Downy mildew

Symptoms

Yellow or angular (vein-bounded) spots on the upper leaf surface, matched underneath by a grey-violet or brown coating of spores. It differs from powdery mildew, whose white coating sits on the upper surface. It develops in humidity and cool nights and is dangerous to cucumbers, onions and grapevines. The spots are initially light green or yellowing, and their angular, mosaic-like shape arises because the pathogen's spread is halted by the larger leaf veins. The delicate, felt-like coating on the underside (hence the name 'mildew') is the sporulation of the oomycete — best seen in the morning or after a period of high air humidity. Over time the spots turn brown, dry out and merge, the leaf dies prematurely, which weakens the whole plant and lowers the yield. The disease is highly host-specialised: cucumbers and squashes are attacked by Pseudoperonospora cubensis, onion by Peronospora destructor, grapevine by Plasmopara viticola, and lettuce by Bremia lactucae — so infection of one species usually does not spread to unrelated crops.

Treatment

Remove infected leaves, improve air movement and reduce humidity, and if needed apply fungicides (different from those used against powdery mildew). Acting quickly is important. A key point: sulphur and triazole fungicides that are effective against powdery mildew do NOT work against downy mildew, because it is caused by an oomycete (a fungus-like organism), not a fungus — misapplying these products is a wasted treatment. Downy mildew is controlled by products specific to oomycetes: contact, multi-site copper products (protectant, applied preventively) and systemic fungicides — phenylamides (metalaxyl/mefenoxam), phosphonates (fosetyl-aluminium, phosphites) and carboxylic acid amides CAA (e.g. mandipropamid). Treatments have the greatest protective value — ideally before infection or at the first symptoms, at intervals adjusted to the weather. Because oomycete pathogens quickly become resistant, especially to phenylamides and strobilurins, rotating groups with different modes of action and alternating with a contact product is mandatory.

Prevention

Airy planting spacing, watering at the base in the morning, avoiding wetting the leaves in the evening, resistant varieties, and crop rotation. Prevention matters more here than cure, because spore germination requires water on the leaf — the shorter the leaves stay wet, the lower the risk. It is therefore worth providing wide spacing and good air circulation (in tunnels and greenhouses — ventilation and limiting condensation), irrigating by drip or at the base in the morning rather than spraying the plants in the evening. Debris from infected plants must be removed and destroyed, because many pathogens overwinter as resting oospores in the soil and residues — hence the value of crop rotation and avoiding growing the same species after itself. It pays to watch the weather: cool nights with dew and long-lasting humidity are a signal for a protective treatment. The foundation remains the choice of resistant or tolerant cultivars, especially in cucumber, onion, lettuce and grapevine, where the disease can be most severe.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.