Plant fungus: 8 common types + how to identify each

Plant fungus: 8 common types + how to identify each

Plant fungus is a catch-all term for dozens of fungal diseases that infect leaves, stems, and roots. Not every fungus in the garden is a problem, though — many soil fungi are beneficial decomposers, and our guide to the types of mushrooms covers the harmless lawn and mulch fruiting bodies that worry gardeners but cause no damage. Eight pathogenic fungi account for ~90% of disease cases on houseplants and garden plants — and each has a distinctive visual signature you can spot once you know what to look for. This guide is the identification reference: 8 fungi, 8 visual fingerprints, and the treatment protocol for each, all sourced from university Extension and RHS research.

Try Growli: Snap a photo of the affected leaf in the Growli app and the AI matches the symptom to one of the 8 fungal patterns in 60 seconds — plus sends a treatment protocol tailored to your species.

The 8 fungal diseases at a glance

The treatment protocols share common elements: improve airflow, remove infected tissue, apply a fungicide. The specific fungicide and protocol vary by pathogen.

#1 — Powdery mildew

Causal organisms: Multiple genera (Erysiphe, Sphaerotheca, Podosphaera, Oidium).

Telltale signs: White or grey powdery coating on the upper leaf surface, starting as small circular spots and expanding to cover the whole leaf. Often mistaken for dust or talc. Affected leaves yellow, distort, and eventually fall. Smears slightly when wiped with a damp cloth and grows back within a week.

Conditions favouring it: Poor air circulation, low light, cooler indoor temperatures around 20-22°C (68-72°F), and high humidity. Unusually for a fungus, powdery mildew does NOT need wet leaves — it thrives in still humid air.

Plants commonly affected: Cucurbits (cucumber, squash, pumpkin), roses, phlox, monarda, peonies. Indoors: African violets, jade, kalanchoe, begonia, ivy, rosemary.

Treatment:

1. Isolate the affected plant.
2. Remove and bin (don't compost) the worst-affected leaves.
3. Improve airflow with a small oscillating fan running 4-8 hours per day.
4. Spray weekly for 3-4 weeks with:
   • Potassium bicarbonate (organic, most effective)
   • Sulfur (effective but skip in temperatures above 27°C / 80°F)
   • Bacillus amyloliquefaciens (biological, pet-safe)
   • Neem oil (early-stage only)

See powdery mildew for the full deep-dive.

#2 — Anthracnose

Causal organisms: Colletotrichum and Gloeosporium species.

Telltale signs (per Clemson HGIC): Dark sunken lesions on stems, leaves, and fruit. The centres of mature spots often produce pink, gelatinous masses of spores during moist warm weather. Look closely with a hand lens for tiny black hair-like structures (setae) arranged in circular groups — these are diagnostic for anthracnose.

Conditions favouring it: Warm wet weather, overhead watering, dense foliage with poor airflow.

Plants commonly affected: Tomato, pepper, bean, cucumber, strawberry. Indoor: ficus, dieffenbachia, dracaena, monstera, peace lily, philodendron.

Treatment:

1. Remove all infected leaves with sharp clean scissors. Dispose in a sealed bag — do not compost.
2. Improve airflow and stop overhead watering.
3. Apply copper soap (universal first-line), chlorothalonil, mancozeb, myclobutanil, or tebuconazole. Repeat every 7-10 days.
4. For severe cases on edibles, discard the worst-affected plants — anthracnose persists in soil and on debris for 1-2 years.

#3 — Leaf spot (Cercospora, Septoria, Alternaria)

Causal organisms: Cercospora, Septoria, Alternaria, and many others.

Telltale signs: Small circular brown spots, often with a yellowish margin or a concentric ring (target) pattern. Spots range from 2 mm to 1 cm. Some species produce visible black dots (the fruiting bodies, called pycnidia) within the brown area. Spots may merge as the disease progresses.

Conditions favouring it: Wet foliage, splash from overhead watering, contaminated tools or debris.

Plants commonly affected: Tomato (Septoria leaf spot is the classic), basil, beetroot, ornamental cherry. Indoor: dieffenbachia, dracaena, ficus, peace lily, philodendron.

Treatment:

1. Remove all affected leaves; dispose in a sealed bag.
2. Water at the soil line — never overhead.
3. Improve air circulation.
4. Apply copper soap (universal), chlorothalonil, mancozeb, or Bacillus amyloliquefaciens. Repeat every 7-10 days for 3-4 weeks.

See brown spots on plant leaves for the full leaf-spot vs bacterial-spot diagnostic flow.

#4 — Rust

Causal organisms: Many genera (Puccinia, Uromyces, Phragmidium, Pucciniastrum, Melampsora).

Telltale signs (per RHS): Yellow spots on the upper leaf surface, corresponding to orange-brown, yellow, or rust-coloured powdery pustules on the lower leaf surface. Pustules often appear in concentric rings. Severely affected leaves yellow, distort, and fall.

