Small Stem Assays as a
Screening Tool for American
Chestnut Blight Resistance
Article and photos by Tom Saielli, TACF Mid-Atlantic Regional
Science Coordinator and Bruce Levine, MD-TACF Chapter
This article from Chestnut magazine used with permission
of The American Chestnut Foundation (TACF). Please see
TACF's website for an explanation of how the backcross
breeding program works, yielding terms like "BC
3
F
3
."
The American Chestnut Foundation (TACF) is
screening American chestnut backcross fami-
lies for blight tolerance using small stem assays
(SSAs). In the small stem assay, the stems of con-
tainerized chestnut seedlings are inoculated with
the fungal pathogen Cryphonectria parasitica
and assessed for differences in blight tolerance
between families. For sixteen weeks post-inocu-
lation, volunteers and TACF staff assessed blight
canker severity and survival to detect differences
in blight tolerance across families. We also wanted
to determine whether a strong correlation exists
between the SSA and standard field inoculations.
Since 2017 TACF staff and cooperators have
been experimenting with SSAs. After some initial
hurdles, we now see promising results. In SSAs
conducted at TACF's Meadowview Research Farms
and at Penn State, differences in canker severity
and survival were detected among BC
3
F
3
fami-
lies screened at both locations. There was also
a correlation between survival in containerized
SSAs and canker severity in field test seedlings.
2018 Progeny Test Results
In 2018 TACF, Penn State University, and several
cooperators assisted in SSA progeny tests of
Meadowview seed orchard BC
3
F
3
families. The SSAs
were performed on approximately 30-40 seed-
lings each of 107 BC
3
F
3
families, as well as Chinese
chestnut, F
1
hybrid, and American chestnut con-
trols. The technique involved making a 1 x 5 mm
incision with a cork borer and inoculating with
the highly virulent Ep155 strain of C. parasitica.
Tolerance was measured by monitoring time to wilt
over 16 weeks. Time to wilt measures the number
of days before seedlings wilt due to susceptibility to
the infection. The blight tolerance of the BC
3
F
3
hybrid
families and controls all segregated as expected,
with the Chinese chestnut family being the most tol-
erant and the American chestnut family being the
most susceptible. The F
1
family had intermediate
blight tolerance. The blight tolerance of BC
3
F
3
chest-
nuts ranged from susceptible to intermediate. >>
www.urban-forestry.com 33
Seedlings in the SSA
trials produced a range
of canker severity from
A) small healing cankers,
B) Intermediate cankers,
to C) severe cankers.
The observed variability
indicates good genetic
segregation among the
chestnut families tested and
help us determine the best,
and the worst, trees and
families being tested in the
breeding program.
b a c
