Plant pathogens

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

  14. 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
  16. 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
  18. 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.

  20. 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.
  21. Parasites
  22. live in or on another living organism and obtain nutrients (food) from it.
  23. Saprophytes
  24. obtain nutrients from dead organic matter.
  26. (obligate parasites)

    • have narrow host ranges
    • cannot grow as saprophytes
    • attack healthy host tissue at
    • any stage
    • kill host cells slowly
    • penetrate directly or
    • via natural openings
  28. (facultative parasites/facultative saprophytes)

    • have wide host ranges
    • can grow as saprophytes
    • attack young, weak, or senescent tissues
    • kill host cells rapidly by producing toxins or enzymes
    • penetrate through wounds or natural openings
  29. Common Diseases Caused by Biotrophs:
  30. nematode diseases
    • • phytoplasma diseases
    • • virus diseases
    • • downy mildews
    • • powdery mildews
    • • rusts
  31. Common Diseases Caused by Necrotrophs:
  32. • anthracnoses
    • • cankers
    • • fruit rots
    • • leaf spots and blights
    • • root rots
    • • vascular wilt
  33. Monocyclic Diseases:
  34. Dutch elm disease
    • Stinking smut
    • Verticillium wilt
  35. Polycyclic Diseases:
  36. Apple scab
    • Coffee rust
    • Late blight of potato and tomato
  37. Environmental factors affect plant diseases in all stages of the disease cycle. They influence
  38. plant development and the ability of the plant to mount defenses against invasion,
    • dispersal of inoculum, both primary and secondary,
    • the ability of the parasite to penetrate the plant, and
    • the survival of the parasite in the absence of the host plant
  39. Water is a major environmental factor in disease, because it
  40. 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.
  41. Approaches to Plant Disease Management
    • Focused on the plant
    • protection

    • Focused on the pathogen
    • avoidance
    • exclusion
    • eradication

    • Focused on the environment
    • protection
  42. Fungi are composed of
  43. hyphae, which form mycelia
  44. Fungi cell walls
  45. predominantly chitin with glucan (a complex sugar)
  46. Fungi storeage compound
  47. glycogen
  48. fungi cell structure
  49. eukaryotic (with haploid nuclei)
  50. fungi nutrition
  51. heterotrophic, nutrients absorbed through cell walls
  52. fungi reproduction
  53. spores (sexual and asexual)
  54. fungal groups
  55. ascomycetes
    • basidiomycetes
    • zygomycetes
  56. ascomycetes
  57. septate hyphae
  58. basidiomycetes
  59. septate hyphae; some but not all species have clamp connections
  60. zygomycetes
  61. nonseptate (coenocytic, aseptate) hyphae
  62. The scientific names of organisms are Latin binomials (two names):
  63. a genus name (capitalized) and a species name, or specific epithet (not capitalized).
    Example Armillaria gallica
  64. After the entire species name has been mentioned in a text,
  65. the genus can then be abbreviated to its first letter if the name is used again.
    Example A. gallica
  66. When the genus name is followed by sp.,
  67. the organism has not been identified to species.
    Example Armillaria sp.
  68. A genus name followed by spp.
  69. is a plural form that refers to more than one species belonging to that genus.
    Example Armillaria spp.
  70. A genus name that is used in the name of a disease is
  71. capitalized but not italicized.
    Example Armillaria root disease
  72. teleomorph
  73. perfect stage
    • sexual spores only
  74. anamorph
  75. • imperfect stage
    • asexual spores only
  76. holomorph
  77. • spores in all stages, sexual and asexual

    • septate mycelium
    • sexual spores: ascospores in an ascus (sac)
    • asexual spores: conidia

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

    • nonseptate hyphae
    • sexual spores: large, dark zygospores
    • asexual spores: sporangiospores, produced in a sporangium

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


    • Choose planting sites and planting times to avoid environmental
    • conditions favoring disease.
  83. Management Strategies for Fungal Diseases


    • Impose quarantines (local and international).
    • Plant pathogen-free seed or stock.
  84. Management Strategies for Fungal Diseases


    • Rotate to nonhost crop.
    • Remove weed hosts.
    • Destroy infested plant debris.
    • Apply fungicides.
  85. Management Strategies for Fungal Diseases


    • Apply fungicides.
    • Minimize leaf wetness.
    • Plant resistant hosts.
  86. Spores and Their Functions in Heteroecious Rust Fungi
    basidiospores (IV)
  87. • are haploid
    • infect first host species to form pycnia
  88. Spores and Their Functions in Heteroecious Rust Fungi
    pycniospores (0)
  89. • combine with receptive hyphae in pycnia
    • result in formation of dikaryotic mycelium (plasmogamy)
  90. Spores and Their Functions in Heteroecious Rust Fungi
    aeciospores (I)
  91. • are dikaryotic spores produced in an aecium
    • infect alternate host
  92. Spores and Their Functions in Heteroecious Rust Fungi
    urediniospores (II)
  93. • are dikaryotic spores produced in uredinia
    • infect the host species on which they are produced (repeating stage)
  94. Spores and Their Functions in Heteroecious Rust Fungi
    teliospores (III)
    • • are diploid spores produced in telia (dikaryotic, becoming diploid)
    • • produce haploid basidiospores (after karyogamy and meiosis)
    • • serve as the survival stage
  95. Autoecious rust fungi require
  96. a single host species to complete their life cycle (similar to most fungal pathogens).
  97. Heteroecious rust fungi require
  98. two unrelated plant species to their complete life cycle.
  99. Important Oomycetes

