Plant pathogens

  1. Symptoms
    are the expression of disease by a plant as a response to a pathogen.
  2. Signs
    are structures or products of a pathogen on or in diseased plants.
  3. Parasite-
    derives its food from a living host
  4. Pathogen-
    causes disease in a living host
  5. Biotic pathogens:
    • • infectious
    • • transmissible
  6. Abiotic factors:
    • • noninfectious
    • Image Upload 1• nontransmissible
  7. Koch's Postulates for Proof of Pathogenicity

    • 1. The suspected pathogen must be consistently associated with diseased plants.

    • 2. The suspected pathogen must be isolated in a pure culture and its characteristics noted.
    • 3. The disease must be reproduced in a healthy plant inoculated with the isolated organism.
    • 4. The same pathogen characterized in step 2 must be isolated from the inoculated plant.

    Step 1. THE PLANT
    Identify the species and cultivar of the plant, when possible. Determine what a healthy specimen should look like, so that you can recognize abnormalities of the plant in question. Carefully observe aboveground symptoms and also belowground symptoms, if necessary. In addition to observing symptoms expressed by a single plant, look for disease patterns in the plant population. Note the apparent host range (i.e., what plant species are affected).

    Step 2. THE PATHOGEN
    Look for signs, evidence of a pathogen or its parts, both above ground and below ground, if necessary. A hand lens is often helpful, but laboratory evaluation is commonly necessary to check for signs. Signs are often more obvious under moist conditions (for example, before dew has dried in the early morning or after rain or irrigation). Abiotic factors produce no signs, and there may be no visible signs of some biotic pathogens, such as viruses. Abiotic factors may affect nearly every plant species in an area to some degree, but biotic pathogens tend to be restricted to a single species or members of the same family of plants. Some signs may be evidence of secondary invaders, which are not the initial cause of the disease.

    Disease will develop in susceptible plants in the presence of a pathogen only if the environment is favorable. Several environmental factors are important, including natural factors and those imposed by human activities. Determine the recent conditions for plant growth, including chemical and fertilizer applications, characteristics of the soil or growing medium, and environmental factors, such as temperature and water supply. Look for factors that commonly affect disease development, such as the timing and amount of irrigation or rainfall, air movement, and the nutrient status of the plant.
  11. Parasites
    live in or on another living organism and obtain nutrients (food) from it.
  12. Saprophytes
    obtain nutrients from dead organic matter.
    • (obligate parasites)

    • Image Upload 2 have narrow host ranges
    • Image Upload 3 cannot grow as saprophytes
    • Image Upload 4 attack healthy host tissue at
    • any stage
    • Image Upload 5 kill host cells slowly
    • Image Upload 6 penetrate directly or
    • via natural openings
    • (facultative parasites/facultative saprophytes)

    • Image Upload 7 have wide host ranges
    • Image Upload 8 can grow as saprophytes
    • Image Upload 9 attack young, weak, or senescent tissues
    • Image Upload 10 kill host cells rapidly by producing toxins or enzymes
    • Image Upload 11 penetrate through wounds or natural openings
  15. Common Diseases Caused by Biotrophs:
    • nematode diseases
    • • phytoplasma diseases
    • • virus diseases
    • • downy mildews
    • • powdery mildews
    • • rusts
  16. Common Diseases Caused by Necrotrophs:
    • • anthracnoses
    • • cankers
    • • fruit rots
    • • leaf spots and blights
    • • root rots
    • • vascular wilt
  17. Monocyclic Diseases:
    • Dutch elm disease
    • Stinking smut
    • Verticillium wilt
  18. Polycyclic Diseases:
    • Apple scab
    • Coffee rust
    • Late blight of potato and tomato
  19. Environmental factors affect plant diseases in all stages of the disease cycle. They influence
    • Image Upload 12 plant development and the ability of the plant to mount defenses against invasion,
    • Image Upload 13 dispersal of inoculum, both primary and secondary,
    • Image Upload 14 the ability of the parasite to penetrate the plant, and
    • Image Upload 15 the survival of the parasite in the absence of the host plant
  20. Water is a major environmental factor in disease, because it
    allows parasites to move to infection courts and penetrate host tissue. Fungal spores landing on a plant surface commonly require water for germination and the production of infection structures. Zoospores are motile and can swim to plants or on plant surfaces. Nematodes can move toward plants and on plant surfaces in a thin film of water. Bacteria require water for multiplication and infection. Seeds of parasitic plants require water for germination.
  21. Approaches to Plant Disease Management
    • Focused on the plant
    • protection

