May 2022 - 9

Breeding cucurbits for
disease resistance sought
By Dean Peterson
VGN Correspondent
Phytophthora fruit rot and downy
mildew have been identified by
cucumber commodity groups as
the disease priorities for researchers
developing resistant lines.
That message has been carried
to genetic researchers across the
country through USDA's Specialty
Crop Research Initiative Cucurbit
Coordinated Agricultural Project
(CucCAP).
Rebecca Grumet, professor in
Michigan State University's (MSU)
Department of Horticulture, is
CucCAP's project director and works
on finding resistance to phytophthora
fruit rot in cucumbers. She talked
about the process of selecting for
disease resistance in the pickling
cucumber session of the 2021 Great
Lakes Fruit, Vegetable & Farm
Market EXPO held in Grand Rapids,
Michigan.
Finding resistant genes in
cucumbers is a worldwide search with
cucumbers grown on nearly 5 million
acres in more than 130 countries.
" There's a lot of variation in what
these pickling cucumbers look like
around the world, " Grumet said.
The original plant carrying the
resistant gene(s) may not look
anything like a modern pickling
cucumber.
China, by far, grows more
cucumbers than any other country
in the world. Turkey and Russia are
also leading producers. A challenge
is that in many local geographies,
cultivars evolved with narrow genetic
bases and limited capacity to improve
through crossbreeding. The demand
for uniformity in plant type, fruit
type and flowering time is often
accompanied by a need for uniformity
in disease resistance. This also
restricts genetic diversity.
The biology of cucurbits is another
challenge. Cucurbits are insect
pollinated and can outcross with other
large, spreading vines, which can make
it difficult to regenerate and maintain
genetic lines.
This makes a primary CucCAP
objective the collection and analysis of
biological data - and, more specifically,
the genetic data - of cultivars.
" We have to test the cucumbers for
sources of resistance, " Grumet said.
Researchers then identify, map and
develop markers for the resistance
genes. These molecular marker tags
can track genes for specific traits at
numerous locations throughout the
genome.
" There are regions on the
chromosome that show resistance, "
Grumet said. " These are the parts of
the chain we want to move. We want
all of the regions that are important for
phytophthora fruit rot to be with the
regions that are important for downy
mildew. "
Identified resistances must then be
introduced into advanced breeding
lines, and then the lines evaluated.
MSU researchers grow their breeding
lines on trellises even though it's not
how cucumbers are commercially
grown.
" They're grown on trellises so there's
no accidental disease inoculation in
the field or contamination by mud, "
Grumet said.
The fruit is harvested, brought into
the lab and inoculated with the target
pathogen. The
process is slow,
but researchers
have identified
a promising
cucumber line
from Turkey that
has the potential
for resistance
on multiple
chromosomes.
CucCAP has
REBECCA
GRUMET
several teams,
each with multiple investigators.
Yiqun Weng is CucCAP's team leader
for cucumber and is the principal
investigator on downy mildew
resistance in cucumbers. Weng is
with the USDA Agricultural Research
Service (ARS) at the University of
Wisconsin.
Michael Mazourek of Cornell
University is the team leader for
squash and pumpkins. Commodity
groups have identified phytophthora
fruit rot and powdery mildew to be the
disease priorities for these crops.
For watermelon, commodity groups
have determined fusarium wilt,
gummy stem blight, phytophthora
rot, powdery mildew, papaya ringspot
virus and cucumber green mottle
mosaic virus to be the disease
priorities. Amnon Levi of USDA-ARS
in Charleston, South Carolina, is the
team leader for watermelon.
The priorities for melon are powdery
mildew, cucumber mosaic virus and
cucurbit yellow stunting disorder,
and Jim McCreight of USDA-ARS in
Salinas, California, is the team leader.
Zhangjun Fei of Cornell University
is CucCAP's team leader for
bioinformatics. Bioinformatics is the
science of collecting and analyzing
complex biological data, such as
genetic codes.
Lina Quesada-Ocampo of North
Carolina State University is the
team leader for integrated disease
management. This team seeks to
provide timely disease control
information and incorporate new
genetic materials into disease
management programs.
More information on CucCAP and
the latest information on developing
disease resistance in cucurbits can be
found at https://cuccap.org/. VGN
VGN | MAY 2022 | 9
TAKE FAST-ACTING CONTROL
OF KEY DISEASES IN YOUR
VEGETABLES INCLUDING
* ALTERNARIA LEAF SPOT
* BLACK DOT
(SUPPRESSION)
* DOWNY MILDEW
* EARLY AND LATE BLIGHT
* PURPLE BLOTCH
* WHITE RUST
* CAVITY SPOT
Reason® is a registered trademark of Gowan Company, L.L.C. Always read and follow label directions.

https://www.cuccap.org/

May 2022

Table of Contents for the Digital Edition of May 2022