Finding and Observing Plant Pests, Diseases and Disorders

Introduction

Healthy plants are at the centre of successful horticulture. A gardener needs to be able to recognise when a plant is growing normally and, equally importantly, when something is beginning to go wrong. Changes in colour, growth, leaf shape, stems, roots, flowers or fruit can provide valuable evidence about the health of a plant.

Plant problems can have many causes. A plant may be damaged by an animal or insect pest, affected by a disease-causing organism, or suffer from a disorder caused by environmental or growing conditions. The visible symptom may be similar even when the underlying cause is completely different. For example, yellow leaves can result from nutrient deficiency, waterlogging, drought, root damage, pest feeding or disease.

This means that successful plant-health observation is not simply a matter of looking at a leaf and naming the problem. It involves careful observation, gathering evidence, considering possible causes and making a reasoned identification.

Plant health is an important part of the RHS Level 2 curriculum. The RHS identifies environmental conditions, plant nutrition, competition, pests and pathogens, biosecurity and maintaining plant health as key areas of plant-health knowledge. (RHS) The RHS Level 2 practical qualification also includes identification of plants, pests, diseases, pathogens and disorders. (RHS)

1. What is plant health?

Plant health describes the condition of a plant and its ability to grow, develop and reproduce successfully.

A healthy plant will normally show characteristics appropriate to its species and growing conditions. These might include:

healthy, appropriately coloured foliage
normal shoot growth
strong stems
functioning roots
appropriate flowering
normal fruit or seed development
freedom from significant pest damage
freedom from serious disease
appropriate response to environmental conditions.

It is important to remember that there is no single appearance that defines a healthy plant. A mature Fagus sylvatica will naturally look very different from a young Lavandula angustifolia. A plant growing in full sun may have different characteristics from the same species growing in shade.

Therefore, observation begins with knowing what normal growth looks like for the particular plant.

2. Pest, disease or disorder?

One of the most important Level 2 plant-health skills is understanding the difference between a pest, a disease and a disorder.

Plant pest

A plant pest is an organism that damages a plant through its feeding or other activities.

Common groups include:

insects
mites
molluscs
nematodes
mammals
some other animals.

Examples include:

aphids
whitefly
vine weevil
thrips
spider mites
slugs and snails
caterpillars
rabbits
mice and other mammals.

Plant disease

A plant disease is a condition caused by a harmful pathogen or other infectious agent.

Important groups of plant pathogens include:

fungi
Oomycetes (fungus-like organisms)
bacteria
viruses
viroids
some nematodes.

Examples familiar to horticulturists include:

rose black spot
powdery mildew
grey mould
damping-off
honey fungus
potato blight.

The RHS Level 2 specification includes plant diseases such as grey mould, strawberry powdery mildew, damping-off, honey fungus, rose black spot and potato blight within its plant-health content. (RHS)

Plant disorder

A plant disorder is a problem caused by a non-infectious factor rather than by a pest or pathogen.

Possible causes include:

nutrient deficiency
nutrient toxicity
drought
waterlogging
frost
excessive heat
unsuitable pH
poor soil structure
compaction
root damage
poor light levels
chemical damage
inappropriate cultivation.

For example, a plant with yellow leaves may have a nutrient deficiency, but yellowing does not automatically mean that the plant has a disease.

3. Symptoms and signs

A useful distinction when examining plants is between a symptom and a sign.

Symptoms

A symptom is a change in the plant caused by the problem.

Examples include:

yellowing
wilting
leaf curling
distorted growth
leaf spots
holes in leaves
premature leaf fall
dieback
stunted growth
poor flowering
lesions
discolouration
reduced vigour.

Signs

A sign is physical evidence of the pest or pathogen itself.

Examples include:

an aphid on a shoot
whitefly adults flying from foliage
caterpillar larvae
slug slime trails
insect eggs
fungal mycelium
fungal fruiting bodies
bacterial ooze
insect honeydew
cast insect skins
webbing produced by some mites.

The distinction is important because a symptom tells you that something may be wrong, whereas a sign can provide evidence about what is causing the problem.

4. Start with observation, not treatment

One of the most common mistakes in plant-health diagnosis is deciding on a treatment before identifying the problem.

A better approach is:

Observe → gather evidence → identify the likely cause → decide on action.

For example, suppose a rose has curled leaves.

