Cucurbit downy mildew is a water mold or oomycete disease of cucurbit crops caused by Pseudoperonospora cubensis. Symptoms first appear on the upper leaf surface as angular, vein-bounded, yellow to pale-green spots, turning brown and coalescing to turn entire leaves brown with disease progression. In very humid conditions, the underside of leaves may appear fuzzy as the pathogen produces numerous spores which enable the pathogen to spread. This foliar disease can very rapidly destroy above ground plant parts reducing potential for yield and quality, and making fruit more susceptible to sunscald and secondary pathogen infection.
Primary Source: Living cucurbit plant tissue
Spread: Windborne spores, rain and irrigation splash, human spread on equipment and hands
Favorable Conditions: Very wet, humid conditions, moderate temperatures (59-68° F)
Infection & Disease Cycle
Pseudoperonospora cubensis does not overwinter on plant debris in Wisconsin, and can only survive on living plant tissue. No soilborne, long-term survival structures of the pathogen have been identified in our growing region. For this reason, the pathogen generally overwinters in warmer climates and in protected greenhouses. Spores spread northward on airborne spore-like structures called “sporangia”. The pathogen infects cucurbit leaves, producing lesions that create more spores when leaf wetness and humidity are high. These spores spread to nearby plants via water splash and human spread, and can travel longer distances via wind currents. The pathogen does not directly infect cucurbit fruits. Currently, two types of the cucurbit downy mildew pathogen are known. One type will infect cucumber and melon (“Clade 2”) and seems to be much more aggressive on these select cucurbit types. Clade 2 also has resistance to some currently used fungicides. The second type of downy mildew pathogen will infect pumpkin, watermelon, winter squash, bittermelon, and balsam apple (“Clade 1”). Clade 1 seems to arise a bit later in the production season than Clade 2.
Scouting regularly allows early identification of disease before significant spread and damage. The following practices can also help prevent disease development:
- Plant resistant varieties when possible
- Avoid overhead irrigation
- Maintain proper spacing between plants
- Plant in areas with good airflow
Keep track of locations of known cucurbit downy mildew infection, and the cucurbit types infected, to best understand your risk and prescriptively manage this disease. For many years this disease was tracked and field reports were used to generate a disease forecast: https://cdm.ipmpipe.org/forecasting/. While this service is currently suspended, the website offer useful resources for management. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.
- Cucurbit Downy Mildew Forecast from ipmPIPE. View latest reports of CDM from across the country, report new infestations, and sign up for text or email alerts.
- Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
- UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.
Written by Ariana Abbrescia, Amanda Gevens, Russell Groves, and Ben Bradford. Last updated Nov 2023