Chestnut Blight Cryphonectria parasitica

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Chestnut BlightCryphonectria parasitica
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The American Chestnut(Castanea dentata)
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American Chestnut Range

• Maine to Georgia and west to Ohio and Tennessee.
  (Braun, 1950)
• Commonly made up 25 or more of mixed stands
• Formed pure stands on many dry Appalachian
  ridgetops and near densely populated areas.

Historical Range of Castanea dentata (Saucier,
1973)
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American Chestnut Habitat

• Common on midslopes and other moderately dry
  soils
• Shared moist meso-phytic soils with many other
  species
• Tap root 4 to 5 ft down

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Redwoods of the East

• Mature chestnuts could be 600 years old and
  average up to five feet in diameter and 100 feet
  tall
• Many specimens of 8 to 10 feet in diameter were
  recorded

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American Chestnut Ecological Importance

• Wildlife depended on the abundant crop of
  chestnuts
• Many species of insects fed on the leaves,
  flowers, and nuts

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American Chestnut Economic Importance

• Throughout much of the range chestnut had the
  most timber volume of any species
• Half the standing timber volume of CT
• Was the major source of tannin for leather
  production (6-11 tannin content)
• Chestnuts

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From cradle to casket

• Fast growing
• reached half ultimate height by 20th year
• Resistant to decay
• Straight and tall
• often branch free for 50 feet
• Only white pine tulip poplar could grow taller

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From cradle to casket

• Posts railroad ties
• Telephone poles (65 feet)
• Construction
• Fuel
• Fine furniture musical instruments

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American Chestnut Economic Importance

• Scientific forest management in the US was just
  getting started when the country lost its most
  important hard wood species (Smith, 2000)
• Foresters had begun to develop comprehensive
  plans for intensive management

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• Near densely populated areas Chestnut often
  formed nearly complete stands
• due to rapid growth from stump sprouts
• repeated coppicing for fuelwood

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Pure stand of Chestnut in CT 90 years after
clear-cutting, 1905.

• Experts estimate that American Chestnut
  represented half the commercial value of all
  Eastern North American hardwoods

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• the most valuable and usable tree that ever
  grew in the Eastern United States.

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Introduction of Cryphonectria parasitica

• In 1904, Herman Merkel, a forester at the New
  York Zoological Garden, found odd cankers on
  American chestnut trees in the park

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Introduction of Cryphonectria parasitica

• "rapid sudden death of many branches stems
  trees"

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Introduction of Cryphonectria parasitica

• American Chestnut produces a sweet but small nut 
  
• Chinese chestnut produces a large but generally
  tasteless nut

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Introduction of Cryphonectria parasitica

• Thomas Jefferson
• imported European or Spanish chestnut (Castanea
  sativa)
• grafted it onto native root stocks at
  Monticello.
• In 1876, a nurseryman in Flushing, NY, imported
  the Japanese chestnut (C. crenata).
• More were brought over in 1882 and 1886.
• Chinese chestnut (C. Molissima) was brought here
  from Ichang in 1900.
• to hybridize for ornamentals and nut production

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Cryphonectria parasitica

• Ascomycete
• Produces both conidia ascospores
• Pycnidia stromata break through the lenticels and
  produce conidia and perithecia producing
  ascospores are formed

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Cryphonectria parasitica Life Cycle
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Dispersal

• Animals and insects
• Ascospores are shot into the air after rain
  storms in the fall
• Rain (conidia)

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active growth sporulation

• Infects trunk and branches
• Only above ground parts of trees

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How does it kill the tree?

• Enters through fissures or wounds in the bark
• Grows in and under the bark, girdling the
  cambium.
• Kill the tree above the point of infection.

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• Causes swollen or sunken orange-colored cankers
  on the limbs and trunks of the chestnut trees.

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How does it kill the tree?

• The leaves above the point of infection die,
  followed by the limbs.
• Within two to ten years the entire tree is dead.
• Not uncommon to find many cankers on one tree

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How does it kill the tree?

