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Plant & Pest Advisory – July 11, 2025 – Andy Wyenandt
As the summer heats up in New Jersey, the control of Phytophthora blight in cucurbit plantings can
be extremely difficult (even with the use of fungicides) as hot, wet weather has set in these past few
weeks. Mitigating losses to Phytophthora blight in cucurbit crops begins with long crop rotations,
where recommendations suggest crop rotations longer than 5 years if possible. Other cultural practices
include avoid planting in low areas of the field where water may persist after rain or overhead
irrigation, rogueing out infected plants as soon as possible, cutting the plastic to help dry out the soil,
proper weed control, and proper preventative fungicide programs.
Searching for sources of resistance to Phytophthora blight in cucurbit crops
Research is continually ongoing for discovering and introducing phytophthora-resistance into cucurbit
crops; where resistance has been identified and used breeding programs in crops such as bell
pepper for decades now. In cucurbit crops, on to genic, or age-related resistance, to the fruit rot
phase of Phytophthora blight has been identified; however, its onset differs by crop. Where summer
squash, zucchini, and melon fruit remain susceptible over time; developing fruit of acorn, butternut,
some cucumber, and pumpkin become less susceptible to P. capsici as they age (i.e., mature) (Perla
et al., 2023). Pumpkin fruit with hard, gourd-like rinds are less susceptible to Phytophthora fruit rot
than pumpkins producing conventional(e.g., softer) rinds (Sanogo et al., 2023). Research for introducing
phytophthora blight resistance into squash is ongoing (LaPlant et al., 2020). Watermelon germ
plasm with resistance Phytophthora blight has been identified and used in breeding programs in the
past few years, unfortunately, its been difficult to bring this resistance to market (Sanogo etal., 2023).
Recently, the USDA has developed and released five resistant germ plasm lines which should prove
useful for developing watermelon cultivars with broad resistance to P.capsici (Kousik et al., 2022).
Fungicides for controlling Phytophthora blight in cucurbit crops
Over the past few years a number of new fungicides, with new active ingredients, have become
commercially-available for use on cucurbit and other crops for Phytophthora control. Mefenoxam or
metalaxyl, both once widely-used to effectively control Phytophthora blight has been hit by resistance
issues around much of Southern New Jersey the past decade. Growers with a known history of
mefenoxam-insensitivity on their farm should use Presidio, Previcur Flex, or Ranman plus a Phosphite
fungicide in rotation in their drip application programs. Importantly, if mefenoxam has not been
used in particular fields on any crop for a number of years (more than 5+) the fungus may revert back
to being mefenoxam-sensitive and control with these products may return. Mefenoxam, metalaxyl,
Previcur Flex, and the phosphites are the most systemic of the group and should readily be taken
up the by plant via drip application. Presidio has locally systemic and has trans laminar activity and
should offer some protection of the root system via drip. Ranman has protectant activity and thus will
offer some root protection where it comes into contact with roots in the soil. Orondis Gold (oxathiapiprolin mefenoxam, 49 +4) is the newest fungicide available with a new active ingredient in a new FRAC group. Additionally, in past research trials, mefenoxam, Orondis Gold, Presidio, Previcur Flex,
Ranman, Revus and the phosphites in rotation and/or tank mixes have offered very good control of
the fruit rot phase of phytophthora blight.
Fungicide control recommendations
mefenoxam –1.0 pt Ridomil Gold 4SL/A or 1.0 qt Ultra Flourish 2E/A or metalaxyl (MetaStar) – 4.0-8.0
pt 2E/A at transplanting via drip and 30 days later.
Orondis Gold (oxathiapiprolin + mefenoxam, 49 +4) at 4.8 to 9.6 fl oz/A 1.67S at transplanting and 30
days after. If applied as drip application it can not be applied as a foliar.
Presidio (fluopicolide, 43) at 3.0-4.0 fl. oz 4SC/A at transplanting via drip and in rotation.
Ranman (cyazofamid, 21) at 2.75 fl oz 400SC at transplanting via drip and in rotation. (Ranman can
be added to transplant water, see label for specific crop uses)
Previcur Flex (propamocarb HCL, 28) at 1.2 pt/A 6F at transplanting via drip or directed spray at base
of plant. (Previcur Flex can be added to transplant water, see label for specific crop uses).
Use in rotation.
Phosphite materials (FRAC code 33) such as Rampart, ProPhyt, or K-Khite may also be tank mixed
with one of the above to help suppress Phytophthora blight.
If mefenoxam-insensitivity is present, only use Presidio, Previcur Flex, Ranman, Revus, and/or
phosphite fungicides.
For more information on these fungicides and specific crop use please see the 2024/2025Mid-Atlantic
Commercial Vegetable Production Recommendations Guide.
References:
Kousik, C.S., Ikerd, J.L., Wechter, W.P., Branham, S., Turechek, W. 2022. Broad resistance to
post-harvest fruit rot in USVL watermelon germ plasm lines to isolates of Phytophthora capsici
across the United States. Plant Dis. 106:711-719.
LaPlant, K.E., Vogel, G., Reeves, E., Smart, C.D., and Mazourek, M. 2020. Performance
and resistance to phytophthora crown and rot in squash lines. Hort Technology 30:608-618.
Perla, D.E., Hayden, Z.D., and Hausbeck, M. 2023. Commercial hard squash cultivars exhibit
differences in susceptibility to Phytophthora crown rot. Plant Health Progress: First Look (online) 4
May 2023
Sanogo et al., 2023. Phytophthora capsici, 100 years later: research mile markers from 1922to 2022.
Phytopathology: First Look (online)