Dealing with Downy Mildew in a Hydroponic System

What Downy Mildew Does

Downy mildew pathogens infect leaves and stems, producing yellow or angular lesions on the upper surface and a corresponding grayish to white downy growth of spores on the undersides, leading to chlorosis, necrosis, and reduced photosynthesis. [1]

Latin Name

Various oomycetes in genera Peronospora, Plasmopara, Bremia (e.g. Peronospora belbahrii on basil). [1]

Identification

Angular yellow to pale green spots on upper leaf surface [2]
Downy gray-white to purple sporangia on leaf undersides in high humidity [2]
Lesions turn brown and necrotic as disease progresses
Stem lesions and wilting in severe cases [3]

Alternative Names

Oomycete leaf blight
Angular leaf spot (in some hosts)
Water-mold disease

Varieties (Pathogen Species)

Peronospora belbahrii (basil downy mildew) [3]
Bremia lactucae (lettuce downy mildew) [2]
Plasmopara viticola (grape downy mildew) [4]

Climates Where They Thrive

Cool to moderate temperatures (15-23 °C) with high leaf wetness (> 85 % RH) [2]
Sporulation favored by nighttime humidity > 95 % [3]
Poor development under hot (> 30 °C) or very dry (< 60 % RH) conditions

Companion Planting as Deterrents

Cultivar	Hydroponic Methods	Effectiveness
Chrysanthemum (Tanacetum cinerariifolium)	All (DWC, NFT, Ebb & Flow, Drip)	Moderate - pyrethrins may inhibit spore germination [5]
Garlic (Allium sativum)	Drip, DWC, NFT	Moderate - sulfur volatiles show antifungal properties [5]
Marigold (Tagetes spp.)	All methods	Low - root exudates may reduce inoculum [5]
Basil (Ocimum basilicum)	All	Varies - some resistant cultivars available [3]

Natural Predators / Antagonists

Ampelomyces quisqualis (hyperparasite of downy mildew) [6]
Beneficial bacteria (e.g., Bacillus subtilis) that suppress oomycetes [4]
Trichoderma spp. - fungal antagonists in media-based systems [4]

Remedies

Treatment	Hold Period	Natural?	Effectiveness
Copper fungicides (e.g., Bordeaux mix)	7 days	No	High - broad-spectrum protectant [4]
Potassium bicarbonate spray	3 days	Yes	Moderate - raises leaf surface pH [2]
Hydrogen peroxide drench (1-3 %)	0 days	No	Moderate - oxidizes sporangia on roots and trays [4]
Phosphorous acid (fosetyl-Al)	3 days	No	High - systemic oomycete control [4]

Other Remedies

UV-C exposure: Brief low-intensity UV-C to inhibit spore viability [7]
Essential oil mist: Tea tree or thyme oils to disrupt pathogen membranes [2]
Airflow pulsing: Intermittent fans to reduce leaf wetness periods [2]
Silica supplementation: Dissolved potassium silicate to strengthen cell walls [4]

Tips and Tricks

Maintain RH below 85 % and avoid overhead misting [2]
Ensure 4+ hours of leaf surface drying each day [3]
Remove and destroy infected foliage immediately
Sanitize reservoirs, trays, and tools between crops
Use resistant cultivars when available (e.g., basil 'Devotion') [3]

Common Hosts

Basil, Lettuce, Spinach [2]
Cucumbers, Grapes, Pumpkins [4]
Impatiens, Coleus, Other ornamentals [1]

Citations

[1] Downy mildew - Wikipedia - https://en.wikipedia.org/wiki/Downy_mildew
[2] Downy Mildew in Lettuce - UC IPM - https://ipm.ucanr.edu/PMG/PESTNOTES/pn7416.html
[3] Downy Mildew on Basil - UConn Extension - https://s.uconn.edu/basildownymildew
[4] Downy Mildew Control - Wisconsin Horticulture
[5] Companion planting - Wikipedia - https://en.wikipedia.org/wiki/Companion_planting
[6] Ampelomyces quisqualis - Wikipedia - https://en.wikipedia.org/wiki/Ampelomyces_quisqualis
[7] UV-C Disinfection of Plant Pathogens - Journal of Plant Pathology