It would be premature to immediately conclude that the plant needs an insecticide. The leaves could have been affected by aphids, frost, drought, herbicide drift or another factor.

The first question should therefore be:

What evidence can I find?

Look carefully at:

the upper surface of the leaves
the underside of leaves
young shoots
stems
buds
flowers
fruit
the crown of the plant
the soil surface
the root zone where appropriate.

A hand lens can be extremely useful when examining small insects, mites, eggs, fungal structures or other signs.

5. Observe the whole plant

A plant should not normally be diagnosed from one leaf alone.

Begin by standing back and looking at the whole plant.

Ask:

Is the problem affecting the whole plant?

If the entire plant is showing similar symptoms, consider environmental or root-related causes.

For example:

drought
waterlogging
nutrient deficiency
unsuitable growing conditions
root damage.

Is the problem affecting only one branch?

A localised problem may indicate:

physical damage
pruning damage
a canker
localised disease
an insect infestation
damage to a branch or stem.

Is the problem affecting one part of the plant?

If only young leaves are affected, the cause may differ from a problem affecting older leaves.

If only roots are affected, examination of the root system may be necessary.

The distribution of symptoms is therefore an important diagnostic clue.

6. Look at the leaves

Leaves are often the first part of a plant to show visible signs of stress.

Look for:

yellowing
browning
mottling
spots
holes
mines
curling
distortion
silvering
scorch
wilting
premature leaf fall
unusual growth.

Holes in leaves

Holes may indicate chewing or feeding damage.

Possible causes include:

caterpillars
beetles
weevils
slugs
snails
other chewing pests.

However, the shape and distribution of the holes can provide further evidence.

Examine the leaf carefully for the pest itself, droppings, eggs or feeding remains.

7. Examine the underside of leaves

The underside of a leaf is often overlooked.

It is an important place to search for:

aphids
whitefly
thrips
mites
eggs
larvae
fungal growth
insect cast skins.

Many sap-feeding insects are found on young shoots or the underside of leaves because these areas can provide suitable feeding sites and protection.

A plant should therefore be examined from both sides, rather than simply viewed from above.

8. Recognising common pest damage

Different pests produce different patterns of damage.

Aphids

Aphids are small sap-feeding insects.

They may cause:

curled leaves
distorted young shoots
weakened growth
sticky honeydew
sooty mould developing on honeydew
colonies of insects around young growth.

Aphids are often concentrated around:

shoot tips
young leaves
flower buds.

Look closely rather than relying solely on the symptom.

Whitefly

Whiteflies are small, winged sap-feeding insects often associated with protected cultivation, although some species are found outdoors.

Signs may include:

small white adults flying when foliage is disturbed
nymphs on the underside of leaves
honeydew
sooty mould
weakened or discoloured foliage.

Vine weevil

Vine weevil can produce two very different types of damage.

Adult beetles may create:

characteristic notches around leaf edges.

The larvae can damage roots, particularly in container-grown plants.

A plant suffering root damage may show:

wilting
poor growth
yellowing
reduced vigour
collapse despite apparently adequate watering.

This demonstrates why examining only the leaves can give an incomplete diagnosis.

Slugs and snails

Slugs and snails may produce:

irregular holes
shredded foliage
damage to young plants
damage to seedlings
slime trails.

The damage may be particularly noticeable overnight or following wet conditions.

Mites

Some mites are extremely small and may require a hand lens to observe.

Damage can include:

fine mottling
bronzing
leaf distortion
stippling
webbing in some infestations.

A useful observation technique is to inspect the underside of the leaf carefully and use a hand lens.

9. Recognising disease symptoms

Plant diseases can produce symptoms that overlap with pest damage and disorders.

Common disease symptoms include:

leaf spots
blotches
mildew
mould
cankers
dieback
rotting
wilting
damping-off
root decay
distorted growth.

A disease may also progress through the plant over time.

For example, a small leaf spot may initially appear insignificant but develop into a larger necrotic area as the disease progresses.

10. Common diseases

Rose black spot

Rose black spot produces dark or black spots on rose leaves. (It is a disease affecting mainly those plants with the recessive gene that makes them susceptible.)

Affected leaves may:

develop black lesions
turn yellow
fall prematurely.

The pattern of symptoms is important. Do not identify every dark mark on a rose as black spot without examining the plant carefully.

Powdery mildew

Powdery mildew typically produces a characteristic pale or white powdery growth on plant surfaces.

