April 2021 - 12

Pear rootstocks for replant
in presence of armillaria
By Crystal Nay
Western Editor
Common in soils that once held oak
trees and other native forests, Armillaria
mellea has a strong presence in California
and the Pacific Northwest. This species
is the most widely spread, is lethal to
susceptible fruit trees, and currently has no
method for eradication.
With this in mind, Caroline Tweedy,
a master's graduate from the University
of California (UC), Davis, set out to find
viable rootstocks that would be good,
resistant replants in an orchard infested
with armillaria. The research remained
in the lab, where 185 seedling and 85
clonal genotypes were exposed to high
concentrations of armillaria and analyzed.
" When the seeds were removed from
the cold room, some did not germinate at
all, or were contaminated, or were already
dead. Some genotypes grew into great little
seedlings, some had only one seedling, and
there weren't as many replicates as there
were in the clonal, " said Tweedy.
There were 20 replicates propagated
for each clonal rootstock. Seedlings were
based on germination success.
Tweedy used the Area Under the
Disease Progress Curve, or AUDPC, to
score each of the genotypes in terms of
disease development - from how much
media was covered in fungus to the parts
of the plant that became necrotic. The
plants vary throughout time, with some
showing symptoms of disease as early as
the first week and others several weeks
later.
" Some could be green, green, green,
then boom, on week six they turned
completely black, " said Tweedy. " If I only
looked at week six, then I'd think that
plant was bad, that it took the armillaria
poorly and was susceptible. But looking
throughout time, it was more resistant, in
a way, until week six. "
Because the curve incorporates time,
Tweedy was able to assess whether the
disease progressed faster or slower within
a genotype.
She also used a genome-wide association
study (GWAS), which is used to determine
whether certain specific genetic variants
are associated with particular traits. The
genetics were already sequenced for all of
the pear varieties she used in her study.
Using both the genetic and phenotypic
data she had, Tweedy was hoping to find
a certain section of the genome that was
associated with disease resistance.
" From that GWAS test, there was no
section that showed significant resistance, "
Tweedy said.
Extreme example of development
roots produced by the in vitro plant.
OHxF 87 six weeks after armillaria
inoculation.
Line graph of a single plant's disease development over six weeks. Each plant's
AUDPC value was calculated by area under the curve, and used to determine which
genotypes performed best.
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April 2021

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