April 2021 - 13

The AUDPC averages showed that
OHxF 87 and OHxF 97 were the
rootstocks most likely to have resistant
qualities. Thought to be a cross between
Old Home and Farmingdale varieties, later
genetic studies revealed that the OHxF
rootstocks are actually a cross of Old Home
and Bartlett, but still use the OHxF label.
OHxF 87 is less vigorous and will grow
slightly smaller than its full-sized, vigorous
OHxF 97 counterpart. Both rootstocks are
also resistant to fire blight.
Tweedy's research comes at what is the
beginning of an increased concern focused
on armillaria, and what growers can do
to combat it. Most growers are unfamiliar
with the fungal pathogen and don't know
if it's in their fields nor how to control
it. Though its lesser prevalence in pear
has caused it to take a back seat to other
diseases, such as fire blight, its lethality in
a variety of crops - particularly in peach,
which is highly susceptible - is causing
more people to take notice.
Armillaria can cause a steady decline in
tree health and, thus, yield. And while one
tree may show symptoms of infection, it
may take multiple years for the tree next to
it to begin showing signs as well. During
field sample collection, Tweedy found
armillaria infection in the bark of dead tree
stumps - trees that had likely been killed
by armillaria - yet its neighboring replant
had yet to show symptoms.
" It's patient, " said Tweedy of the
pathogen.
Once a tree is infected, the root system
of that tree is also infected. One theory
is that in an orchard system, where the
trees are planted closely together, the root
system of the infected tree can touch that
of the neighboring uninfected tree, causing
the spread of the pathogen.
How this in vitro lab study translates to
the field has yet to be determined. Field
studies on armillaria are difficult because
plants can take years to show symptoms of
infection. Though unofficial, Tweedy did
stumble upon some interesting findings
during her study when she decided on
an impromptu side experiment: planting
nearly dead plants.
Tweedy gave her armillaria-infected
plants teetering on the edge of death to
Wes Hackett, retired UC Davis researcher,
who then planted the seedlings in soil and
placed them in his greenhouse.
" They started to grow back normally, "
said Tweedy. " There are other bacteria and
fungus in soil, so maybe the armillaria
fungus is waiting for the plantlet to show
signs of weakness or susceptibility again,
or the armillaria itself is waiting to get
stronger. It's a slow-growing fungus. "
There is other research Tweedy would
like to see in regards to armillaria, its
effect on seeds, and if a seed's phenolic
compounds that protect it from disease
early in life remain consistent throughout
its lifetime.
" There are still rootstocks that are
offered to growers that are in the seedling
form versus the clonal form, so that
would be something else nice to look at, "
Tweedy said.
Until there's further research - both in
the lab and in the field - Tweedy stands
by her recommendations not only for
replants but for new growers considering
a pear rootstock. FGN
FGN | APRIL 2021 | 13
Boxplot of average AUDPC scores for all 85 clonal genotypes inoculated, ordered from most resistant to most susceptible.
Image: Carolina Tweedy

April 2021

Table of Contents for the Digital Edition of April 2021