Root Rot in Hydroponic & Soil Grows: Causes, Signs & Fixes

Root rot is a fungal infection, most often caused by Pythium or Phytophthora species, that turns healthy white roots brown, mushy, and foul-smelling, usually triggered by overwatering, poor oxygenation, or warm, stagnant water. It shows up as wilting despite wet medium, slowed growth, and yellowing leaves. Treating it means removing affected roots, improving oxygen and drainage, and correcting the conditions that let the pathogen establish in the first place.

What Causes Root Rot

Root rot pathogens are present in practically all growing environments, but they only become a problem under specific conditions.

• Low oxygen at the root zone: waterlogged soil or a poorly aerated hydroponic reservoir starves roots of oxygen, weakening them and making infection far more likely.
• Warm, stagnant water: in hydroponic reservoirs, water above roughly 24°C (75°F) with little movement creates ideal conditions for pathogen growth.
• Overwatering in soil or coco: according to the University of California's Statewide IPM Program, drainage in container growing becomes poor as potting media ages, settles, and loses aeration, and root rot is commonly associated with poor drainage and overwatering in these conditions. See UC IPM's guidance on houseplant problems and root rot for the full picture.
• Pythium specifically: UC IPM notes that Pythium species attack juvenile tissues such as root tips and newly germinated seedlings, and once established can cause a rapid, brown to black rot of the entire primary root, sometimes moving up into stem tissue. Full detail is available in UC IPM's Pythium Root Rot guidelines.
• Phytophthora specifically: a related pathogen that behaves similarly, favoured by the same wet, poorly drained conditions. UC IPM's Phytophthora Root and Crown Rot guidance covers symptoms and management in more detail.

Root Rot in Hydroponics vs Soil

Factor	Hydroponic systems	Soil / coco
Main trigger	Warm, low-oxygen, stagnant reservoir water	Overwatering and compacted, poorly draining medium
Spread speed	Fast – roots share the same water, so infection can spread to the whole system quickly	Slower – often confined to overwatered plants or pots
Early warning sign	Reservoir water turns cloudy or develops a foul, sulphurous smell	Medium stays wet far longer than expected between waterings
Key prevention lever	Oxygenation (air stones), reservoir temperature control	Watering discipline, drainage, pot sizing

Signs and Symptoms Checklist

• Roots turn from firm and white to brown, black, slimy, or mushy.
• A foul, sulphurous, or rotten-egg smell coming from the reservoir or medium.
• Wilting or drooping leaves despite the medium or reservoir being adequately wet.
• Slowed or stalled growth without an obvious nutrient or pest cause.
• Yellowing lower leaves that progress upward as the root system fails.
• In hydroponics, cloudy or discoloured reservoir water, sometimes with a slimy biofilm on the roots or reservoir walls.

If you're not sure whether you're looking at root rot or a nutrient issue, our guide on nutrient deficiency vs pest damage covers other look-alike symptoms above ground, though root inspection is the most reliable way to confirm root rot specifically.

How to Treat Root Rot Once It Starts

1. Inspect the roots directly. In hydroponics, lift the net pot or plant and check root colour; in soil, gently remove the plant from its pot if wilting is severe enough to warrant it.
2. Trim away affected roots using clean, sterilised scissors, cutting back to healthy white tissue where possible.
3. Sterilise the system. In hydroponics, drain and clean the reservoir fully, then refill with fresh, pH-balanced water and nutrients. A dilute hydrogen peroxide flush is commonly used to knock back remaining pathogen load.
4. Increase oxygenation. Add or upgrade air stones and air pumps in hydroponic reservoirs; in soil, allow the medium to dry out more between waterings to restore oxygen to the root zone.
5. Lower reservoir temperature toward 18-22°C if it's been running warm, since cooler, oxygen-rich water is far less hospitable to Pythium and Phytophthora.
6. Reduce watering frequency in soil or coco and check that pots are draining freely rather than sitting in standing water.
7. Monitor closely for 1-2 weeks after treatment, since root rot can re-establish if the underlying oxygen or temperature problem isn't fully corrected.

How to Prevent Root Rot Long-Term

• Keep hydroponic reservoirs oxygenated continuously with an air stone and pump sized to your reservoir volume. Our hydroponic systems range includes reservoirs and air pumps built for this.
• Control reservoir temperature, keeping it below roughly 24°C where possible, since warm water holds less dissolved oxygen and favours pathogen growth.
• Don't overwater in soil or coco. Let the top layer of medium dry before watering again, and make sure pots have adequate drainage holes.
• Use clean, sterile equipment between grows – reservoirs, net pots, and tubing can harbour pathogens from a previous infected grow if not properly cleaned.
• Choose a well-draining medium and avoid compacted, aged potting mix. Our grow mediums range includes options selected for good aeration and drainage.
• Maintain good air circulation around the base of plants and reservoirs, since stagnant air can contribute to the same warm, humid conditions that favour root pathogens above the waterline too. See our air circulation fans range.

Frequently Asked Questions

What does root rot look like?

Healthy roots are firm and white; root rot turns them brown, black, slimy, or mushy, often accompanied by a foul, sulphurous smell in hydroponic reservoirs.

What causes root rot in hydroponic systems?

Warm, stagnant, low-oxygen reservoir water is the main trigger, creating ideal conditions for Pythium and Phytophthora pathogens to infect roots.

Can overwatering cause root rot in soil?

Yes. According to UC IPM, root rot in container growing is commonly associated with poor drainage and overwatering, which starves roots of oxygen and allows pathogens to establish.

How do I treat root rot once I see it?

Trim away affected roots, sterilise the reservoir or improve drainage, increase oxygenation, and correct the underlying temperature or watering issue that caused it.

Can a plant recover from root rot?

Often, yes, if caught early and the healthy portion of the root system is still substantial. Severe, advanced infections that have destroyed most of the root mass are much harder to reverse.

How is root rot different from powdery mildew?

Root rot attacks the root system below the growing medium or waterline, while powdery mildew is a fungal disease that appears on leaf surfaces above ground. Both are fungal issues favoured by poor airflow, but they affect different parts of the plant and need different treatments.

How do I stop root rot from coming back?

Keep reservoirs oxygenated and cool, avoid overwatering in soil or coco, use clean equipment between grows, and maintain good air circulation around the root zone.

Spotted signs of root rot or want to prevent it before it starts? Browse our hydroponic systems range for reservoirs and air pumps built for proper oxygenation, add air circulation fans to keep airflow moving around your root zone, and check our pest control range for treatment products. Not sure if it's root rot or something else? WhatsApp us on 0718837026 with a photo of your roots and we'll help you diagnose it.