It may be associated with:

distorted leaves
reduced vigour
poor appearance
affected young growth.

Environmental conditions can influence disease development.

Grey mould

Grey mould is associated with the pathogen Botrytis cinerea.

It can produce:

soft rotting tissue
brown or dead plant material
grey fungal growth.

It is particularly associated with conditions where plant material remains damp and air movement is poor.

Damping-off

Damping-off commonly affects young seedlings.

Symptoms can include:

poor emergence
seedlings collapsing
soft or rotted tissue around the base of the stem
death of young seedlings.

The growing environment is particularly important when considering damping-off.

Honey fungus

Honey fungus can affect the roots and lower stems of plants.

Symptoms can include:

reduced vigour
yellowing
dieback
wilting
decline over time.

Diagnosis requires careful examination because several different problems can produce similar above-ground symptoms.

11. Recognising disorders

Disorders are especially important because they can easily be mistaken for pests or diseases.

Nutrient deficiency

Nutrient deficiencies may cause:

yellowing
poor growth
unusual leaf colour
leaf-edge scorching
premature leaf fall
poor flowering.

The pattern of the symptom can provide useful information.

Ask:

Are older leaves affected first?
Are younger leaves affected first?
Is the symptom between the veins?
Are the leaf margins affected?
Is the entire plant affected?

However, visual symptoms alone do not always provide a definitive diagnosis.

Drought

Drought stress may cause:

wilting
leaf curling
leaf scorch
premature leaf fall
reduced growth.

Check the soil rather than assuming that wilting always means lack of water.

A wilted plant may also be suffering from:

root damage
waterlogging
disease
root pests.

Waterlogging

Too much water can be just as damaging as too little.

Waterlogged conditions reduce the amount of oxygen available around roots and may result in:

yellowing
poor growth
wilting
root death
increased susceptibility to some root diseases.

The surprising appearance of a wilted plant in wet soil is therefore an important diagnostic clue.

12. The importance of the growing environment

Plant health cannot be considered separately from the environment.

When observing a problem, record:

soil type
soil moisture
drainage
aspect
light levels
exposure
temperature
recent weather
watering
feeding
planting density
air movement
surrounding vegetation.

The RHS identifies environmental conditions, nutrition and competition alongside pests and pathogens as important components of plant health. (RHS)

For example, a densely planted border may have reduced air movement. Leaves may remain wet for longer, potentially creating conditions favourable to some diseases.

Similarly, a plant growing in the wrong place may become stressed and subsequently more vulnerable to pest or disease problems.

This leads to one of the most important principles in plant health:

The symptom is not always the cause.

13. Pattern is evidence

The pattern of damage can be extremely useful.

One plant affected

Consider:

localised pest activity
physical damage
plant-specific disease
planting problem.

Several plants of the same species affected

Consider:

pest outbreak
disease
unsuitable conditions for that species
nutrient problem affecting a particular plant type.

Several different plant species affected

Consider:

environmental conditions
drought
waterlogging
chemical exposure
nutrient problems
weather damage.

Plants throughout an entire area affected

Consider:

irrigation problems
soil conditions
drainage
fertiliser application
chemical application
environmental stress.

The distribution of symptoms can therefore help distinguish between possible causes.

14. Check the plant’s history

Good observation includes finding out what has happened to the plant.

Ask:

When was it planted?
Has it recently been moved?
Has it been pruned?
Has it been watered?
Has fertiliser been applied?
Have chemicals been used nearby?
Has the weather recently changed?
Has the plant been damaged?
Has the plant been repotted?
Are neighbouring plants affected?
Has the problem occurred previously?

A plant is not an isolated object. Its history can provide essential diagnostic information.

15. Use a systematic inspection method

A useful Level 2 inspection can follow a consistent sequence.

Step 1 – Identify the plant

Record:

common name
botanical name where known
location.

Correct plant identification matters because different plants have different pests, diseases and environmental requirements.

Step 2 – Observe from a distance

Look at:

overall shape
vigour
colour
distribution of symptoms
affected and unaffected areas.

Step 3 – Inspect closely

Examine:

leaves
stems
buds
flowers
fruit
crown
soil surface.

Step 4 – Look underneath

Check the undersides of leaves and hidden parts of the plant.

Step 5 – Look for signs

Search for:

insects
eggs
larvae
webs
honeydew
fungal growth
fruiting structures
slime trails.

