How do I tell bacterial from fungal?

Shape. Bacterial spots are angular because veins block them; fungal spots are round and cross veins. If you can see fuzz or powder, it is fungal.

Does copper spray help?

It can slow spread as a protectant on healthy tissue, but it does not cure infected leaves and it is no substitute for keeping the foliage dry.

Is the fruit safe to eat?

Yes, after cutting away any lesions. It is a plant pathogen and not a human one.

Will it spread to other species?

Most of these bacteria are fairly host-specific, but the conditions that caused it affect everything nearby, so neighbours are still at risk.

Not from an affected plant — several of these bacteria are seed-borne and will carry straight into next season.

Plant guide:🍃 Calathea

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Bacterial & Fungal Leaf Spot — Xanthomonas & Cercospora spp. — calathea plant care

⚠ Moderate

Irregular necrotic lesions with surrounding chlorotic halos on forest-floor foliage — consistent with Xanthomonas bacterial infection or Cercospora fungal leaf spot.

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Fungal

Bacterial & Fungal Leaf Spot — Xanthomonas & Cercospora spp.

Commonly affects: Ficus lyrata (Fiddle Leaf Fig), Calathea, Dracaena, Dieffenbachia

Dark brown to black spots with yellow halos appear on leaf surfaces, often water-soaked in early stages. Bacterial spots have irregular margins and a translucent, greasy appearance when held to light. Fungal spots (Cercospora, Alternaria) develop concentric rings with a tan, dry centre. Spots coalesce under humid conditions to form large necrotic patches.

The short answer

Small dark spots with a yellow halo, angular rather than round because the leaf veins stop them spreading, are bacterial leaf spot. The angular shape is the whole diagnosis — fungal spots are round and ignore the veins. Bacteria enter through wet leaves and through wounds, so the treatment is to stop wetting the foliage, improve airflow, and remove affected material. Fungicides do not work on bacteria.

Confirm it before you treat it

1Look at the shape of a spot. Angular and boxed in by the veins means bacterial. Round with a smooth outline means fungal.
2Hold the leaf up to light. Bacterial spots often look water-soaked and slightly translucent at the edge early on.
3Check for the halo. A yellow ring around a dark centre is characteristic, though not exclusive.
4Recall the conditions. Warm and wet, with splashing water or handling wet plants, is how it establishes.
5Look at the undersides for anything growing. Bacterial spot has no fuzz, mould or powder — if you find any, it is a fungus instead.

Potential Causes

1Overhead watering or misting leaving free water on leaf surfaces for extended periods
2Introduction of infected plant material without proper quarantine
3High humidity combined with poor air circulation encouraging pathogen dispersal
4Physical damage from repotting or handling creating entry points for bacterial infection
5Infected potting soil or contaminated tools spreading Pseudomonas or Xanthomonas

Solution Steps

1Remove and dispose of all visibly spotted leaves in a sealed bag — do not compost.
2Sterilise pruning tools with 70% isopropyl alcohol between cuts to prevent cross-contamination.
3Apply a copper-based bactericide/fungicide (copper oxychloride or copper hydroxide) to all remaining foliage according to label rates.
4Water at the base of the plant — never overhead — using a spouted can or drip system.
5Increase spacing between plants and improve air movement to reduce leaf surface moisture.
6For bacterial infections, a preventive spray of dilute neem oil + copper solution every 10 days during high-risk periods is effective.

What else looks like this

Could be	How to tell it apart
Fungal leaf spot	Round spots that cross veins freely, often with visible fungal growth or concentric rings in the centre.
Downy mildew	Also angular on top, but with grey-purple fuzz underneath. Always check the underside before deciding.
Sun scorch	Large pale bleached areas on exposed leaves with no defined edge and no halo.
Chemical or fertiliser burn	Spots follow where droplets landed, appear all at once after an application, and do not spread afterwards.
Early blight	Larger lesions with concentric rings like a target, usually starting on the oldest leaves of tomatoes and potatoes.

Recovery timeline

Day 1Remove affected leaves with clean tools and bin them. Stop overhead watering entirely and water at the base.
Days 1–3Thin crowded growth to open up airflow, and mulch to stop soil splashing spores back onto the lower leaves.
Week 1Existing spots stay as they are. Only new growth tells you whether it has stopped.
Weeks 2–3Clean new foliage means it is under control. Continued spotting on new leaves means the foliage is still getting wet.
Next seasonRotate crops, use clean seed, and avoid working among the plants while they are wet — handling wet foliage moves bacteria plant to plant.

If you do nothing

Spots merge into large dead patches, leaves yellow and drop, and the plant loses the photosynthetic area it needs. On fruiting crops the bacteria reach the fruit and cause raised scabby lesions that make it unmarketable and unpleasant to eat. Because it spreads by water splash and by hands and tools, an untreated plant reliably infects its neighbours, and the bacteria overwinter on plant debris left in the bed to start the following season early.

Common mistakes

Spraying fungicide. This is a bacterium; fungicide has no effect and the delay lets it spread.
Continuing to water overhead. Splash is the main transmission route, and this single habit undoes everything else.
Working among wet plants. Brushing past wet foliage carries bacteria on your clothes and hands down the whole row.
Leaving debris in the bed over winter. The bacteria survive on it and reinfect next year's crop.
Removing too many leaves at once in a panic. Stripping a plant bare stresses it and slows its recovery more than the disease would have.

Prevention Tip

Always quarantine new plants in a separate room for a minimum of 14 days before introducing them to your collection. Inspect undersides of leaves and new growth carefully. This single practice eliminates the majority of disease introductions.

Questions people actually ask

How do I tell bacterial from fungal?: Shape. Bacterial spots are angular because veins block them; fungal spots are round and cross veins. If you can see fuzz or powder, it is fungal.
Does copper spray help?: It can slow spread as a protectant on healthy tissue, but it does not cure infected leaves and it is no substitute for keeping the foliage dry.
Is the fruit safe to eat?: Yes, after cutting away any lesions. It is a plant pathogen and not a human one.
Will it spread to other species?: Most of these bacteria are fairly host-specific, but the conditions that caused it affect everything nearby, so neighbours are still at risk.
Can I save seed?: Not from an affected plant — several of these bacteria are seed-borne and will carry straight into next season.