Plants commonly affected: Pelargonium (geranium), fuchsia, hollyhock, snapdragon, daylily, mint, hypericum. Indoor: pelargoniums on a sunny windowsill, fuchsias overwintered indoors.

Treatment:

1. Carefully remove infected leaves without shaking spores onto healthy parts. Bin in a sealed bag.
2. Clear away all debris around the plant.
3. Improve airflow and reduce humidity.
4. Apply tebuconazole, triticonazole, or myclobutanil for pelargonium rust. Caution: Fuchsia is sensitive to several fungicide actives — check the label includes fuchsia before spraying, and test a small area first.
5. For repeat offenders, consider replacing with rust-resistant cultivars.

Important note: Fungicides are not effective once rust symptoms are visible — they prevent new infections rather than curing existing ones. Sustained control requires the leaf-removal + airflow protocol combined with preventive sprays.

#5 — Sooty mould

Causal organisms: Capnodium, Cladosporium, and other non-pathogenic moulds.

Telltale signs: Black powdery coating on leaves and stems, often dripping or smudging on touch. Distinct from the other 7 fungi because sooty mould doesn't infect plant tissue — it grows on honeydew excreted by sap-sucking pests below the affected area.

The cause is the pest, not the fungus. Sooty mould is a symptom of an active aphid, mealybug, scale, or whitefly infestation. The fungus disappears once the pest is killed and the honeydew is washed off.

Treatment:

1. Identify and treat the pest (see sticky leaves on a houseplant for the diagnostic flow).
2. Once the pest is dead, wipe sooty mould off leaves with a damp soft cloth. For stubborn deposits, use a diluted insecticidal soap solution.
3. The mould stops growing once honeydew is removed and pests are dead. No fungicide needed.

#6 — Downy mildew

Causal organisms: Oomycetes (water moulds) including Peronospora, Plasmopara, Bremia, Hyaloperonospora. Despite the name, downy mildew is more closely related to algae than to true fungi — but it's grouped with fungal diseases in horticulture.

Telltale signs: Yellow or pale green angular patches on the upper leaf surface (constrained by leaf veins, giving them an angular shape), with a corresponding grey-white, blue-grey, or purple-grey fluffy growth on the underside in damp conditions. Severely affected leaves brown and die.

Conditions favouring it: Cool wet weather, dense plant canopy, overhead watering, low light.

Plants commonly affected: Lettuce (the classic), cucumber, basil, impatiens (devastating outbreaks 2010-present), brassicas, grapevine, onion. Less common on houseplants but does affect impatiens kept indoors.

Treatment:

1. Remove and dispose of all affected leaves.
2. Improve airflow, reduce humidity, stop overhead watering.
3. Apply copper soap or mancozeb. Phosphite (potassium phosphonate) is particularly effective on oomycetes.
4. For impatiens grown indoors, downy mildew can be a recurring problem — consider switching to New Guinea impatiens or other resistant species.

#7 — Root rot (Pythium and Phytophthora)

Causal organisms: Oomycetes — Pythium, Phytophthora, sometimes secondary Rhizoctonia and Fusarium.

Telltale signs: The plant wilts despite wet soil. Lower leaves yellow and drop. The soil smells musty or sour. Base of the stem feels soft and squishy. When unpotted, roots are brown or black, soft, and mushy — they may slip off in a "wet sock" pattern, leaving a thin thread-like core. Healthy roots, by contrast, are white or cream and firm.

Conditions favouring it: Overwatering, poor drainage, pots without drainage holes, cold wet soil, contaminated soil mix.

Plants commonly affected: Almost any plant if overwatered. Particularly prone: peace lily, pothos, snake plant, orchids, succulents, seedlings (damping-off).

Treatment:

1. Stop watering immediately.
2. Unpot the plant and inspect roots.
3. Snip all brown, slimy, mushy roots with clean sharp scissors.
4. Repot into fresh dry well-draining mix in a clean pot with drainage holes. Drop down a pot size if rot was severe — less soil means faster drying.
5. Wait 5-7 days before the first watering to let cuts callus.
6. For severe seedling damping-off, apply a copper soap drench or Trichoderma harzianum biocontrol.

See root rot for the full rescue protocol and musty potting soil smell for early detection.

#8 — Botrytis (grey mould)

Causal organism: Botrytis cinerea.

Telltale signs (per RHS): Fuzzy grey-brown coating on dying or damaged plant tissue, especially on flowers, soft fruit, and stems. Starts on senescing or wounded tissue and spreads to healthy tissue from there. Often begins on dropped petals or dying flowers that sit on healthy leaves below.

Conditions favouring it: Cool damp still air. The fungus needs three things simultaneously: moisture on tissue surfaces, cool temperatures (around 15-21°C / 60-70°F), and still air.

Plants commonly affected: Strawberry, tomato, lettuce, cyclamen, begonia, geranium, roses. Indoor: any flowering houseplant in cool damp conditions — cyclamen, begonias, gloxinia, orchids in autumn/winter.