Step 6 – Examine the environment

Check:

soil
moisture
drainage
light
shelter
recent weather.

Step 7 – Compare

Compare the affected plant with:

a healthy plant of the same species
unaffected parts of the same plant
neighbouring plants.

Step 8 – Record

Write down what has actually been observed.

Step 9 – Identify

Use reliable reference material to identify the most likely cause.

Step 10 – Decide on action

Only after identification should appropriate management be considered.

16. Recording observations

Good records are an important part of professional horticulture.

A simple plant-health record might include:

Observation	Example
Plant	Rose
Location	South-facing border
Date	28 September
Symptoms	Black spots and yellowing leaves
Distribution	Several lower leaves affected
Signs	No obvious insects observed
Environment	Dry soil, sunny position
Recent activity	Plant pruned two months previously
Initial identification	Possible rose black spot
Further investigation	Compare symptoms with reliable reference
Action	Monitor and follow appropriate plant-health guidance

The purpose of recording is not simply to create paperwork. Records allow gardeners to identify patterns over time.

For example, if the same area repeatedly develops disease symptoms after periods of wet weather, this may indicate that the environmental conditions are contributing to the problem.

17. Observation and monitoring

Finding a problem once is not necessarily enough.

Monitoring means observing a plant or area repeatedly over time.

This can reveal:

whether the problem is increasing
whether new growth is affected
whether a pest population is increasing
whether symptoms are spreading
whether natural enemies are present
whether environmental conditions are changing.

The RHS emphasises surveillance and improved detection of plant pests and diseases as important aspects of plant health management. (RHS)

Regular monitoring can therefore allow a problem to be recognised before serious damage occurs.

18. Beneficial organisms must not be mistaken for pests

Not every insect found on a plant is harmful.

A garden can contain many useful organisms, including:

ladybirds
lacewings
hoverflies
parasitoid wasps
predatory beetles
spiders
ground beetles.

Some of these organisms feed on pests.

For example, a plant covered with aphids may also contain ladybird larvae or hoverfly larvae feeding on them.

A gardener who sprays every insect encountered may remove both harmful and beneficial organisms.

This is one reason why identification should come before intervention.

The RHS recommends cultural control, suitable plant selection and encouraging natural predators as important elements of preventing pest and disease problems. (RHS)

19. Biosecurity and plant health

Plant-health observation is also part of horticultural biosecurity.

New pests and diseases can spread through:

movement of plants
contaminated growing media
tools
clothing
footwear
water
plant debris
vehicles
traded plant material.

The RHS identifies plant biosecurity and plant passports as part of its Level 2 plant-health curriculum. (RHS)

Good horticultural practice therefore includes:

inspecting incoming plants
isolating suspect material where appropriate
cleaning tools
removing infected material responsibly
avoiding unnecessary movement of contaminated material
reporting serious or unusual plant-health concerns through appropriate channels.

Plant health is not only about protecting one plant. It can involve protecting an entire collection, garden, nursery or wider landscape.

20. Avoiding diagnostic mistakes

There are several common mistakes when identifying plant-health problems.

Mistake 1: Assuming every hole is an insect

Holes may result from:

slugs
snails
caterpillars
beetles
weather
physical damage.

Better approach: inspect the damage and search for evidence.

Mistake 2: Assuming every yellow leaf is a nutrient deficiency

Yellowing can have many causes.

Better approach: consider the plant, leaf age, soil, roots, watering and environmental conditions.

Mistake 3: Treating before identifying

Applying a product without knowing the cause can waste time and potentially harm beneficial organisms.

Better approach: diagnose first.

Mistake 4: Looking only at the leaves

Root problems can produce symptoms above ground.

Better approach: consider the whole plant and its growing environment.

Mistake 5: Ignoring normal seasonal change

Some plants naturally change colour or shed leaves.

Better approach: know what normal seasonal development looks like.

Mistake 6: Identifying from one symptom

Many different problems can produce similar symptoms.

Better approach: combine several pieces of evidence.

21. A practical diagnostic framework

A useful way to remember plant-health observation is:

P-L-A-N-T

P – Plant
What plant is it? What does healthy growth normally look like?

L – Look
Observe the whole plant before concentrating on the damaged area.

A – Assess
Assess symptoms, signs, distribution and environmental conditions.