    • Focused on the pathogen
    • avoidance
    • exclusion
    • eradication

    • Focused on the environment
    • protection
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  26. Fungi are composed of
    hyphae, which form mycelia
  27. Fungi cell walls
    predominantly chitin with glucan (a complex sugar)
  28. Fungi storeage compound
  29. fungi cell structure
    eukaryotic (with haploid nuclei)
  30. fungi nutrition
    heterotrophic, nutrients absorbed through cell walls
  31. fungi reproduction
    spores (sexual and asexual)
  32. fungal groups
    • ascomycetes
    • basidiomycetes
    • zygomycetes
  33. ascomycetes
    septate hyphae
  34. basidiomycetes
    septate hyphae; some but not all species have clamp connections
  35. zygomycetes
    nonseptate (coenocytic, aseptate) hyphae
  36. The scientific names of organisms are Latin binomials (two names):
    • a genus name (capitalized) and a species name, or specific epithet (not capitalized).
    • Example Armillaria gallica
  37. After the entire species name has been mentioned in a text,
    • the genus can then be abbreviated to its first letter if the name is used again.
    • Example A. gallica
  38. When the genus name is followed by sp.,
  39. the organism has not been identified to species.
    Example Armillaria sp.
  40. A genus name followed by spp.
    • is a plural form that refers to more than one species belonging to that genus.
    • Example Armillaria spp.
  41. A genus name that is used in the name of a disease is
    • capitalized but not italicized.
    • Example Armillaria root disease
  42. teleomorph
    • perfect stage
    • • sexual spores only
  43. anamorph
    • • imperfect stage
    • • asexual spores only
  44. holomorph
    • spores in all stages, sexual and asexual

    • Image Upload 24 septate mycelium
    • Image Upload 25 sexual spores: ascospores in an ascus (sac)
    • Image Upload 26 asexual spores: conidia

    • Image Upload 27 septate hyphae; some species have clamp connections at some septa
    • Image Upload 28 sexual spores: basidiospores, generally produced in a group of four external to the basidium (the cell that produces them)
    • Image Upload 29 asexual spores: vary considerably within this group

    • Image Upload 30 nonseptate hyphae
    • Image Upload 31 sexual spores: large, dark zygospores
    • Image Upload 32 asexual spores: sporangiospores, produced in a sporangium

    • Image Upload 33 nonseptate hyphae
    • Image Upload 34 cell wall containing cellulose (no chitin)
    • Image Upload 35 diploid nuclei
    • Image Upload 36 sexual spores: oospores, produced after contact between an oogonium and an antheridium
    • Image Upload 37 asexual spores: zoospores, with two flagella, produced in a sporangium
  49. Management Strategies for Fungal Diseases


    • Image Upload 38 Choose planting sites and planting times to avoid environmental
    • Image Upload 39 conditions favoring disease.
  50. Management Strategies for Fungal Diseases


    • Image Upload 40 Impose quarantines (local and international).
    • Image Upload 41 Plant pathogen-free seed or stock.
  51. Management Strategies for Fungal Diseases


    • Image Upload 42 Rotate to nonhost crop.
    • Image Upload 43 Remove weed hosts.
    • Image Upload 44 Destroy infested plant debris.
    • Image Upload 45 Apply fungicides.
  52. Management Strategies for Fungal Diseases


    • Image Upload 46 Apply fungicides.
    • Image Upload 47 Minimize leaf wetness.
    • Image Upload 48 Plant resistant hosts.
  53. Spores and Their Functions in Heteroecious Rust Fungi
    basidiospores (IV)
    • • are haploid
    • Image Upload 49 • infect first host species to form pycnia
  54. Spores and Their Functions in Heteroecious Rust Fungi
    pycniospores (0)
    • • combine with receptive hyphae in pycnia
    • Image Upload 50 • result in formation of dikaryotic mycelium (plasmogamy)
  55. Spores and Their Functions in Heteroecious Rust Fungi
    aeciospores (I)
    • • are dikaryotic spores produced in an aecium
    • Image Upload 51 • infect alternate host
  56. Spores and Their Functions in Heteroecious Rust Fungi
    urediniospores (II)
    • • are dikaryotic spores produced in uredinia
    • Image Upload 52 • infect the host species on which they are produced (repeating stage)
  57. Spores and Their Functions in Heteroecious Rust Fungi
    teliospores (III)
    • Image Upload 53 • are diploid spores produced in telia (dikaryotic, becoming diploid)
    • Image Upload 54 • produce haploid basidiospores (after karyogamy and meiosis)
    • Image Upload 55 • serve as the survival stage
  58. Autoecious rust fungi require
    a single host species to complete their life cycle (similar to most fungal pathogens).
  59. Heteroecious rust fungi require
    two unrelated plant species to their complete life cycle.
  60. Important Oomycetes

