Nightmare on Elm Street

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Nightmare on Elm Street
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The Elms (genus Ulmus)

• - gt30 species in genus
• Europe has 5
• N. America 8
• Asia has 23 or more.
• - 2 ssp live in tropics
• - 6 spps native to the northeastern U.S.,
• including Ulmus
• americana, the
• American elm.
• Many cultivars too.
• - New species are still
• being found in China,
• the center of diversity.

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The perfect shade tree

• Street liners fast-growing,
• easily transported, tolerant
• of soil compaction and
• different soil types. Dense
• canopy borne high above
• ground, few low branches.
• Shade, shelter When
• planted in rows, they
• overhang the street
• forming a Gothic-style arch. Good for
  windbreaks.
• 1 urban tree in U.S east of the Rockies, and in
  large parts of Europe and Asia (Heybroek, 1993)

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Elms in urban/rural settings

• In coastal western Europe, as windbreaks
• In dry continental places, the Siberian elm is
  important. Planted as shelterbelts to prevent
  erosion during the Dustbowl in the 30s. The
  most important shelterbelt tree species in the
  U.S.

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In Nature

• In general, a riparian, river bottom species
  (goes through periods of anoxia, explaining its
  tolerance to over-watering and soil compaction).
• Long lived (up to 300 years).
• Largest trees seem to be most susceptible.
• Moller (1992), in Netherlands, 79
• ssp of insect are specialized or
• dependent on elm. Elm seeds are important.

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Dutch Elm Disease

• Why Dutch? First
• isolated in 1920 by a
• Dr. Schwarz in the
• Netherlands.
• - Wilt disease that
• attacks elm (Ulmus ssp)
• caused by ascomycete
• fungi (genus Ophiostoma,
• formerly Ceratosystis).
• Vectored by beetles (fam. Scolytidae) and root
  graft. Has a saprophytic and a pathogenic stage.

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Life Cycle of Ophiostoma ulmi
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Life Cycle of the Disease

• Saprophytic stage (in the bark, beetles emerge
  and carry to healthy tree) and a pathogenic stage
  (once introduced to a healthy host tree, moves
  from bark to xylem and begins to attack. May then
  go back to bark to reinfect beetles).
• Obligate outcrossers with two sexual
  compatibility types.

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Life cycle of beetles and Ophiostoma are closely
matched
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1) Native elm bark beetle (Hylurgopinus rufipes)
(above) is the primary vector in parts of the
northern United States, New England, and all of
Canada. However, temperatures below -6F kill the
larvae. 2) European elm bark beetle (Scolytus
multistriatus Marsh.) (below) is the major vector
of the disease.
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Vectors of disease

• Insects 1) the native elm beetle 2) the smaller
  European elm beetle. The beetles can fly for
  several miles, allowing the disease to spread
  over a wide area.
• Root grafts when elms are within 50 feet of one
  another, their roots can grow together and
  disease passes easily along. Important in urban
  settings.
• Infected logs Often transferred long distances
  in logs.

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Management Sanitation

• Includes removing bark from elm logs which are
  being stored for use as fuel and/or covering or
  burning all downed wood (so that beetles cant
  get in it). AND, removing dead or diseased
  branches of standing trees (again because of the
  beetles).
• Needs to be community-wide, and coupled
  w/fungicide use.
• Thought of as the most effective way of curbing
  DED.

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Management Innoculations

• Systemic fungicides labeled for preventative
  control, injected into root flares. Effective on
  trees showing lt 5-10 crown symptoms.
• Need new injections every 3 years, expensive.

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Management Spraying

• Best when coupled w/sanitation methods.
• Timing of spraying is important

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Other Management Methods

• Development of resistant hybrid elms
• Additional treatments breaking up root grafts
  is commonly used and efffective.
• Timing of pruning wounded trees attract the bark
  beetle vectors of DED (Byers et al., 1980), so
  routine pruning should be done in the dormant
  season or during periods of beetle inactivity.

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History of the Disease (Brasier, 2001)

• -unknown in Europe and N. America pre-1900. Since
  then, 2 major pandemics.
• -caused by 2 different species
• 1) Ophiostoma ulmi
• 2) Ophiostoma novo-ulmi
• (in both cases, geographic origins unknown-
  probably Asia)

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Pandemic 1 (Ophiostoma ulmi)

• Appears in Europe in 1910s (sweeps across Europe
  and into Asia) arrives in eastern U.S. in late
  1920s on infested elm timber transported to
  Ohio in 1928 via diseased logs.
• In Europe it killed 10-40 of the elms in most
  countries but by the 1940s it had slowed,
  because of the of spread of deleterious viruses.
  These viruses did not show up in the U.S. and O.
  ulmi continued to kill trees.

