing plant problems: Plant Diseases
Koch’s Postulates Note constant association of organism with diseased plants and consistent, observable symptoms. Isolate and characterize organism in pure culture. Inoculate healthy plant with organism, and observe the same disease and symptoms. Re-isolate same organism.
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Plant Disease Diagnosis Steps
MODULE 4 – Diagnosing plant problems: Plant Diseases Plant Disease Diagnosis Steps 14. Final Diagnosis All information compiled and analyzed Organism identified, found to be pathogen of host OR Lab tests, other information reveal an abiotic cause 15. Develop control recommendations and present to grower.
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Authors Meg Williamson, Clemson University
MODULE 4 – Diagnosing plant problems: Plant Diseases Authors Meg Williamson, Clemson University Melissa Riley, Clemson University Otis Maloy, Washington State University
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Publication Date: December 2006
MODULE 4 – Diagnosing plant problems: Plant Diseases This publication can be used for non-profit, educational use only purposes. Photographers retain copyright to photographs or other images contained in this publication as cited. This material was developed as a core training module for NPDN First Detector Training. The authors and website should be properly cited. Images or photographs should also be properly cited and credited to the original source. Publication Number: 0013 Publication Date: December 2006
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