    Pythium species

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

    Phytophthora species (phyto = plant, phthora = destroyer)

    • Nonseptate hyphae
    • Sexual spores: oospores; paragynous or amphigynous antheridium; usually only one antheridium per oogonium
    • Asexual spores: zoospores produced by typically lemon-shaped sporangia
    • 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.
  101. Important Oomycetes

    Downy mildew pathogens (biotrophs)

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

    • Nonseptate hyphae
    • Sexual spores: oospores, produced in infected aboveground plant tissue
    • 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
    • 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
  102. acervulus
  103. amphigynous
  104. apothecium
  105. cleistothecium
  106. paragynous
  107. perithecium
  108. pycnidium
  109. sporodochium
  110. synnema
  111. Hypha with a septum and a clamp connection (arrow).
  112. Septate (A) and nonseptate hyphae (B). Note the cross-walls in the septate hypha.
  113. Fruiting body (conk) of a wood decay fungus, a sign of the pathogen. Ganoderma applanatum.
  114. Examples of asci and ascospores. The shape and size of the ascus and ascospores vary greatly.
  115. 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.
  116. Corn smut, caused by Ustilago maydis, a basidiomycete.
  117. Clamp connection and dikaryotic hypha formed during mitosis.
  118. 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.
  119. Ascomata with bitunicate asci. A, Ascostroma with multiple locules filled with asci. (Apiosporina morbosa) B, Pseudothecium. Note the double wall of the bitunicate asci.
  120. 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.
  121. Agrobacterium

    • aerobes, peritrichous flagella, abundant EPS
    • Example: Agrobacterium tumefaciens
  122. Erwinia and closely related newer genera: Brenneria, Pantoea, Pectobacterium

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

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

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

    • aerobes, irregularly shaped rods, nonmotile
    • Examples: Clavibacter michiganensis subsp. michiganensis, C. michiganensis subsp. sepedonicus
  126. Streptomyces

    • aerobes, branched filaments, spore-forming (actinomycete)
    • Example: Streptomyces scabies
  127. phytoplasmas

    • aerobes, pleomorphic
    • Example: aster yellows phytoplasma
  128. spiroplasmas

    • aerobes, helical shape
    • Example: Spiroplasma citri
  129. 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
  130. Phloem-limited bacteria

  131. Symptoms: yellowing, stunting, witches' brooms

  132. Fastidious Vascular-Colonizing Plant-Pathogenic Bacteria
    • Phytoplasmas

    • Spiroplasmas

    • Phloem-colonizing walled bacteria

  133. Xylella fastidiosa
  134. s the most common xylem-limited pathogen.
  135. Management Strategies for Bacterial Diseases


  136. Impose quarantines (local and international).

    • Grow plants in dry environments.
    • Plant pathogen-free seed or stock.
  137. Management Strategies for Bacterial Diseases


    • Rotate to nonhosts.
    • Rogue weed hosts.
    • Destroy infested plant debris.
    • Use heat treatment of seeds or propagative material.
    • Apply antibiotics.
    • Apply insecticides (to kill infective vectors).
  138. Management Strategies for Bacterial Diseases


    • Apply copper chemicals.
    • Apply antibiotics.
    • Apply bacterial antagonists.
    • Minimize leaf wetness.
    • Plant resistant hosts.
  139. plasmologamy
    fusion of cells , bringing two sexually compatible haploid nuclei together in one cell
  140. karyogamy
    nuclear fusion forming a diploid nucleus
  141. meiosis
    formation of four genetically recombined haploid nulei from a single diploid nucleus
  142. A sexual spore used for dispursal
    conidium (conidia, pl)
  143. septum
    a divider hyphae with crossed walls
  144. teleomorph
    the sexual stage of fungi
  145. anamorph
    asexual stage of fungi
  146. ascomycota sexual spores and where stored?
    ascospores, ascus
  147. conidia
    asexual spores often produced by anamorphs
  148. ascoma or ascocarp
    sexual fruiting body
  149. basidioma
    sexual fruiting body of basidiomycete
  150. urediniospores
    asexual spores of rust
  151. teliospores
    dust like spores of smut
  152. Sexual spores of oomycete
  153. zoospores
    small asexual swimming (flegella) spores of chytriomycota and oomycete produced in sporangium
  154. zygospores
    sexual spores of zygomycota
  155. sporangiospores
    asexual spores of zygomycota
  156. I
    aeciospores - dikaryotic
  157. II
    urediniospores -dikaryotic (repeating stage)
  158. III
    teliospores - diploid (karyogamy meiosis)
  159. IV
    basidiospores -haploid
  160. O
    pycniospores + receptive hyphae- dikaryotic (plastogamy)

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Plant pathogens
2010-09-30 18:09:44
plant pathogens diseases

Chpt 1-3
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