Root rot is the leading cause of death in cultivated aloes. It is not a single disease but a collective term for infections caused by several different soil-borne organisms — primarily the oomycetes (water moulds) Pythium and Phytophthora, and the true fungi Fusarium and Rhizoctonia — that attack and destroy the root system of a plant growing in waterlogged or poorly drained substrate. Because these pathogens differ in their biology and their response to fungicides, understanding which group is responsible matters for treatment. This article examines the organisms behind aloe root rot, explains how to diagnose the condition, provides a step-by-step treatment protocol, and describes the preventive measures that keep healthy roots healthy.
The pathogens: not all root rot is the same
This is the point that most online guides miss. “Root rot” is treated as if it were a single problem with a single solution. In reality, four groups of organisms cause root rot in aloes, and they respond to different treatments.
Oomycetes (water moulds) — Pythium and Phytophthora
Pythium species and Phytophthora species are not true fungi. They belong to the oomycetes, a group of organisms more closely related to brown algae than to fungi. This distinction is not academic — it means that many conventional fungicides designed to kill true fungi (e.g., thiophanate-methyl, propiconazole) have no effect on oomycetes. Treating a Pythium root rot with a standard fungicide will fail.
Oomycetes require free water to reproduce: they produce motile zoospores — microscopic swimming spores with a tail (flagellum) — that move through water-saturated soil toward root tips. This is why overwatering is so directly linked to oomycete root rot: without saturated soil, the zoospores cannot swim to new roots.
Pythium preferentially attacks juvenile root tissue — fine root tips and newly formed roots. It causes a rapid brown-to-black soft rot. The outer cortex of the root peels off easily when touched, leaving the central vascular strand intact (a characteristic diagnostic sign). Pythium is the most common cause of root rot in container-grown succulents.
Phytophthora attacks roots but also invades the crown (stem base) and larger structural roots. The rot spreads upward faster than Pythium, and crown collapse is more common. Phytophthora produces oospores — thick-walled survival structures that can persist in soil for years to decades, even in dry conditions.
True fungi — Fusarium and Rhizoctonia
Fusarium species cause a dry, firm rot that is typically slower-progressing than oomycete rot. Roots may turn brown but remain relatively firm rather than collapsing into mush. Fusarium also causes vascular wilt — it can invade the water-conducting tissues of the stem, blocking water transport and causing the plant to wilt even if the roots still look partially healthy. A longitudinal cut through the stem base may reveal brown discolouration in the vascular ring — a diagnostic clue for Fusarium.
Rhizoctonia solani causes reddish-brown, sunken, dry lesions on roots and at the stem base. It is less associated with waterlogging than the oomycetes and can attack in moderately moist conditions. It is a particular risk during propagation, where cuttings are vulnerable at the soil line.
Why the distinction matters
If you apply a fungicide containing mefenoxam or metalaxyl (active against oomycetes) to a Fusarium infection, it will have no effect. Conversely, if you apply thiophanate-methyl (active against true fungi) to a Pythium infection, it will equally fail. Correctly identifying the pathogen group — or at least narrowing it down — determines whether treatment has any chance of success.
How to diagnose root rot in aloes
Above-ground symptoms
The above-ground symptoms of root rot are shared by all four pathogen groups and are not specific enough for pathogen identification. They are, however, the signals that prompt investigation.
Lower (oldest) leaves turn yellow, then brown, and become soft or translucent. The leaf bases may feel mushy where they contact the stem. The plant leans or topples as the root system weakens. Growth stops entirely. In advanced cases, the entire rosette collapses and the stem base becomes dark brown or black and hollow.
A sour or foul odour from the soil is a strong indicator of anaerobic decomposition — the signature of waterlogged, rotting roots.
Below-ground diagnosis
The definitive diagnosis requires unpotting the plant and examining the roots directly. Gently remove the plant from its pot and shake off as much substrate as possible. Rinse the roots under lukewarm water to expose them clearly.
Healthy roots: white to pale tan, firm, springy. The outer layer is intact and does not peel off. No smell.
Oomycete rot (Pythium/Phytophthora): roots are brown to black, soft, mushy, and waterlogged. The outer cortex (the outer tissue layer) separates easily from the inner core — you can slide it off like a sleeve. Strong foul smell. The rot is wet and slimy. In Phytophthora cases, the rot extends upward into the stem base.
Fusarium rot: roots are brown but relatively dry and firm. The rot progresses more slowly. A longitudinal cut through the stem base may reveal a brown ring or streaks in the vascular tissue (the water-conducting tubes). No slimy texture.
Rhizoctonia rot: reddish-brown, dry, sunken lesions on roots or at the stem–soil junction. The affected tissue is firm and dry, not mushy.
Crown rot vs root rot
In rosette-forming plants like aloes, a critical variant of root rot is crown rot — where the pathogen invades the stem base (the compact, shortened stem from which the leaves radiate) from below or from water trapped between the leaf bases. Crown rot can be caused by any of the four pathogen groups but is especially associated with Phytophthora. It is often triggered by watering the centre of the rosette rather than the soil, trapping moisture in the leaf axils.
Crown rot is more dangerous than root rot because the stem is the irreplaceable core of the plant. Roots can regrow; a destroyed stem cannot. If the stem base is soft, dark and hollow, the plant is usually beyond rescue.
Treatment protocol
Triage: can this plant be saved?
Be honest about the extent of the damage before investing time and materials.
More than 50 % of roots still firm and white, stem base firm: excellent chance of recovery. Proceed with surgical treatment.
Fewer than 50 % of roots healthy, but stem base still firm: moderate chance. Proceed with surgery but prepare a cutting as backup.
Stem base soft, dark, hollow: the plant in its current form is lost. Attempt to salvage healthy leaf tissue or offsets for propagation.
