March 2021 - 27

PUMPKINS & SQUASH
Processing squash makes phytophthora a big threat
By Dean Peterson
VGN Correspondent
Squash grown for processing is in
contact with the ground for a long time.
" Processing squash has a longer time in
the field than other cucurbits because
of its longer time to maturity, " said
Mary Hausbeck, vegetable pathologist
in Michigan State University's (MSU)
Department of Plant, Soil and Microbial
Sciences.
That makes phytophthora a serious
issue because the fruit can be in direct
contact with this highly destructive,
soil-borne pathogen.
" Squash fruit may look good at first
glance, but when you pick it up and
turn it over the infection can be seen, "
Hausbeck said. She spoke on managing
phytophthora blight on processing
squash at the recent Great Lakes Fruit,
Vegetable & Farm Market EXPO, held
virtually.
The economics of controlling
phytophthora in processing squash are
difficult. " It's quite a struggle, " Hausbeck
said. " Due to the long time needed for
the processing squash to mature, many
fungicide sprays may be needed to
protect it from phytophthora fruit rot,
and that's expensive. "
The roots, crowns, stems, leaves
and fruits of processing squash are
susceptible to phytophthora. Crown rot
early in the season and fruit rot late in
the season are the two stages where the
disease must be controlled.
" Getting squash to the finish line is a
two-step process, " Hausbeck said. " First,
the plants must be kept alive so that fruit
can be produced. Then, the fruit must be
protected from infection until harvest. "
Crown rot control requires using soilapplied
fungicides early. This includes
soil drenches and banded sprays at
transplanting, and season-long drench
and drip applications targeting the plant
crown and soil. Treat more frequently in
rainy weather.
Orondis, Elumin and Presidio are
options for soil and drip applications.
Ridomil Gold (mefenoxam) is also an
option, but some phytophthora isolates
are known to develop resistance to
mefenoxam-based fungicides if they are
overused.
Some squash varieties have some
resistance to phytophthora and that can
really help.
In MSU trials, the varieties Butternut
Ultra, Buckskin, New England Cheddar
and Dickinson developed crown rot in
less than 20% of the plants in a trial that
included high levels of phytophthora and
no fungicide treatments.
In the same trial, Space Station, NK580,
Autumn Cup and Thunder had
crown rot in about 60% of the plants.
Delica, Sunshine, Sweet Mama and
Golden Delicious had crown rot in more
than 90% of the plants.
Resistance to fruit rot may develop
later in the season during fruit formation
and some varieties become more
resistant as they mature.
MSU researchers evaluated whether
the fruit of different hard squash
varieties would become infected when
challenged with phytophthora at seven,
14 and 21 days after pollination.
The varieties including Buckskin, New
England Cheddar, Butternut Ultra and
Dickinson showed the most dramatic
results. The fruit of these four varieties
became more resistant to phytophthora
rot once they reach an age of 21 days
after pollination than the other varieties
tested. These same varieties were also the
ones more resistant to crown rot.
" Select a variety that is less susceptible
to crown and fruit rot and build a disease
control program from there, " Hausbeck
said. " If you can use some of these
cultivars that my program has identified
as having some resistance to this
destructive pathogen, you can begin with
a good foundation, and the fungicides
can provide the needed additional
control. "
Begin foliar fungicide applications
before symptoms appear when the fruit
are very young and developing, as they
are very susceptible to infection at that
stage. Reapply regularly and remember
to rotate among fungicide classes.
Orondis Ultra, Presidio, Elumin,
Revus are preferred choices for foliar
sprays to the fruit. Ridomil-based, premixed
fungicides may be an option but
resistant phytophthora could develop
and become an issue.
Apply sprays at greater than 40 gallons
per acre because the fungicides must
penetrate the leaf canopy and cover the
fruit in order to provide protection.
Systemic fungicides applied only to the
leaves do not protect the fruit.
Timely harvest so that the fruit is not
left on the soil any longer than necessary
in fields with a known phytophthora
history is an important strategy.
Phytophthora has many vegetable
hosts, so rotation crops should be
carefully chosen.
Managing phytophthora means
managing water to prevent standing
water in the fields following a rainstorm.
Use well-drained sites for growing
processing squash and don't plant in low
areas of the field.
Irrigating with surface water that may
be contaminated with phytophthora
is always an issue. Irrigating with
contaminated surface water is essentially
applying phytophthora to your crop.
Using surface waters as an irrigation
source is not a recommended practice
and allows phytophthora to spread to
clean ground quickly.
Avoid spreading phytophthora from
fields known to be contaminated to
clean fields.
" Work your known phytophthoracontaminated
fields last and then
power wash the equipment to remove
the soil to limit the spread to your
clean fields, " Hausbeck said. VGN
Snowball
Sunlight
Hot Chocolate
Mary Hausbeck is a vegetable pathologist at Michigan State University.
Photo: Gary Pullano
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VGN | MARCH 2021 | 25
March 2021

Table of Contents for the Digital Edition of March 2021
