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 finally sets in. 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, ontogenic, 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 germplasm 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 et al., 2023). Recently, the USDA has developed and released five resistant germplasm 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 translaminar 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 2022/2023 Mid-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 germplasm 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 rot in squash lines. HortTechnology 30:608-618. <doi.org/10.21273/HORTTECH04636-20>
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 <doi.org/10.1094/PHP-01-23-0009-RS>
Sanogo et al., 2023. Phytophthora capsici, 100 years later: research mile markers from 1922 to 2022. Phytopathology: First Look (online) <doi.org/10.1094/PHYTO-08-22-0297-RVW>
