Other pathogens besides fungi

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Other pathogens besides fungi
ESC 432 Forest Pathology
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Other Pathogens

• Viruses, viroids, and prions
• Bacteria
• Phytoplasmas
• Fungi
• Nematodes
• Parasitic plants

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Viruses, viroids, and prions

• Viruses protein coat and nucleic acid
• Viroid low MW RNA
• Prion infectious protein molecule

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Viruses and viroids

• Needs a vector
• Insects
• Nematodes
• Grafting or vegetative propagation
• Can move through plant in phloem or xylem, or
  stay localized in foliage
• Can become part of plant genome

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Symptoms

• Foliage streaking, spotting, mottling
• Brooms or rosettes
• Growth reduction
• No symptoms

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Management

• Vector control
• Prevention of wounding
• Virus diseases are less of an issue in forestry,
  since plants are propagated by seed rather than
  vegetatively

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Bacteria

• 1-celled prokaryotes with a cell wall
• Rods (bacilli), spirals, or spherical (cocci)
• Easily exchange genetic material on plasmids

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Beneficial Bacteria

• Nitrogen-fixers in soil and on roots (Rhizobium,
  Frankia)
• Nutrient cycling OM ? NH4 ? NO3
• Anaerobic N-fixers in wood

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Symptoms

• Bacteria cause disease by enzymes that digest
  cell walls, toxins, or tumors
• Typical symptoms are
• Water soaking
• Wetwood
• Shoot blight
• Bleeding cankers
• Galls

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Bacterial Wetwood

• Very common on elms and poplars
• Caused by anaerobic bacteria in the xylem
  following wounding and wound closure
• May prevent decay fungi from colonizing

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Crown Gall

• Caused by Agrobacterium tumifaciens, a soil
  bacterium
• Usually occurs at the soil line, but sometimes on
  aerial parts
• Requires wounding for infection
• Galls interfere with nutrient and water flow
• Used in genetic engineering can insert DNA into
  plant genome

Elm bacterial gall
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Management

• Bacterial diseases are spread by splashing water,
  wounding caused by insects, humans
• Control by avoiding unnecessary wounding, using
  insect control, and quarantine
• Use disease-free nursery stock

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Phytoplasmas

• Also called MLOs, or Mycoplasm-like organisms
• Like bacteria without cell walls
• Cannot be cultured apart from the host

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Symptoms

• Infect phloem and cause a systemic, lethal
  disease
• Causes elm yellows, X disease of cherry, coconut
  lethal yellowing, and others
• Symptoms yellowing, epinasty, witches brooms,
  defoliation

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Management

• Most phytoplasmas are transmitted by insect
  vectors, especially planthoppers and leafhoppers
• Control vector control, planting resistant
  species, antibiotic injection (being studied)

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Nematodes

• Worm-like animals in Phylum Nemahelminthes
• Various parasitic habits
• Usually in the soil or on roots, more of an issue
  in agriculture

" If all the matter in the universe except the
nematodes were swept away, our world would still
be dimly recognizable..." -N.A. Cobb, 1914    
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Mode of Action

• Plant parasitic nematodes have a stylet to pierce
  plant cell walls
• Cause injury by feeding, toxins
• Vectors of other diseases
• Contributing factors in declines
• Indirect damage

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Pine Wilt Nematode

• Bursaphelenchus xylophilus, vectored by
  Monochamus sp. (Cerambycidae)
• Introduced to Asia from North America

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Life cycle of PWN
Oviposition wounds in bark of stressed tree
Dauerlarvae in spiracles of vector
Vector (M. alternatus) maturation feeding in
shoots
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Management of PWN

• Fell and burn infested trees
• Submerge infested logs in water
• Vector control
• Quarantine
• Destroy all packing wood at the port

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Parasitic Plants

• Evolution towards parasitism has occurred at
  least 8 times in the flowering plants. Many are
  in the order Santalales
• Loranthaceae leafy mistletoes
• Santalaceae root and stem hemiparasites
• Viscaceae
• Phoradendron leafy mistletoes
• Arceuthobium dwarf mistletoes

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Parasitic Plants

• Hemiparasites depend on host for water,
  nutrients
• Holoparasites achlorophyllous, depend on host
  for water, nutrients, carbon
• Parasitic plants use a specialized organ called a
  haustorium to connect with their host plant

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Mistletoes

• Phoradendron sp. -
• Common on oaks
• South of 40-45 lat.
• Cause water stress, usually not lethal
• Spread by birds

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Dwarf Mistletoes

• Arceuthobium spp.
• Host-specific
• Conifer hosts
• Dioecious
• Sticky seeds spread by birds or forcefully
  ejected from plant

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Symptoms

• Parasitic plants are usually visible
• Brooming
• Galls
• Reduced growth

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Management

• Clearcutting or prescribed fire
• Mistletoe brooms are good wildlife habitat
• Thinning can increase dwarf mistletoe

Isolation strip between mature stand and
regeneration