Is this leaf spot a disease or just damage?

The edge of the spot tells you what made it — round with a firm margin, angular and stopped by the veins, or fading out with no margin at all.

Difficulty: intermediate
Time: 18 min
Read: 5 min

Short answer

Look at the boundary of the spot. Circular with a defined margin, often with faint rings and minute black dots in the centre, is fungal. Angular, bounded by the veins into a polygon, water-soaked with a yellow halo, is bacterial. No margin at all, no halo, not spreading, and absent from new leaves, is damage rather than disease.

Spots on leaves are the most over-diagnosed thing in gardening, because a great many of them are not an infection at all. Separating the living causes from finished damage decides whether you need to do anything, and it can be done in three days with a pen.

Fungal leaf spot, bacterial leaf spot and physical damage compared

Step by step

Look at the edge of one spot closely.A fungal spot is roughly circular and has a definite boundary, often a darker ring around a paler dead centre, and sometimes faint concentric rings like a target. Physical or chemical damage fades out with no boundary at all.
Look for the minute black dots.Hold the spot to the light or use a hand lens. Tiny black pinheads scattered in the dead centre are the fungus's fruiting bodies and are as close to proof as a gardener gets.
Check whether the veins have stopped it.Bacterial spots spread through the soft tissue and are halted by the veins, producing an angular, straight-sided patch rather than a circle. Early on they look water-soaked and translucent, and a bright yellow halo often develops around them.
Mark the edge of a spot with a pen and wait three days.The single most useful test there is. Only a living infection is bigger three days later. Damage is exactly the size it was on the day it happened.
Look at leaves that opened this week.A live infection appears on new leaves. Finished damage does not — a plant scorched or splashed in June has clean July leaves.
Check the position for a splash pattern.Both fungal and bacterial leaf spots are spread by water throwing infected soil and debris upwards. Damage concentrated on the lowest leaves, on the side facing a path or a downpipe, or after overhead watering, points strongly at a splash-borne disease.
Rule out the specific mimics.Cold water on hairy or soft leaves in sun leaves discrete round scorch spots. Fertiliser or foliar feed at too strong a dilution burns spots where the droplets sat. Hail leaves torn spots on the upper surfaces and on stems. Sun through glass or a water droplet leaves bleached, papery centres.
Treat the two live causes the same way, because the householder response is identical.Remove and bin affected leaves, clear fallen leaves from beneath the plant, stop watering over the foliage and water at the base instead, mulch so rain cannot splash soil upwards, thin and space to improve airflow, and keep the plant unstressed — a drought-stressed plant is far more susceptible.
Only consider a treatment for a plant that matters and repeats.Fungicides available to gardeners are limited, must list the plant on the label, and are preventive rather than curative — they protect leaves not yet infected. On a rose that gets black spot every year, a resistant variety is a better investment than an annual spray programme.

Tips

Autumn clearance beneath a susceptible plant is the highest-value action against next year's leaf spot, because that is where the spores overwinter.
Bacterial problems get much worse in warm wet weather and after handling wet foliage. Do not prune or tie in during rain on plants prone to them.
A spot with a hole where the dead centre has fallen out is called shot-hole, and on cherries and laurels it is usually a bacterial or fungal leaf spot rather than an insect.
Take a photograph of a marked spot next to a ruler, then repeat in three days from the same distance. It removes all the guesswork about whether it is growing.

Common mistakes

Spraying a fungicide at finished damage — Scorch, hail and chemical burn are not living, cannot spread and cannot be treated. The spray achieves nothing and the reader concludes the product failed.
Watering over the leaves of a plant with leaf spot — That is exactly the mechanism by which the spores reach the clean leaves. Water at the base, in the morning.
Leaving fallen infected leaves on the soil under the plant — They are next spring's inoculum, sitting exactly where the rain will splash them back up.
Removing every marked leaf at once — You strip the plant of the leaf area it needs. Take the worst in stages.

If it doesn't work

Round spots with dark margins and faint rings, tiny black dots in the centre

Cause: Fungal leaf spot, spread by splash — Fix: Remove affected leaves, clear fallen ones, mulch, water at the base, and improve spacing and airflow.

Angular spots bounded by the veins, water-soaked with a yellow halo

Cause: Bacterial leaf spot, worst in warm wet weather — Fix: Remove affected leaves, avoid handling and pruning in the wet, stop overhead watering, and improve air movement.

Spots that stopped growing and never appear on new leaves

Cause: Physical or chemical damage — hail, scorch, splashed feed, cold water — Fix: Nothing. Identify what caused it so it does not recur, and leave the leaves in place.

Dead centres falling out to leave clean-edged holes

Cause: Shot-hole, usually a leaf spot on cherry, plum or laurel — Fix: Rake up fallen leaves, improve vigour and drainage, and prune stone fruit only in dry summer weather.

Questions people ask

Does it matter whether it is fungal or bacterial?

Not much at householder level, because the response is the same and no bacterial treatment is available to gardeners. What matters is separating both of them from damage, which needs nothing at all.

Can I save the spotted leaves?

No. A leaf never repairs. Success is measured by whether the leaves that open next are clean.

Is leaf spot serious?

On a mature shrub or tree, rarely — it costs some leaf area and looks poor. It matters on young plants, on repeatedly defoliated roses, and on crops where the leaf is what you are growing.