Cryphonectria parasitica and Phytophthora cinnamomi at UTC · William Scott Smith and J. Hill...
Transcript of Cryphonectria parasitica and Phytophthora cinnamomi at UTC · William Scott Smith and J. Hill...
Chestnut Breeding for Resistance to Cryphonectria
parasitica and Phytophthora cinnamomi at UTCWilliam Scott Smith and J. Hill Craddock
Department of Biology, Geology, and Environmental Science
Orchard Establishment
Canker Evaluation
Pollination
Germplasm Exploration
METHODSBACKGROUND
CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES
CONTACT INFORMATION
A large part of the
breeding process is
growing and caring
for trees. Figure 4
shows C. dentata
grafted to a Chinese
rootstock.
Thousands of
seedlings were
planted in the
Fortwood
Greenhouse in 2018
(Fig. 5).
After trees have grown in the orchard for several years they are
inoculated with C. parasitica which will cause cankers. The cankers are
then evaluated. Figure 8 is a tree with no resistance while figure 9 is a
tree with the desired resistance trait.
Figure 16: This orchard of B3F2s was planted during the spring
of 2018. It is located at Seven Islands State Birding Park. TACF
volunteers are shown here helping us plant seedlings .
In order to breed for resistance to
diseases, selected parents are crossed by
hand pollination. Figure 6 illustrates one
method of pollinating, using whole catkins
from a male tree to pollinate the female
flowers of a different tree. Figure 7 shows
the process in action on large trees in an
orchard setting at TTU. We observed large
surviving C. dentata in
Cannon County, TN
(Fig. 15). Flowers were
collected to be used in
the breeding program,
and leaves were
collected from wild
trees (including this
one) to be used in a
range-wide genomic
diversity study. Scion
wood collected during
the winter was used for
grafting (Fig. 4) (Deason,
2018).
The purpose of the small stem assay is to check for resistance in younger trees, which can shorten the breeding process. First, C. parasitica
strain EP-155 is grown on potato-dextrose agar in the lab (Fig. 10). In 2018, we inoculated 600 trees by first cutting two 2mm x 10mm x 1mm
wounds on each tree. The C. parasitica mycelium is place into the wound (Fig. 11). The inoculated wound was then covered with Parafilm (Fig.
12). Within a few days the orange fungus can be seen spreading from the wound site (Fig. 13). Trees with resistance will develop a smaller
canker around the wound, while trees with little to no resistance become girdled and die (Fig. 14) (Gentner, 2018).
RougingAfter canker evaluations, only the
very most resistant trees are
selected to be used in the breeding
process. Only 14 of 125 trees in the
orchard shown in Figure 17 were
selected. We rouged the rest from
the population.
Phytophthora cinnamomi
Cryphonectria parasitica
History
Another infectious agent that affects C.
dentata is the Oomycete Phytophthora
cinnamomi (Rands), which causes ink
disease, or Phytophthora root rot (PRR),
in chestnut trees and several other tree
species (Robin, 2012). P. cinnamomi was
first described in 1922 in Sumatra, but had
likely spread across the world long before
(Robin, 2012). It has the broadest host
range of any Phytophthora species, but is
particularly destructive in the
southeastern United States (Robin, 2012).
Figure 3 shows PRR infected roots (left)
next to healthy roots (right) during our
2018 screening of hybrid chestnut progeny
(Hein, 2018; Robinson, 2016).
In 1904, Cryphonectria parasitica (Murr.),
the causal agent of the chestnut blight,
was first reported in New York. Within
50 years, it had spread across the
chestnut’s natural range killing almost
every individual, reducing the trees to
basal shoots, which eventually succumb
to the blight too (Anagnostakis, 1987,
2001). This fungus attacks the cambium
and eventually girdles the branch or
trunk, resulting in tree death. We found
blight cankers on wild C. dentata in
Tennessee in 2018 (Figure 2).
The American chestnut tree (Castanea dentata Borkh.) was once one of the
dominant tree species of deciduous forests of the eastern United States,
comprising as much as 25% of the total timber volume in some areas of
Appalachia. C. dentata was an important source of lumber, food, and wildlife
fodder. Figure 1 shows the massive size that these trees could attain. The
Chestnut Project at UTC is actively participating in all aspects of current
efforts to restore chestnuts to the Appalachian forest.
Developing American chestnut populations that are resistant to Cryphonectria
parasitica and Phytophthora cinnamomi will allow natural selection to resume for
this valuable tree species that is now functionally extinct. This, in turn, will
increase biodiversity in our forests, increase food for wildlife, allow cultivation for
nut production, and allow the harvest of high quality timber. The American
Chestnut Foundation uses backcross breeding to introgress genes for disease
resistance from the Asian chestnut species into C. dentata in order to get trees
that are phenotypically identical to Americans chestnuts, but carry the resistance
found in the Chinese and Japanese species. The current generation of resistant
hybrids are roughly 96% American.
Anagnostakis SL. 1987. Chestnut blight: the classical problem of an introduced pathogen. Mycologia 79: 23–37.
Anagnostakis SL. 2001. The effect of multiple importations of pests and pathogens on a native tree. Biological Invasions 3:
245–254.
Deason, Trent. 2018. Conservation and collection of Castanea dentata germplasm in the South. Honors Thesis. UTC.
Gentner, Kevin. 2018. Evaluation of blight resistance in chestnut F2 half-sibling and full-sibling families via small stem assay.
Honors Thesis. The University of Tennessee at Chattanooga.
Hein, Kirsten. 2018. Implementing early screening methods to detect resistance to Phytophthora cinnamomi in backcross
Chinese-American Chestnut hybrids. Honors Thesis. U. Tenn. Chattanooga.
Robin, C., Smith, I., Hansen, E.M. 2012. Phytophthora cinnamomi. Forest Phytophthoras 2(1). doi: 10.5399/osu/fp.2.1.3041.
Robinson, Anna C. 2016. Measuring Phytophthora resistance phenotypes in segregating testcross families of hybrid
American chestnut trees. Honors Thesis. The University of Tennessee at Chattanooga.
WSS would like to thank Dr. Hill Craddock for being his mentor and sharing his extensive knowledge and experience. WSS and JHC
are thankful to the URACE program for giving us the opportunity to collaborate in this research. A big thanks must be given to all the
TACF volunteers who helped out and a special thanks to all the UTC students who helped us at the Fortwood Greenhouse.
William Scott Smith – [email protected]
Dr. J. Hill Craddock – [email protected]
The American Chestnut Foundation – https://www.acf.org
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Nursery Culture
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Small Stem Assay
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Photo by Trent Deason
Forest History Society