Step 1: surgery
Using sterilised tools (dip in 70 % isopropyl alcohol between each cut), remove all brown, soft, slimy or discoloured roots. Cut back to firm, white tissue with generous margins. Remove any leaves that are soft, translucent or detaching from the base.
If the stem base shows brown discolouration, slice upward in thin cross-sections until you find clean, firm, green-white tissue with no brown streaks in the vascular ring.
Step 2: disinfection
After surgery, the cut surfaces must be treated to suppress the pathogens that are certainly still present on the tissue surface.
Hydrogen peroxide (3 %, undiluted): soak the trimmed root system and stem base for 5 to 10 minutes. Hydrogen peroxide is an oxidising agent that kills oomycetes and fungi on contact. It breaks down rapidly into water and oxygen and leaves no toxic residue.
Sulphur powder or cinnamon: dust all cut surfaces generously. Both have broad-spectrum antimicrobial properties and help the wounds callous cleanly.
Step 3: drying
Allow the plant to air-dry for 24 to 72 hours in a warm (20–25 °C), shaded, well-ventilated location. The cut surfaces must form a dry callous — a hard, sealing layer — before replanting. This callous is the plant’s first line of defence against reinfection.
For severely pruned plants (most roots removed), extend the drying period to three to five days. The thicker the remaining stem, the longer the callous needs to form.
Step 4: repotting
Repot in completely fresh substrate. Never reuse old substrate from a rotted plant — it is saturated with pathogen spores (oospores, chlamydospores) that can survive for years.
Use a highly mineral substrate: 60 to 70 % coarse inorganic material (pumice, perlite, coarse sand, fine gravel) and 30 to 40 % fresh organic component. The goal is maximum aeration and rapid drying.
Use a clean pot with drainage holes. Terracotta is strongly preferred over plastic — its porous walls allow lateral evaporation, accelerating the drying cycle.
Plant at the same depth as before or slightly higher. Deep planting, where substrate covers the stem base, promotes Phytophthora crown infection.
Step 5: post-treatment care
Do not water for 7 to 14 days after repotting. The roots (or the calloused stem if roots were entirely removed) need time to begin new growth without encountering saturated soil.
After the dry period, resume watering very conservatively — small volumes, allowing complete drying between waterings. Place in bright indirect light (not full sun, which stresses a recovering plant). Normal light exposure can resume once new root growth is evident (typically four to six weeks), signalled by the resumption of leaf turgidity and the emergence of new leaves from the centre of the rosette.
Optional: fungicide drench
For high-value plants, a fungicide drench at the time of repotting can provide insurance against residual pathogen inoculum.
For suspected oomycete rot (Pythium/Phytophthora): mefenoxam (trade name Subdue MAXX), metalaxyl (Ridomil), or fosetyl-aluminium (Aliette). Phosphonate products (potassium phosphonate, marketed as Agri-Fos or Fosphite) move both upward and downward in the vascular system and have documented efficacy against Phytophthora.
For suspected true fungal rot (Fusarium/Rhizoctonia): thiophanate-methyl (trade name Cleary’s, Topsin-M) or propiconazole.
If the pathogen is unknown (the most common situation for home growers): a broad-spectrum biological product containing Trichoderma harzianum (e.g., RootShield, Trianum) provides suppression of both oomycetes and true fungi without chemical toxicity. Research has documented up to 88 % disease control efficacy for Trichoderma-based products in some systems.
Important: drying alone does not eliminate the infection. Oomycete oospores and Fusarium chlamydospores survive drought and reactivate when moisture returns. A fungicide drench or biological amendment provides an additional layer of protection during the vulnerable recovery period.
Prevention
Substrate and drainage
The single most effective preventive measure is a well-draining, highly aerated substrate. Standard potting compost holds too much moisture for aloes. A minimum of 50 % coarse mineral material (pumice, perlite, coarse sand) is essential. Composted pine bark, when used at 20 % of the mix, has documented suppressive effects against both Pythium and Phytophthora (UC IPM, Cornell University).
Watering discipline
Water only when the substrate is completely dry to a depth of at least 5 cm (2 in). Water the soil, never the rosette. Allow excess water to drain completely; never let the pot sit in a saucer of water.
Container selection
Terracotta pots with drainage holes. The porous material wicks moisture laterally, accelerating drying. Avoid plastic pots for root-rot-prone species. Choose a pot only slightly larger than the root ball — oversized pots hold excess moisture around the roots.
Quarantine and hygiene
Inspect new acquisitions for root health before adding them to a collection. Sterilise tools between plants. Never reuse substrate from a plant that has died of root rot.
Environmental management
Good air circulation around the base of the plant reduces humidity at soil level. Avoid placing aloes in enclosed, humid, poorly ventilated spaces. If growing indoors, a small fan providing gentle air movement can significantly reduce the conditions that favour root rot pathogens.
References
Erwin, D.C. & Ribeiro, O.K. (1996). Phytophthora Diseases Worldwide. APS Press, St. Paul, Minnesota.
Martin, F.N. & Loper, J.E. (1999). Soilborne plant diseases caused by Pythium spp.: ecology, epidemiology, and prospects for biological control. Critical Reviews in Plant Sciences 18(2): 111–181.
Moorman, G.W. (2016). Root and Crown Rots of Floriculture Crops. Penn State Extension.
UC IPM (2023). Pythium Root Rot / Floriculture and Ornamental Nurseries. University of California Agriculture and Natural Resources.
Cornell University (2023). Root Rot Diseases. Greenhouse Horticulture Fact Sheet.
Wisconsin Horticulture (2024). Root and Crown Rots. University of Wisconsin–Madison, Division of Extension.