SUMMARY

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SUMMARY

• Cypress canker
• Infectious propagules called conidia are
  multicelled, genetically identical to parent, and
  need a wound to infect
• Genetic studies reveal one clone in Europe and
  many in California
• Disease caused by exotic pathogen in Europe and
  by off site planting and exotic host in
  California

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Conidia of Seiridium cardinale observed by
optical microscope and SEM
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SUMMARY

• Heterobasidion
• Primary infection caused by airborne genetically
  distinct basidiospores
• Secondary infection of adjacent trees caused by
  direct root to root contact
• Host specificity of H.irregulare vs. H.
  occidentale
• Stumps main infection courts for H. irregulare on
  pine/juniper, wounds for true fir/sequoia
• Environmental changes increase abundance of this
  root and bole pathogen
• Armillaria (Honey mushroom)
• Mostly secondary infection thanks to rhizomorphs,
  largest organism in the world

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True firs
Pines
Each spore is a genetically different
individual In pines we found the same genetic
individual in stumps and adjacent trees
indicating direct contagion between the two In
true firs and true firs/sequoias we find same
individual in adjacent standing trees indicating
infection not linked to stumps but to wounds on
standing trees
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Emergent diseases3 exotic pathogens

• 99 of times human responsible for their
  introduction

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Like the conquistadores brought diseases that
were lethal to those who had never been exposed
to them, so do exotic diseases cause true
devastation in plant communities because of lack
of coevolution between hosts and microbes
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California invaded 1849 A.D.
Port Orford Cedar Root Disease 1950s
New hybrid root pathogen 1990s
Manzanita/madrone die-back
Sudden Oak Death 1990s
White pine blister rust 1930s
Canker-stain of Sycamores 1980s
Dutch Elm Disease 1960s
Pitch canker disease 1980s
Oak root canker 2000
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How can people transport pathogens

• By transporting plants and plant parts
• Crops, and seeds
• Raw food
• Ornamental plants
• Untreated lumber
• Soil
• Insects vectoring fungi
• Military activity

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The Irish Potato Famine

• From 1845 to 1850
• Phytophthora infestans
• Resulted in the death of 750,000
• Emigration of over 2 million, mainly to the
  United States.

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Girdling aerial cankers removed from roots
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Big Sur 2006 K. Frangioso
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Wickland et al., unpublished
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P. ramorum growing in a Petri dish
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Organism new to science

• Origin unknown
• Biology unknown
• Symptoms caused unknown
• Immediately though highly regulated

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Rhododendron In EU mostly a nursery issue, but
also present in nurseries in US and Canada
Stem canker
Leaf necrosis
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Phytophthora ramorum
Sporangia
Chlamydospores
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Is it exotic?

• Our studies have indicated that California
  population is extremely simplified, basically two
  strains reproducing clonally as expected of an
  introduced organism
• Many hosts appear to have no resistance at all
• Limited geographic distribution

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Where does it come from?

• It is unknown where pathogen originally comes
  from, but previous studies have shown that
  California forest population is derived from a
  relatively genetically diversified US nursery
  population, indicating ornamental nurseries were
  the most likely avenue for pathogen introduction

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Lets look at its genetic structure

• Need a number of independent and neutral DNA
  markers
• Used AFLP, a technique that scans the entire
  nuclear genome
• Are our isolates the same as the European ones?
• Is the genetic structure suggestive of an
  introduced or native species?

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• US forest isolates clearly distinct from EU
  nursery isolates, also have different mating type
• Isolates from nurseries in WA, OR, BC both of
  the US and EU types
• Potential for XXX sex and recombination in US
  nurseries
• US forest population is genetically very
  homogeneous, trademark of an introduced species

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The entire genome was sequenced in less than 3
years since discovery of organism
12 SSR loci (di- and tri- repeats identified)
Loci selected to be polymorphic both between
and within continental populations 500
representative isolates analyzed
CCGAAATCGGACCTTGAGTGCGGAGAGAGAGAGAGACTGTACGAGCCCGA
GTCTCGCAT
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Mating Type A1 A2 A2
Growth Rate Fast Slow Fast
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Terminology Genotype Lineage Population
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Results of 1st microsatellite study

• There actually three distinct (genotypically and
  phenotypically) lineages of P. ramorum
• Very low diversity in US forests (microsats
  cannot discriminate among individuals, clonality
  confirmed), only one lineage
• Several genotypes but only one lineage in EU
  nurseries
• Three lineages in US nurseries

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Was the pathogen first in US forests or in US
nurseries?
Slide 12
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Was the pathogen first in US forests or in US
nurseries?
Slide 12
nurseries
forests
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Where was it introduced?

• First reports mid 90s
• Pathogen identified in 2000
• By then, the pathogen was widespread
• CLUES severity of symptoms and anedoctal stories
  

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We found same genotypes in nurseries and forests
proving origin of wild outbreak
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Introduction phase 1- Escape of pathogen
from Infected nursery plants at two locations
Mount Tamalpais (Marin County), and
Scotts Valley (Santa Cruz County) 2- Nurseries
and two sites have identical strain composition,
but distance between sites is impossible for
natural spread of organism
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What favors invasion of exotic fungi ?

• Density of host increases severity of disease
• Corridors linking natural habitats
• Synchronicity between host susceptibility and
  pathogen life cycle
• Ecological and environmental conditions

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Bay/Oak association
Bay
Coast Live Oak (no sporulation)
Canker margin in phloem
Bleeding canker
Sporangia
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Mantel test among all individuals. Morans I vs
ln (geographic distance)
Site ID Correlation coeff. (r) P-value (1000,000 perm)
ALL -0.2153 lt0.000001