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Don't let phytophthora rot your hard squash
MARY K. HAUSBECK
Michigan State University
The Phytophthora capsici microbe
known to rot the crown and fruit of hard
squash may have a funny name that's
hard to spell, but it is a limiting factor for
many growers. The crowns, petioles and
fruits of hard squash are susceptible to this
devastating pathogen.
Symptoms of crown rot include water
soaked and brown tissue of the plant
and/or fruit followed by a rot that becomes
soft. In some cases, a water-soaked blight
may be seen on the leaves during times
of high rain and humidity. Infected fruit
typically show patches of white spores that
look similar to powdered sugar; these areas
expand as the rot develops (Figure 1). It is
possible to harvest healthy-appearing fruit
with rot developing days later while the crop
is in transit or on grocers' shelves.
Major crop losses often occur despite
following recommended rotation and other
control strategies. Since survival spores
(oospores) of phytophthora are long-lasting
(10 years or more) in soil, fields infested
with this pathogen are sometimes taken
out of vegetable production because many
important crops are susceptible (see
Table 1). An important advance in the
battle against this pathogen was made when
research showed how easily phytophthora
can spread by irrigating with surface water
contaminated with the pathogen.
While it was known that phytophthora
is favored by rain and spreads readily in the
field via water, it was not widely recognized
that the pathogen could be in the creeks,
ponds, ditches and culverts used to irrigate
squash, pumpkins and other vegetables.
Our research showed that phytophthora
in these surface water irrigation sources is
likely to be greatest during July and August
when the need to water crops is often the
greatest. Recognizing that phytophthora
may be introduced to new fields via
irrigation water and cause squash and
pumpkins to rot in fields without a
history of phytophthora or vegetable
production has prompted many growers
to establish wells.
Known as a water mold, phytophthora
may form many sporangia (spores) on the
surface of infected fruit (see Figure 1). If
these sporangia come into contact with
water, each will develop into 20 to 40
swimming zoospores. These zoospores
may swim through water-saturated soil or
may be carried in flowing water, including
irrigation water. Swimming zoospores use
electrical and chemical signals to hone in on
a susceptible plant. Zoospores in water can
survive and cause infection for days, even
after they have stopped swimming.
Phytophthora is not likely to be found in
surface water when the water temperature
is greater than 77˚ F or less than 57˚ F. Since
vegetable producers usually irrigate during
the warm summer months, the temperature
of irrigation water is unlikely to limit
infection. If high numbers of zoospores are
12 | VegetableGrowersNews.com
Figure 1. Rotting squash infected by
Phytophthora capsici and showing
the " powdered sugar " coating of the
pathogen's spores. Photo and charts: MSU
present in the water, infection of susceptible
crops like squash and pumpkins is more
likely, even when the water is cooler.
Older zoospores still caused infection of
pickling cucumber fruits in Michigan State
University (MSU) studies. The amount
of infection observed depended on the
number of zoospores in the water; more
fruit became infected at higher zoospore
numbers. Zoospores were able to infect fruit
even after being in water for five days. Well
water or water from well-fed ponds should
be used to irrigate squash and pumpkins.
While treatment options for disinfecting
surface water sources are not currently
available, MSU research found that
algaecides work well in the laboratory
setting to zoospores in water. Researchers
in Virginia found that chlorine effectively
killed zoospores in a lab setting. These
products have the added advantage that
they are already used to clear algal growth
from filter intakes and irrigation lines.
Managing water in the field is key to
managing phytophthora, and drip irrigation
systems are recommended to reduce
splashing of infested soil onto the fruit and
plant crown. If overhead irrigation must be
used, reduced watering during fruiting may
limit infection.
If you do not have Phytophthora
capsici in your fields, do everything you
can to prevent it from being introduced
and established. If there is a history of
phytophthora in a particular field, take
preventive measures. Do not plant winter
squash or pumpkins in that field. Soils
should be well-drained. Leave the lowlying
areas of the field unplanted as that is
often where disease can begin. Plant winter
squash and pumpkin into raised beds,
allowing excess water to drain away from
the plant and fruit. If phytophthora crown
rot develops, remove the diseased plants,
including the adjacent healthy-looking
plants, as they are likely already infected
and will show symptoms soon. Work
fields without a history of phytophthora
first and finish with problem fields. Clean
all equipment used in an infested field to
prevent spread to other fields. Discard
infected and rotting fruit culls in an area
where crops are not grown.
