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Downy Mildew

Downy Mildew

Pseudoperonospora cubensis and related water molds

Water moldalso: Downy mildew

A fast-moving water mold disease that blankets the undersides of leaves with gray or purple fuzz while the tops show angular yellow blotches. It hits cucurbits like cucumber, melon, and squash hard, and other forms strike basil, lettuce, spinach, and grapes. In cool, wet, humid weather it can brown out a planting in days.

🔎 How to spot it

Look for pale green to yellow spots on the upper leaf surface that are angular, boxed in by the leaf veins, rather than round. In humid conditions the undersides directly below the spots carry a downy gray, purple, or brown fuzz. Spots run together, and whole leaves turn brown and die as if frosted, while stems and fruit usually stay symptom free.

🥀 Damage it causes

Downy mildew kills leaves quickly, and the loss of foliage starves the plant, reduces and delays the harvest, and leaves fruit exposed to sunscald. On fast cycles it can defoliate a cucumber or basil planting within a week or two of the first spots.

🔬 What causes it

Downy mildews are caused by water molds, most famously Pseudoperonospora cubensis on cucurbits, that need cool, wet, humid conditions and leaf moisture to infect, doing best around 59 to 68 F. The cucurbit form cannot survive cold winters or live on dead debris in the north; instead spores blow in on air currents from the south each year. It spreads fast on wind and splashing water.

🛡️ Prevent it

Plant resistant varieties, which are the single best defense for cucumber and basil. Space plants widely, trellis vines, and water at the base so leaves dry fast. Grow in full sun with good air movement. Watch regional downy mildew forecasts so you can scout closely and act the moment it is reported in your area.

🧯 If it is already here

Remove and destroy symptomatic leaves and plants promptly. Once it arrives, approved organic fungicides such as copper, applied preventively to protect healthy foliage, can slow it, but they will not cure infected tissue; follow the label and rotate products. Because the disease moves so fast, resistant varieties and sanitation matter more than spraying.

💡 Good to know

Downy mildew is often confused with powdery mildew, but the two are opposites in key ways: downy makes angular spots bounded by veins with fuzzy growth only on the leaf underside and favors cool wet weather, while powdery makes round white patches on both surfaces and tolerates dry, warm conditions. Each downy mildew is specialized to its host group.

🌱 Plants it affects

339 plants in the library can be affected by this problem

Acorn SquashAfrican Blue BasilAfrican MarigoldAgapanthusAgeratumAlice du Pont MandevillaAmethyst Falls WisteriaAnemoneAngelique TulipAngeloniaAnnabelle Smooth HydrangeaAnnual VincaApeldoorn TulipApril Tryst CamelliaArizona Sun Blanket FlowerArmenian CucumberArugulaAtlantic Giant PumpkinAugust Beauty GardeniaAutumn Joy SedumBachelor's ButtonBarbara Karst BougainvilleaBecky Shasta DaisyBee BalmBenarys Giant ZinniaBengal Tiger Canna🥬Bibb LettuceBitter MelonBlack Beauty Zucchini🍉Black Diamond Watermelon🥬Black Seeded Simpson Lettuce🥕Black Spanish RadishBlack-Eyed PeaBlack-eyed Susan VineBlood OrangeBloomsdale SpinachBlue Bird DelphiniumBlue Bird Rose of SharonBlue FescueBok Choy🥒Boston Pickling CucumberBroccoli RabeBrunneraBrussels SproutsBurning BushButtercrunch LettuceButtercup SquashButterfly Blue Pincushion FlowerButterfly Marguerite DaisyButterfly WeedButternut SquashCafe au Lait DahliaCalabrese BroccoliCaladiumCalendulaCalibrachoaCalifornia Giant ZinniaCalifornia PoppyCampanulaCantaloupeCardinal FlowerCarolina GeraniumCarolina JessamineCasaba MelonCatawba Grape🥬Champion CollardsCharleston Gray WatermelonCheddar CauliflowerCherry Belle RadishChicoryChinese BroccoliCinderella PumpkinCinnamon BasilCipollini OnionClimbing HydrangeaClimbing Prairie RoseCocktail Vodka BegoniaColeusCollard GreensCommon ChivesConcord GrapeCoral Drift Groundcover RoseCosmosCostata Romanesco ZucchiniCreeping PhloxCrenshaw MelonCrimson Sweet WatermelonCupani Sweet PeaDaikon RadishDandelionDavid Garden PhloxDe Cicco BroccoliDelft Blue HyacinthDelicata SquashDenim n Lace Russian Sage🥒Diva CucumberDusty MillerDutch Master DaffodilDwarf Blue Curled Vates Kale🥬Dwarf Siberian Kale

For educational and informational purposes only. Disease management advice is general guidance drawn from university cooperative extension sources; always identify a problem positively and read and follow the label on any product before use, especially around food crops, children, and pets.