• The fungus has girdled the tree and is
  producing yellow conidia asexual spores

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Host Range

• Like most cankers - fairly specific host range
• Serious pathogen American European (infects
  Japanese and Chinese much less)
• Moderate pathogen Chinquapin Live Oak 
• Can also be found infecting/living on numerous
  oak species in the US

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Rate of Spread

• Aggressive attempts to halt the spread of the
  blight were made by PA and NY
• removed chestnut over a large area to halt
  southward spread
• In 1911-1913, the U.S. Congress appropriated
  special funds to enable foresters to study and
  control the blight

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Rate of Spread

• Horticulturalists, found a blight-free area in
  Pennsylvania and quickly imported trees to form
  an experiment station
• transported the blight and created a new
  epicenter
• Accelerated spread in PA
• Cuts in funding for Chestnut blight research
• With the onset of World War I in 1914
• The evident futility of control efforts

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• By 1926, fungus reported throughout native range
• By 1940, virtually all (an estimated 4 billion)
  were dead or infected with the blight
• Chestnut was the dominant wood processed at PA
  sawmills in the early 1920s,
• salvage logging to make use of the dead and
  dying trees

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• a tragic loss, one of the worst natural
  calamities ever experienced by this nation

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Cummulative Impacts

• Chestnut in Southern range was first affected by
  Phytophtera cinnamomum
• Now affecting hybrids

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Cummulative Impacts

• In 1974, the Oriental Chestnut Gall Wasp
  (Dryocosmus kuriphilus) was brought to the US
• Female lays eggs in chestnut vegetative buds
• Galls suppress shoot elongation and reduce
  fruiting
• Heavy infestations can kill the trees (afflicts
  both American and Chinese chestnuts at the
  southern end of their ranges)
• Threatening complete extinction
• (Anagnostakis, 1994)

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Varying Outcomes Europe

• The fungus was later introduced into Europe (for
  tree breeding) from America
• Moved through Europe killing European Chestnut
• However, it was observed that many trees, while
  infected and full of cankers, did not die

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• Instead of sunken diffuse cankers, surviving Europ
  ean chestnuts had swollen cankers with evidence
  of "healing" along the margins.

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• Many forest pathologists began working on
  this healing canker
• Speculation that
• European Chestnut was less susceptible
• That the fungus had mutated
• That it was a different fungus altogether 

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• Noticed that a different colored fungus was
  recovered from "healing cankers"  
• Instead of the typical orange colored
  Cryphonectria parasitica fungus, a white-colored
  fungus was found. 
• White fungus was slower growing and produced
  fewer spores
• When you "sprayed" the white fungus on a "killing
  canker" the "killing canker"  became a "healing
  canker" (Europe)

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• Determined that the white hypovirulent strains
  had become infected with a simple dsRNA  virus
• This virus was making the fungus "sick
• A slower fungus allowed the tree to respond to a
  point where the tree could survive infection

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Varying Outcomes Europe

• Grente reported in 1965 that hypovirulent
  strains from Italy did not kill chestnut trees
• Began a program of active intervention when
  blight was found in France
• blight strains with dsRNA passed hypovirulence to
  lethal strains
• Treatment of new cankers as they formed resulted
  in a successful biological therapy of the
  disease.
• treat every canker for several years

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• For a number of reasons biological control of
  chestnut blight does not work as well in the US
• Different mating types of the fungus
• Lack of chestnut to support conversion of the
  fungus by the virus
• The many different types of virus in the United
  States

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Varying Outcomes Michigan

• Hypovirulent strains were found in the United
  States
• Most notably in Michigan
• Successful because
• Few mating types
• High number of Chestnut
• Isolated from the native range   
• Less diversity of pathogen in MI so that
  hypovirulence can transfer more readily

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• The transmission of hypovirulence from strain to
  strain of the fungus is restricted by a genetic
  system of vegetative incompatibility
• Six loci, each with two alleles in a system of
  heterogenic incompatibility which keep the
  strains of the fungus from fusing and passing
  hypovirulence (Huber and Milgroom)
• Virus transfer is restricted when there are
  different alleles at the vegetative
  incompatibility loci

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Current Status

• Reduced to a short lived sprouting understory
  tree
• Fungus can not survive below the ground. 
• roots continue to live and they send up stump
  sprouts.   

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Current Status

• Stump sprouts grow until infected
• the stump re-sprouts again
• Little chance for resistance to evolve
• sprouts typically killed before they become
  sexually mature
• sexual reproduction rare

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Last remaining stand of American Chestnut

• Largest living (gt3 ft dbh) about 20 miles east
  of La Crosse, WI.
• 10 chestnuts planted in 1885
• Seeds propagated around 50 acres and more than
  3000 trees

• Trees were blight free due to isolation until a
  canker was found in 1986
• Now over 1600 cankers are present on 530 trees.  
  
