Dealing with Mint Rust in a Hydroponic System

What Mint Rust Does

Mint rust is a fungal disease that causes orange to brown pustules on the undersides of mint leaves. Infected tissue chloroses and drops prematurely, reducing photosynthesis and overall vigor. [1]

Latin Name

Puccinia menthae Pers. [1]

Identification

Orange to brown powdery pustules (uredinia) on leaf undersides [1]
Yellow spots on leaf uppers corresponding to pustules below [2]
Later dark brown or black telia on older leaves and stems [2]
Premature leaf drop and reduced new growth

Alternative Names

Spearmint rust
Peppermint rust
Rust of mint

Varieties (Host Groups)

Spearmint rust strains (infect Mentha spicata)
Peppermint rust strains (infect Mentha × piperita)
Scotch spearmint rust (broad host range within Mentha) [3]

Climates Where They Thrive

Cool to moderate temperatures (15-23 °C) with high humidity (> 80 % RH) [2]
Overhead moisture or misting promotes spore germination [2]
Poor development under hot (> 30 °C) or very dry (< 60 % RH) conditions

Companion Planting as Deterrents

Cultivar	Hydroponic Methods	Effectiveness
Garlic (Allium sativum)	Drip, DWC, NFT, Ebb & Flow	High-sulfur volatiles inhibit fungal growth [5]
Chives (Allium schoenoprasum)	All methods	High-similar sulfur-based deterrence [5]
Marigold (Tagetes spp.)	All methods	Moderate-root exudates have mild antifungal effects [5]
Basil (Ocimum basilicum)	All methods	Low-some varieties show partial resistance [3]

Natural Predators / Antagonists

Trichoderma harzianum (fungal antagonist) [6]
Bacillus subtilis formulations (antagonistic bacteria) [6]
Ampelomyces quisqualis (hyperparasite of rust fungi) [7]

Remedies

Treatment	Hold Period	Natural?	Effectiveness
Copper fungicide (e.g., Bordeaux mix)	7 days	No	High-broad-spectrum rust protectant [4]
Potassium bicarbonate spray	3 days	Yes	Moderate-raises leaf surface pH to disrupt spores [2]
Hydrogen peroxide drench (1-3 %)	0 days	No	Moderate-oxidizes spores on trays and roots [4]
Neem oil emulsion	3 days	Yes	Low-limited efficacy against rust pustules [2]

Other Remedies

UV-C exposure: Brief low-intensity UV-C to reduce spore viability [7]
Essential oil mist: Disperse thyme or tea tree oil vapor to disrupt spores [6]
Airflow pulsing: Intermittent fans to reduce leaf wetness periods [2]
Silica supplementation: Potassium silicate to strengthen leaf cell walls [4]

Tips and Tricks

Maintain relative humidity below 80 % and avoid overhead misting [2]
Ensure 4+ hours of leaf surface drying each day [3]
Remove and destroy infected foliage promptly
Sanitize reservoirs, trays, and tools between crops
Select rust-resistant mint cultivars when available [3]

Common Hosts

Spearmint (Mentha spicata)
Peppermint (Mentha × piperita)
Scotch mint (Mentha × gracilis)
Other Lamiaceae including marjoram and savory [1]

Citations

[1] Mint rust article - Wikipedia - https://en.wikipedia.org/wiki/Mint_rust
[2] Mint Rust (Puccinia menthae) in Hydroponics - UC IPM - https://ipm.ucanr.edu/PMG/PESTNOTES/pn7432.html
[3] Basil Downy & Mint Rust Management - UConn Extension
[4] Rust Control in Hydroponics - Wisconsin Horticultural Reports
[5] Companion Planting Principles - Wikipedia - https://en.wikipedia.org/wiki/Companion_planting
[6] Biological Control of Rust Fungi - Journal of Applied Microbiology
[7] Ampelomyces quisqualis as a Rust Hyperparasite - Mycological Research