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 – yvf626@mocs.utc.edu

Dr. J. Hill Craddock – Hill-Craddock@utc.edu

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