    Pythium species

    • Image Upload 56 Nonseptate hyphae
    • Image Upload 57 Sexual spores: oospores; paragynous antheridia, often more than one antheridium per oogonium
    • Image Upload 58 Asexual spores: zoospores released from vesicles produced by lumpy to round sporangia
    • Image Upload 59 Diseases: seedling damping-off; stem, crown, and root rots; diseases of lower tissues reached by water splashing up from the soil (the "splash zone")
  61. Important Oomycetes

    Phytophthora species (phyto = plant, phthora = destroyer)

    • Image Upload 60 Nonseptate hyphae
    • Image Upload 61 Sexual spores: oospores; paragynous or amphigynous antheridium; usually only one antheridium per oogonium
    • Image Upload 62 Asexual spores: zoospores produced by typically lemon-shaped sporangia
    • Image Upload 63 Diseases: stem, crown, and root rots; diseases of lower tissues reached by water splashing up from the soil (the "splash zone").
    • Phytophthora infestans causes late blight of potato and tomato, the disease that led to the Irish potato famine in the 1840s and the beginning of the science of plant pathology.
    • Phytophthora ramorum causes the devastating ramorum blight (sudden oak death) on the U.S. west coast and was probably introduced into the United States on rhododendron nursery stock.
    • Phytophthora cinnamomi is a widespread, destructive root and crown pathogen throughout the tropical world.
  62. Important Oomycetes

    Downy mildew pathogens (biotrophs)

    • Plasmopara, Peronospora, Pseudoperonospora, Bremia, and other genera

    • Image Upload 64 Nonseptate hyphae
    • Image Upload 65 Sexual spores: oospores, produced in infected aboveground plant tissue
    • Image Upload 66 Asexual spores: sporangia, formed on treelike sporangiophores, which typically emerge through stomata on the lower leaf surface; some sporangia produce zoospores, and others germinate directly
    • Image Upload 67 Diseases: blue mold of tobacco; crazy top of corn; yellow tuft of turfgrasses; downy mildews of brassicas, curcurbits, grape, hops, onion, snapdragon, and other plants
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    Hypha with a septum and a clamp connection (arrow).
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    Septate (A) and nonseptate hyphae (B). Note the cross-walls in the septate hypha.
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    Fruiting body (conk) of a wood decay fungus, a sign of the pathogen. Ganoderma applanatum.
  76. Image Upload 82
    Examples of asci and ascospores. The shape and size of the ascus and ascospores vary greatly.
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    Reproduction of a basidiomycete. A, Fruiting body. B, The lower surface of a fruiting body, containing numerous pores. C, Formation of basidiospores following meiosis. D, Basidia and basidiospores emerging from pores in a fruiting body.
  78. Image Upload 84
    Corn smut, caused by Ustilago maydis, a basidiomycete.
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    Clamp connection and dikaryotic hypha formed during mitosis.
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    Conidiomata. A, Sporodochium. (Myrothecium sp.) B, Synnema. (Didymostilbe sp.) C, Acervulus. (Marsonina sp.) D, Pycnidium. (Phyllostica sp.). Note that the conidia are not in asci.
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    Ascomata with bitunicate asci. A, Ascostroma with multiple locules filled with asci. (Apiosporina morbosa) B, Pseudothecium. Note the double wall of the bitunicate asci.
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    Generalized process of ascus formation. A, Male structure (often an antheridium) and female structure (ascogonium). B, Plasmogamy: the male structure contributes nuclei to the female structure. C, A limited dikaryotic mycelium develops. D–F, Through a complex mechanism, the ascus mother cell forms. G, Karyogamy: the two nuclei fuse to form a single diploid nucleus. H, Meiosis produces four haploid nuclei. I, In many ascomycetes, mitosis produces eight haploid nuclei and the ascus elongates. J, A spore wall forms around each nucleus to produce eight ascospores.
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  85. Agrobacterium

    • Image Upload 93 aerobes, peritrichous flagella, abundant EPS
    • Image Upload 94 Example: Agrobacterium tumefaciens
  86. Erwinia and closely related newer genera: Brenneria, Pantoea, Pectobacterium

    • Image Upload 95 facultative anaerobes, peritrichous flagella; pectolytic enzymes (some species)
    • Image Upload 96 Examples: Erwinia amylovora (no pectolytic enzymes), Pectobacterium carotovorum (pectolytic enzymes)
  87. Pseudomonas and closely related newer genera: Acidovorax, Burkholderia, Herbaspirillum, Ralstonia