• Virus was introduced in 1992 not successful

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Where are we now?

• Upper slopes scarlet oak, hickory, black gum
• Mid slopes red and white oak, red maple,
  hickory
• Coves Poplar, hard maple, beech
• Understory - American chestnut sprouts still
  persist, however they become infected between
  1-12 yrs of age.

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Blight Control and Restoration

• Approaches
• Hypovirulent strains
• Asian blight resistance
• Natural resistance
• Forest management practices

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Hypovirulent Strains

• Italian and French scientists observed non-lethal
  cankers growing on trees in Italy (1960s)
• Found that strains of the fungus associated with
  the blight produced colonies of abnormal shape
  and pigment
• Demonstrated that these strains contained some
  contagious factor responsible for the
  inability to produce lethal infections (i.e.,
  Hypoviruses)
• In North America, hypovirus-infected strains have
  been found in stands in Michigan.

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Hypovirulent Strains

• In the last two decades, scientists have
  attempted to debilitate the fungus by infecting
  it with a virus, a process called hypovirulence.
• Hypovirulence gives chestnut trees a much less
  potent form of the disease and gives chestnuts a
  fighting chance for survival (i.e., fungus is
  restricted to the outer bark).
• Once introduced into a few trees, hopes are that
  hypovirulence will spread throughout the forest,
  offering hope to surrounding trees as well.

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• Varied success
• increase in stem size and stem number
  (Anagnostakis 2001)
• strains do not persist (Peever et al. 1997)

Virulent strains
Hypovirulent strains
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Intensively treated - cankers sampled, paired
with hypovirulent strains, and reintroduced into
the canker.
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Limited treatment - cankers were sprayed with a
mixture of conidia from Hypovirus-infected
strains that had been used for treatment of the
intensively treated plot.
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Control Treatment
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Factors contributing to failure

• high blight susceptibility
• abundance of virulent inoculum
• restricted movement of the hypovirulence viruses
  among the many strains
• Europe and Michigan strains

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Factors contributing to failure

• The transmission of hypovirulence from strain to
  strain of the fungus is restricted by a genetic
  system of vegetative incompatibility.
• Genetic studies found that there are six loci,
  each with two alleles in a system of heterogenic
  incompatibility which keep the strains of the
  fungus from fusing and passing hypovirulence
  (Milgroom and Cortesi).

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More factors contributing to failure

• environmental stress
• superficial canker instability (i.e.,
  hypovirulent cankers produced change back into a
  killing canker after one or more winters)

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Asian blight resistance

• Early breeding efforts unsuccessful
• Poor form
• lt 50 AC parentage
• Poor survival
• 1981 backcross breeding method proposed (Burnham)
  
• Better form
• Field blight resistance

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• Resistant Asian X Susceptible American
• Partially resistant X American again
• 1 out of the 4 will have 1 copy of both resistant
  genes
• Process repeated until a final cross of 2 trees
  with partial resistance yields 1 having 2 copies
  of both resistant genes making it fully resistant

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American chestnut resistance

• Breeding programs
• Scions were grafted into chestnut rootstocks to
  establish seed orchards
• Seeds and seedlings have been distributed that
  have low levels of blight resistance by
  artificial inoculation with a standard virulent
  strain

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Site Factors

• High vs. low elevation
• High elevation sites contain the highest density
  of chestnut sprouts
• Studies found the superficiality rating of
  cankers to decrease greatly (disease developed at
  the vascular cambium) after several winters at
  high elevation sites
• May be a result of physiological stress from low
  temperatures in mid- to late winter which may
  decrease host defense mechanisms in chestnut
  towards weak pathogens, such as hypovirulent
  strains

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Site Factors

• Xeric vs. mesic sites
• Blight control greatest on mesic sites
• Competition- high levels of hardwood competition,
  especially on mesic sites
• Browse damage

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Restoration

• Combination of the four approaches can bring the
  chestnut back
• Individual or group selection openings- an
  integrated management system using grafted trees,
  inoculating them with hypovirulent strains, and
  controlling hardwood competition
• Timber production- backcross approaches