What a photograph can — and cannot — diagnose in a plant

A photograph makes it possible to recognise patterns, locate symptoms and decide what information is missing. It rarely confirms the cause on its own.

Wilted leaves, spots, chlorosis and branch dieback can result from water, soil, pests, disease, mechanical damage or a combination. The image shows the visible effect; it does not measure moisture, compaction or the integrity of the roots.

A good photographic record makes triage more useful and helps to decide whether a visit or laboratory analysis is needed.

The first image should show the plant in its context.

What matters is seeing whether the problem affects the whole crown, one side, the upper branches or only the inner leaves.

The relationship with buildings, paving, sun, wind and other plants provides clues.

A photograph of a single leaf loses this distribution.

After the general view, record the affected leaves, branches, trunk or fruit.

The image should be in focus and include a size reference where necessary.

Photograph the upper and lower sides of the leaves, the transition between healthy and damaged tissue and different stages of the symptom.

Do not choose only the worst leaf. It is useful to show the average pattern.

The trunk at ground level, the staking and the surface should all appear.

Wet soil, piled-up mulch, cracks and weeds help in understanding water and depth.

When it is safe, open a small inspection point next to the roots and photograph the moisture.

This image is often more informative than another close-up of a leaf.

The photograph needs a date. It also matters when the symptom started and how quickly it advanced.

Comparing images taken from the same angle over days or weeks shows progression.

A stable spot means something different from dieback moving up through the crown.

Saying whether there has been a storm, heat, transplanting or a change in watering helps to relate events.

Species, planting date, origin and previous interventions should accompany the images.

The questions are how it is watered, what volume it receives, whether there is drainage, whether the soil has been disturbed and what products have been applied.

Without this information, the diagnosis stays open to many hypotheses.

A photograph does not confirm whether the rootball is dry.

A dark surface may be the result of recent watering, while the centre remains without water. A dry surface may hide saturation below.

The soil has to be checked. Giving the irrigation run time without the flow rate does not allow the volume to be calculated either.

Where there is a controller, photographing the zone, the emitters and the settings can help.

Some insects, eggs, galleries and droppings are visible. A macro photograph may allow an approximate identification.

Other organisms are too small, or the signs are common to several species.

The presence of an insect on a leaf does not prove that it is responsible for the general decline.

Identification should consider quantity, damage and distribution.

Leaf spots, cankers and rots can have suggestive patterns.

Confirming fungi, bacteria or nematodes may require microscopy, culture or a laboratory test.

A primary agent also has to be distinguished from secondary colonisation. A weakened tree can show fungi without their having started the problem.

Applying fungicide on visual resemblance alone can be useless and delay correction of the environment.

Chlorosis and changes in colour can suggest a deficiency, but they are influenced by pH, water and roots.

A photograph does not measure nutrients. The pattern between new and old leaves helps, but soil and leaf analysis may be needed.

Fertilising without a diagnosis can increase salinity.

Broken branches, cuts, abrasion and tight straps are often visible.

Sunscald, frost, hail and wind leave patterns linked to exposure and timing.

Weather information and the orientation of the plant complete the picture.

It can allow a simple intervention to be recognised, such as an over-tight stake tie, a broken branch or a visible caterpillar.

It also helps in recommending safe checks: measuring moisture, observing drainage, removing a tie or isolating an area.

The answer should still state uncertainty where it exists.

Instability, rapid decline, large trees close to people, trunk wounds and root problems justify an inspection in person.

A visit makes it possible to touch the soil, check resistance, observe from several sides and relate levels.

For trees with structural risk, photographs do not replace assessment by a qualified professional.

Persistent symptoms with no clear environmental cause, unknown organisms and suspected regulated organisms may require a laboratory.

The sample should include tissue from the transition zone and follow the instructions for preservation.

The DGAV, Portugal’s food and veterinary authority, and the plant health services should be contacted when a quarantine organism is suspected.

A useful set includes a general view, several details, the base, the soil and the irrigation, accompanied by the date, approximate location, species, planting and progression.

The description should say what has changed, and not just ‘the plant is ill’.

The photograph is a triage tool. It makes the conversation quicker and more focused, but diagnosis still depends on evidence beyond what the camera can see.

• Post-transplant stress
• How to monitor adaptation
• How to assess the soil

1. EPPO Global Database.
2. Direção-Geral de Alimentação e Veterinária — plant health information.
3. Principles of plant diagnosis and laboratory sampling.
4. Image triage method used in Diaplant monitoring.