    • Image Upload 97 aerobes, polar flagella; some fluoresce under ultraviolet light and chelate (remove) iron from their environment
    • Image Upload 98 Example: Pseudomonas syringae (many pathovars)
  88. Xanthomonas

    • Image Upload 99 aerobes, one polar flagellum, yellow colonies; source of "xanthan gums"
    • Image Upload 100 Examples: Xanthomonas axonopodis pv. citri, X. axonopodis pv. vesicatoria
  89. Clavibacter

    • Image Upload 101 aerobes, irregularly shaped rods, nonmotile
    • Image Upload 102 Examples: Clavibacter michiganensis subsp. michiganensis, C. michiganensis subsp. sepedonicus
  90. Streptomyces

    • Image Upload 103 aerobes, branched filaments, spore-forming (actinomycete)
    • Image Upload 104 Example: Streptomyces scabies
  91. phytoplasmas

    • Image Upload 105 aerobes, pleomorphic
    • Image Upload 106 Example: aster yellows phytoplasma
  92. spiroplasmas

    • Image Upload 107 aerobes, helical shape
    • Image Upload 108 Example: Spiroplasma citri
  93. Xylem-limited bacteria

    • Fastidious Vascular-Colonizing Plant-Pathogenic Bacteria
    • Symptoms: water-stress symptoms including wilt, stunting, scorch on leaves

    • Pathogen: Xylella fastidiosa
    • Shape: bacilliform (rod-shaped)
    • Vectors: insects that feed in xylem, e.g., sharpshooters and spittlebugs
    • Example diseases: bacterial leaf scorch of shade trees, citrus variegated chlorosis, Pierce's disease of grapevine
  94. Phloem-limited bacteria

    Symptoms: yellowing, stunting, witches' brooms

    • Fastidious Vascular-Colonizing Plant-Pathogenic Bacteria
    • Phytoplasmas

    • Spiroplasmas

    • Phloem-colonizing walled bacteria

  95. Xylella fastidiosa
    s the most common xylem-limited pathogen.
  96. Management Strategies for Bacterial Diseases


    • Impose quarantines (local and international).

    • Image Upload 109 Grow plants in dry environments.
    • Image Upload 110 Plant pathogen-free seed or stock.
  97. Management Strategies for Bacterial Diseases


    • Image Upload 111 Rotate to nonhosts.
    • Image Upload 112 Rogue weed hosts.
    • Image Upload 113 Destroy infested plant debris.
    • Image Upload 114 Use heat treatment of seeds or propagative material.
    • Image Upload 115 Apply antibiotics.
    • Image Upload 116 Apply insecticides (to kill infective vectors).
  98. Management Strategies for Bacterial Diseases


    • Image Upload 117 Apply copper chemicals.
    • Image Upload 118 Apply antibiotics.
    • Image Upload 119 Apply bacterial antagonists.
    • Image Upload 120 Minimize leaf wetness.
    • Image Upload 121 Plant resistant hosts.
  99. plasmologamy
    fusion of cells , bringing two sexually compatible haploid nuclei together in one cell
  100. karyogamy
    nuclear fusion forming a diploid nucleus
  101. meiosis
    formation of four genetically recombined haploid nulei from a single diploid nucleus
  102. A sexual spore used for dispursal
    conidium (conidia, pl)
  103. septum
    a divider hyphae with crossed walls
  104. teleomorph
    the sexual stage of fungi
  105. anamorph
    asexual stage of fungi
  106. ascomycota sexual spores and where stored?
    ascospores, ascus
  107. conidia
    asexual spores often produced by anamorphs
  108. ascoma or ascocarp
    sexual fruiting body
  109. basidioma
    sexual fruiting body of basidiomycete
  110. urediniospores
    asexual spores of rust
  111. teliospores
    dust like spores of smut
  112. Sexual spores of oomycete
  113. zoospores
    small asexual swimming (flegella) spores of chytriomycota and oomycete produced in sporangium
  114. zygospores
    sexual spores of zygomycota
  115. sporangiospores
    asexual spores of zygomycota
  116. I
    aeciospores - dikaryotic
  117. II
    urediniospores -dikaryotic (repeating stage)
  118. III
    teliospores - diploid (karyogamy meiosis)
  119. IV
    basidiospores -haploid
  120. O
    pycniospores + receptive hyphae- dikaryotic (plastogamy)
Card Set
Plant pathogens
Chpt 1-3