Hard squash types that become more
resistant to rot as the fruits mature (21
days following pollination) including
Butternut Ultra, Buckskin, New England
Cheddar, Dickinson, spaghetti squash and
acorn squash. Some cultivars that remain
susceptible as the fruit mature include
Hubba Hubba Hubbard squash, Autumn
Cup, Sweet Mama, Thunder, Delica,
Table 1. Vegetables that can be infected by Phytophthora capsici.
Cucumber
Gourd
Pumpkin
Honeydew
Muskmelon
Summer squash
Winter squash
Watermelon
Zucchini
Eggplant
Bell pepper
Management Strategies
Plant into well-drained, tiled fields.
Use raised beds and drip irrigation.
Avoid using surface water for irrigation.
Irrigate sparingly from a well.
Rotate crops.
Scout fields regularly for phytophthora.
Remove any diseased plants and adjacent healthy-appearing plants.
Apply fungicides preventively and at short intervals when needed.
Remove fruits from field as quickly as possible and store in a warm, dry place.
Powerwash equipment after it has been in infested fields.
Do not dump diseased culls in production fields.
Product
Elumin
Preferred Phytophthora Fungicides for WINTER SQUASH
A.I. FRAC*
Comment
Orondis
Gold 200
Orondis
Ultra
Presidio 4SC
Ethaboxam
Oxathiapiprolin
Oxathiapiprolin/
Mandipropamid
22
49
49/
40
Fluopicolide 43
Revus 2.08SC Mandipropamid 40
**Apron XL
Mefenoxam 4
**Ridomil Gold Mefenoxam 4
Rotate between applications. Apply
as a soil or foliar spray or via drip.
Apply at-plant in-furrow or via drip
(after plant emergence if direct-seeded).
Rotate to a fungicide with a different FRAC
after 2 sequential applications. Use either
soil or foliar applications of oxathiapiprolin
products, but not both for disease control.
Use in a fungicide tank mix. Apply via
drip or as a foliar spray.
Include surfactant.
Seed treatment. Wait 6 weeks after
transplant to apply mefenoxam products.
Apply as a preplant-incorporated, at-plant
soil spray or via drip.
Phytophthora 'B' Team for WINTER SQUASH
Forum 4.18SC Dimethomorph 40
Gavel 75DF
Mancozeb/
Zoxamide
Ranman 400SC Cyazofamid
Zampro 4.4SC
Ametoctradin/
Dimethomorph
21
45/
40
M03/22
Use in a fungicide tank mix.
Relatively long PHI.
See label about surfactant.
Apply via drip or as a foliar spray.
*The FRAC code is an alphanumeric code assigned by the Fungicide Resistance Action Committee
and is based on the mode of action of the active ingredient.
***While mefenoxam is not labeled for phytophthora, it is labeled for control of pythium. Fungicide
resistance has been detected in phytophthora where mefenoxam has been used frequently.
Sunshine, Space Station and Golden
Delicious.
Fungicides can offer help but must
be applied preventively and add
cost. Protecting the young fruit from
phytophthora is critical. On some farms,
phytophthora has become resistant to
fungicide containing mefenoxam or
metalaxyl as their primary active ingredient.
For the best protection, fungicides must
be applied early and often. Many cultivars
produce large, dense canopies and proper
application equipment is usually required
to penetrate the canopy.
To protect fruits, it is recommended to
apply heavy foliar sprays with drop nozzles
at the time of initial fruit formation and
reapply every seven to 14 days, depending
on the weather conditions and the squash
cultivars. Rotate fungicides using those
representing different modes of action to
prevent phytophthora from developing
resistance.
This information is based upon work
that is supported by the National Institute
of Food and Agriculture, U.S. Department
of Agriculture, under award number
2020-51181-32139. VGN
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July 2021

Table of Contents for the Digital Edition of July 2021