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Pandemic 2 (Ophiostoma ulmi-novo)

• In the 1940s, two strains of O. ulmi-novo began
  a second wave of epidemics the EAN (Eastern
  European) strain in Moldova-Ukraine, and the NAN
  (North American) strain in the Great Lakes region
  of U.S. Traveled to Asia, W. Europe, and all
  over the U.S.
• Repeated introductions occurred b/c people didnt
  realize it was a separate species.
• Most mature European elms dead (30 million in UK
  alone). In N. America, hundreds of millions of
  elms dead. In these places and in Asia,
  recurring cycle of recovery of seedlings, and
  then attack by O. novo-ulmi, are predicted well
  into the future.
• In U.S., in all states besides the desert
  Southwest.

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Spread of O. ulmi and O novo-ulmi
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Arrival dates in the U.S.
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Dynamics between O. ulmi and O. novo-ulmi

• O. ulmi arrives first, but O. novo-ulmi then
  arrives and outcompetes and replaces. Why?
• --evolved in tropics vs. temperate
• --levels of aggression (O. ulmi is moderate
  pathogen on European elms, O. ulmi-novo is
  aggressive. American elms are more susceptible
  to both). Different levels of the cerato- ulmil
  protein (see later slide).

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O. ulmi and O. novo-ulmi hybridization?

• They are anciently diverged taxa but seem to be
  able to cross under certain conditions, so rare
  hybrids do occur in nature. These are transient
  (weak and sterile). But they can act as genetic
  bridges- allowing unilateral gene flow from one
  species to the other, when backcrossing occurs.

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Hybridization (cont.)

• Evidence for gene flow
• --has the pathogenicity gene been
  transferred from O. ulmi to O. ulmi-novo??
• Field inoculations of the moderately resistant
  elms Ulmus procera and Ulmus X Commelin were
  carried out with progeny of a genetic cross
  between AST27, a Eurasian (EAN) O. novo-ulmi
  isolate with an unusually low level of
  pathogenicity, and H327, a highly aggressive EAN
  isolate. These confirmed the results of a
  previous study that indicated that the difference
  in phenotype was controlled by a single nuclear
  gene. This pathogenicity gene,designated here
  Pat1, is the first putative pathogenicity gene to
  be identified in O. novo-ulmi. (Linkage
  distances, etc.) suggest that the Pat1 allele
  conferring unusually low aggressiveness in AST27
  may have been acquired from O. ulmi via
  introgression.
• ( Et-Touil, Brasier, Bernier. 1999. Molecular
  and Plant Interactions)

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Hybridization (cont.)

• Evidence for rapid changes in O. ulmi-novo
  population structure
• --Gene acquisition of vegetative
  compatibility (vc) genes from O. ulmi. (occurs
  only where O. ulmi and O. novo-ulmi coexist or
  used to coexist, and the virus is present allow
  for resistance to viruses.)
• --Gene acquisition of virus from O.
  ulmi? (preliminary data suggests that its
  possible)

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Cerato-ulmin

• A secreted protein, isolated in 1975, that seems
  to be directly correlated to aggressive forms of
  Ophiostoma. May be a wilt toxin.
• The nucleotide sequences of the cerato-ulmin (cu)
  genes of two naturally occurring pathogenic
  CU-deficient mutants, PG470 and MAFf8, of the
  Dutch elm disease fungus, Ophiostoma novo-ulmi,
  were determined.. It is likely that the cu gene
  of MAFf8 has been introgressed from O. ulmi,
  probably as a result of rare hybrid formation
  between O. ulmi and O. novo-ulmi, followed by
  backcrossing of the hybrid with O.novo-ulmi. The
  presence of an O. ulmi-like cu gene in MAFf8 is
  consistent with its CU deficiency, since the O.
  ulmi cu gene is known to be poorly expressed and
  O. ulmi isolates secrete little or no CU in
  culture. (Pipe Brasier Buck. 2000. Molecular
  Plant Pathology).
• Results from these trials demonstrated that
  cerato-ulmin was not directly involved in the
  virulence of the pathogen. All of the
  epidemiological data, however, indicated a
  correlation between cerato-ulmin and the
  pathology of Dutch elm disease. We suggest that
  the critical evaluation and consideration of
  these recent data offer opportunities in
  developing biological control strategies for
  Dutch elm disease. (Temple and Horgen. 2000.
  Mycologia)

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Hybridization (cont.)

• The EAN and NAN forms are hybridizing.
• -- swarms of EAN/NAN hybrids are likely to
  emerge in overlapping sites (Brasier, 2001)
• --so O. novo-ulmi is currently undergoing
  rapid evolutionary development in Europe
  (accelerated pathogen evolution when it is
  released from its endemic environment)
• Evidence of hybridization in other species as
  well.
• --Ophiostoma quercus (saprophytic on
  oaks) and O. novo-ulmi?
• --Talk about diversity in O. ulmi and O.
  novo-ulmi

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And..

• The combination of low d-infection frequency, low
  vc type diversity and the presence of a less
  efficient Dutch elm disease vector (Scolytus
  multistriatus) in North America suggests that
  North American novo-ulmi populations might be
  potential targets for attempted biological
  control of Dutch elm disease via the release of
  d-factors. (Brasier, 1996)

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