May 2021 - 20

USDA grant roots out rapid apple decline culprit
By Erin Rodger
Cornell AgriTech
Since 2013, a mysterious phenomenon
has been killing apple trees across North
America, leaving growers panicked and
scientists bewildered. The phenomenon
has been named rapid apple decline
(RAD) for its sudden onset, which causes
apple trees to quickly deteriorate and die.
Now, two scientists from the College of
Agriculture and Life Sciences (CALS) are
on a mission to track down its cause.
This effort is thanks to a recent
$299,000 grant from the USDA's
National Institute of Food and
Agriculture. The three-year project,
" Root Traits and Rapid Decline of Apple
Trees in High-Density Orchards, " is one
of the first federally funded projects
to research rapid apple decline in the
United States.
In Geneva, New York, two researchers
at Cornell AgriTech first teamed up in
2018 to explore potential causes of RAD,
such as pathogens, insects, cold weather
injury and root systems' interactions.
While their findings revealed no direct
above-ground causes, they noticed that
a weaker root system could potentially
contribute to the trees' decline.
" So very little is known about the
below-ground root system of apple
rootstocks right now, " said Awais Khan,
co-principal investigator and associate
professor of plant pathology and plantmicrobe
biology. " By performing a
comprehensive analysis of these systems,
we are hopeful that this project will
uncover the answers the apple industry
has been looking for. "
He and Marc Fuchs, professor of plant
pathology and plant-microbe biology,
will hone in on the complex dynamics
of apple root systems and explore the
behavior of viruses in commercial apple
orchards.
While roots are an important
Awais Khan, associate professor of plant pathology and plant-microbe biology, examines apples in an orchard. A tree killed by rapid
apple decline. Photos: Allison Usavage/Cornell University
component of any apple orchard,
knowledge of their growth and
architecture is scarce due to the difficulty
and cost involved with studying them.
Few technologies exist to help scientists
study these systems, and digging up trees
in an orchard to examine their root traits
is not only expensive but destructive.
Now, Khan and Fuchs will be working
with commercial growers in the Hudson
Valley, Capital District and Lake Ontario
regions to compare physical samples of
both declining and healthy trees from
different orchards and soil systems.
They will also obtain trees from
nurseries in New York state and beyond
for analysis.
As part of his research, Khan will
explore a possible connection between
high-density orchard systems and
RAD. By planting small nursery trees
in a greenhouse with spacing that is
comparable to the close spacing of these
systems, Khan will replicate the way in
which roots may potentially compete with
each other for nutrient uptake.
Fuchs will establish whether the
presence of viruses in these nursery trees
may weaken the root system of declining
trees.
The nursery plant roots will be
examined after one, two and three years
and a 3D root system of each will be
documented. From there, the trees will be
planted and then dug up over a three-year
period to assess how they have grown
over time.
Khan and Fuchs will also examine
a wide variety of commonly used
commercial apple rootstocks to
determine whether there is a connection
between RAD and specific rootstocks
being used by growers.
As part of the project, Fuchs will
specifically analyze whether certain
rootstocks are more impacted by viruses
than others.
" Viruses are all over the place in
orchards, " he said. " What I noticed a few
years ago was that the declining trees are
often affected by two viruses. But that
doesn't mean those are causal viruses. It
might just mean that they are coexisting
with the roots, and that it's really the roots
that are the weak link in the trees' system. "
He suspects that when multiple viruses
are present in an apple cultivar, they may
work their way down to the rootstock.
There, they could debilitate the vascular
connection between the rootstock and
the rest of the tree, further weakening
the roots and limiting nutrients' ability to
travel up to the cultivar.
The most proactive way to maintain
the long-term health of an orchard is to
make sure cultivars are virus free before
being grafted onto apple rootstock. Fuchs
said this could also be an important first
step in preventing the onset of RAD,
and he's working with New York state's
Department of Agriculture and Markets
to develop a virus-free certification
program for apple tree nurseries.
" This project is an opportunity for
CALS to stand out as a leader in helping
the industry overcome a challenging
problem, " Fuchs said. " Ultimately,
we want to help growers make good
decisions about using cultivars that do
not have viruses present on the right
rootstock and in the right site. " FGN
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May 2021

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