Guide to Fungal Diseases in Cacti: How to Prevent and Treat Them

Fungal diseases (caused by fungi) are one of the most common reasons for losses in cactus collections and nurseries. They typically occur when **high humidity**, **poor air exchange**, **too organic substrates**, and/or **wounds** in the tissue are combined.

In this guide, you will learn to recognize typical symptoms, the most common causes, and a practical protocol for prevention and treatment to minimize relapses.

1) How to recognize a fungal problem

Although it is not always easy to distinguish fungi from bacteria or pests, there are quite typical signs:

• →**Brown/black circular spots** that grow slowly.
• →**Sunken areas** (necrosis) and corkiness ( “corked” tissue).
• →**Soft rot** at the base (neck) or in roots, with a bad smell in advanced stages.
• →**Gray mold** on wounds or soft areas (especially in cold and humid environments).
• →**Growth cessation** and general discoloration when the damage is internal.

Important: a cactus can “heal” (cork) an old injury. What is concerning is if the spot **advances**, soft tissue appears, or there are **exudates**.

2) Most frequent causes

In cacti, fungi tend to take advantage of stress conditions:

• →**Excessive watering** or frequent watering without complete drying.
• →**Poorly draining substrate** (too much peat, fine coconut fiber, or compact soil).
• →**Pots without good aeration** or saucers with water.
• →**Poor ventilation** in greenhouses or indoors (stagnant air).
• →**Low temperatures** with high humidity (very typical in autumn/winter).
• →**Wounds** from transplanting, hail, rubbing, insects, or cutting cuttings.
• →**Non-disinfected tools** that transmit pathogens.

3) Common fungi (and what they usually cause)

Without entering into laboratory diagnosis, these are useful categories:

• →**Root/neck rot fungi**: they usually attack when there is excess moisture. The cactus softens from the base and may collapse.
• →**Vascular fungi**: cause internal discolorations and progressive deterioration. Sometimes a brown ring is seen when cut.
• →**Opportunistic molds on wounds**: appear on recent cuts or damaged areas if they do not dry well.

If the damage progresses quickly, there is a bad smell, or the tissue liquefies, act as soon as possible.

4) Prevention protocol (what works best)

Prevention is usually 80% of success:

Substrate and pot - Use a **highly draining substrate**: high mineral percentage (pumice, volcanic gravel, perlite, coarse sand). - Avoid very organic “indoor” mixes. - Ensure **drainage holes** and do not leave standing water.

Watering - Water only when the substrate is **completely dry**. - In cold weather, significantly reduce watering (or suspend it depending on species and temperatures). - Better to water deeply and spaced out than frequent “sips.”

Ventilation and light - Ventilate well: indoors, avoid corners without air movement. - Sufficient light: weakened cacti get sick more easily.

Hygiene - Disinfect tools (isopropyl alcohol) before and after each plant. - Isolate new plants for 2–3 weeks to observe symptoms.

5) What to do if symptoms are already present

Step 1: Isolate and evaluate Separate the plant to avoid contagion. Assess if the lesion is: - **Superficial and dry** (better prognosis). - **Active, soft, or spreading** (intervention is needed).

Step 2: Stop watering If you suspect fungi, **stop watering immediately**. Keeping the substrate moist usually accelerates the problem.

Step 3: Check roots and neck (if applicable) If the problem seems to come from the base:

• →Remove the plant from the pot.
• →Eliminate all old substrate.
• →Check roots: if there are **black/soft** parts, cut them back to healthy tissue.

Let it air dry (bright shade, good ventilation) for 24–72 hours before replanting.

Step 4: Sanitize the affected tissue If the lesion is on the body of the cactus and is advancing:

• →With a clean tool, cut/scrape until you reach firm tissue.
• →Disinfect the tool between cuts.
• →Let the area **dry** and form a protective layer.

Step 5: Treatment (common options) The exact treatment depends on the country and the product available. Still, the general approach is:

• →**Contact fungicide** (preventive): usually used on superficial lesions or as support after sanitization.
• →**Systemic fungicide** (curative): reserved for more serious or recurring cases.

Always follow the product label (dosage, frequency, compatibilities) and avoid treating in full sun or with extreme temperatures.

Step 6: Replanting and quarantine - Replant in **new and sterile substrate**, with a higher mineral proportion. - Do not water for several days (or even 1–2 weeks) to allow for healing. - Keep the plant in **quarantine** and observe if the spot stabilizes or continues to grow.

6) Typical case: rot at the base (saving by cutting)

If the cactus is soft from the bottom, often the most effective approach is:

• →Cut above the affected area until you see clean tissue (without discoloration).
• →Let the cutting **heal** in a dry and ventilated place.
• →Root in a highly mineral substrate, with minimal watering at the start.

7) Quick checklist (to avoid falling back)

• →More mineral and draining substrate
• →Water only when it is 100% dry
• →Real ventilation (moving air)
• →Disinfected tools
• →New plants in quarantine

With these changes, most fungal problems in cacti are drastically reduced, and when they appear, they are controlled much earlier before they advance.