Treatment:

1. Remove all infected tissue — flowers, leaves, fruit — with sharp clean scissors. Bin in a sealed bag.
2. Improve airflow with a small oscillating fan. This is the single most effective intervention.
3. Reduce humidity. Aim for 50-65% indoors.
4. Stop overhead watering and misting completely. Water at the soil line only.
5. Apply copper soap, Bacillus amyloliquefaciens, or fenhexamid weekly until clear.

How to diagnose in 60 seconds

The fastest way to match a fungal symptom to one of the 8 above:

Step 1 — Where on the plant?

• White or grey coating on UPPER leaf surface → powdery mildew (#1)
• Yellow patches above + fluff below → downy mildew (#6)
• Spots on leaves with rings → leaf spot (#3) or anthracnose (#2)
• Powdery pustules on leaf UNDERSIDES → rust (#4)
• Black coating on leaves with sticky residue → sooty mould (#5) — treat the pest
• Fuzzy grey on flowers or dying tissue → botrytis (#8)
• Plant wilting in wet soil → root rot (#7)

Step 2 — What colour and texture?

• White and powdery, wipes off → powdery mildew
• Dark sunken with pink ooze → anthracnose
• Small round with halos → leaf spot
• Orange/brown/yellow rust-coloured pustules → rust
• Black soot → sooty mould
• Grey-white fuzz under leaf → downy mildew
• Grey-brown fuzz on flowers → botrytis
• Brown mushy roots → root rot

Step 3 — How fast is it spreading?

• Days → powdery mildew, downy mildew, botrytis
• Weeks → anthracnose, leaf spot, rust
• Already severe at first sight → root rot (often missed until critical)
• Stops growing once pest is dead → sooty mould

Universal treatment principles

Regardless of which fungus you're dealing with, four interventions help every case:

1. Remove infected tissue immediately. Sharp clean scissors, dispose in a sealed bag (never compost), disinfect tools with isopropyl alcohol between cuts.
2. Improve airflow. A small oscillating fan running on low for 4-8 hours per day is the single most effective intervention against fungal disease indoors. It works on every one of the 8 fungi above.
3. Water at the soil line. Wet leaves are how most fungal spores germinate. Stop overhead watering and misting.
4. Apply a broad-spectrum fungicide. Copper soap and Bacillus amyloliquefaciens are universal safe choices that work on most fungal diseases and are approved for organic gardening.

Chemical safety boilerplate

Always read the label and follow manufacturer's PPE / dosage / re-entry guidance. Approvals change — confirm via UK HSE register or US EPA before use. The UK has restricted all outdoor neonicotinoid uses since 2018, and imidacloprid approvals lapsed entirely by late 2020. For fungal diseases, copper soap, sulfur, potassium bicarbonate, and Bacillus amyloliquefaciens are universal safe choices for home use. Fuchsia is sensitive to several fungicide actives — always check the label includes fuchsia and patch-test before whole-plant treatment.

UK + US regulatory notes

• UK: HSE maintains the Pesticides Register at secure.pesticides.gov.uk. Acetamiprid remains approved as of 2025; imidacloprid is no longer approved for general outdoor or indoor use. Copper-based fungicides, sulfur, neem oil, insecticidal soap, and Bacillus-based biologicals are widely available.
• US: EPA-approved actives for ornamental fungal disease include chlorothalonil, mancozeb, myclobutanil, tebuconazole, and propiconazole. Always check state-specific restrictions and re-entry intervals.

Prevention: 5 rules

1. Improve airflow with a fan. A single oscillating fan running on low 4-8 hours per day cuts fungal disease pressure dramatically.
2. Water at the soil line. Wet leaves are how 80% of fungal infections start.
3. Space plants apart. Crowded plants share spores. Aim for at least 15 cm (6 in) of leaf-tip clearance between adjacent houseplants.
4. Quarantine new plants for 2-3 weeks. Many fungal infections arrive on newly bought plants.
5. Disinfect tools between plants. Wipe scissors and trowels with isopropyl alcohol when moving between plants, especially when pruning out diseased tissue.

Sources and further reading

This guide draws on university Extension and authoritative research:

• Clemson HGIC — Houseplant Diseases & Disorders (anthracnose, leaf spot, powdery mildew identification)
• Iowa State Extension — Diagnosing Houseplant Diseases
• UMass Extension — Powdery Mildew Diseases of Ornamental Plants
• RHS — Rust Diseases (UK-specific cultivar guidance)
• RHS — Grey Mould (Botrytis)
• Penn State Extension — Botrytis (Grey Mould) Management

Related Growli guides:

• Houseplant diseases — disease library hub
• Powdery mildew — deep-dive treatment
• Root rot — full rescue protocol
• White mold on plant soil — the harmless saprophyte lookalike
• Brown spots on plant leaves — leaf-spot vs bacterial diagnostic
• White spots on plant leaves — powdery mildew vs pest diagnostic
• Houseplant pests: 12 identified — adjacent pest hub
• What's wrong with my plant? — full Pillar 1 flowchart

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