N – Notice
Look for insects, fungal growth, eggs, feeding damage, root problems and other evidence.

T – Think
Consider several possible causes before identifying the most likely one.

This encourages students to become observers rather than simply memorising photographs of pests and diseases.

22. From identification to management

Identification is only the beginning of plant-health management.

Once the cause has been established, appropriate action can be considered.

Possible approaches include:

Cultural control

Changing growing conditions or horticultural practice.

Examples:

improving air circulation
removing heavily infected material
improving cultivation
avoiding overwatering
selecting appropriate plants
maintaining plant vigour.

Physical or mechanical control

Examples include:

removing pests by hand
barriers
traps
removing affected plant material.

Biological control

Using or encouraging natural enemies of pests.

Examples include:

predatory insects
parasitoids
biological control organisms.

Chemical control

Where appropriate and legally permitted, plant-protection products may form part of an integrated approach.

However, chemical control should not be the automatic response to seeing a pest or symptom.

The RHS stresses prevention, plant selection, cultural approaches and encouraging natural predators as important ways of reducing pest and disease problems. (RHS)

23. Integrated plant-health thinking

Modern horticulture increasingly considers plant health as a system rather than a series of individual pest problems.

A healthy plant is influenced by:

Plant + Soil + Water + Nutrition + Light + Temperature + Air movement + Biodiversity + Management

Problems often occur when several stresses interact.

For example:

poor soil structure → poor root growth → plant stress → reduced vigour → greater susceptibility to pests or disease

This means that the best plant-health solution may not be to eliminate the organism that is visible on the plant. The more useful response may be to improve the conditions in which the plant is growing.

24. Practical observation exercise

Choose three plants growing in a garden, nursery, glasshouse or college horticultural area.

For each plant:

Identify the plant.
Observe the whole plant from a distance.
Examine the leaves from both sides.
Examine stems and growing points.
Look for pests or other organisms.
Look for disease signs.
Record any abnormal growth.
Examine the soil and growing conditions.
Compare the plant with a healthy specimen where possible.
Decide whether the evidence suggests:
pest
disease
disorder
normal growth
or that further investigation is required.

Do not automatically identify a problem simply because a leaf looks unusual.

Record evidence such as:

“Small black insects observed on the underside of young leaves, with curled new growth and sticky honeydew.”

This is much more useful than:

“Plant has aphids.”

The first statement records the evidence. The second records the conclusion.

25. Key Level 2 knowledge

A competent horticulturist should be able to explain that:

a pest damages a plant through its feeding or other activities
a disease is associated with a harmful pathogen or infectious agent
a disorder results from a non-infectious cause
symptoms are changes shown by the plant
signs provide physical evidence of the pest or pathogen
similar symptoms can have different causes
the whole plant should be examined
the growing environment should be considered
the distribution of symptoms can provide diagnostic evidence
pest identification should not rely solely on damage
beneficial organisms should not automatically be treated as pests
monitoring is an important part of plant-health management
accurate records can help identify patterns
plant-health problems should be identified before management decisions are made
biosecurity helps reduce the spread of harmful pests and diseases.

These principles directly support the City & Guilds Level 2 requirements for identifying pest damage and identifying plant diseases and disorders. (BestLearn VLE)

Conclusion

Finding plant pests, diseases and disorders is a fundamental horticultural skill. Effective observation requires more than recognising a picture from a textbook. It requires the ability to examine a plant systematically, recognise symptoms and signs, consider the growing environment, identify patterns and gather enough evidence to reach a sensible diagnosis.

The most important habit is to slow down and observe before acting.

A hole in a leaf is evidence, not automatically an identification. A yellow leaf is a symptom, not automatically a nutrient deficiency. Wilting does not necessarily mean drought. An insect on a plant is not necessarily a pest.

Professional horticultural observation therefore asks:

What can I see? What evidence can I find? Where is the problem occurring? What has changed? What could cause these symptoms?

Only after these questions have been considered should the horticulturist move from observation to identification and, finally, management.

This approach develops the practical plant-health knowledge expected at Level 2 while also establishing a professional habit that remains important throughout a horticultural career.

References

Royal Horticultural Society (RHS) (2022) RHS Level 2 Certificate in the Principles and Practices of Horticulture: Qualification Specification. London: Royal Horticultural Society. (RHS)

Royal Horticultural Society (RHS) (2022) RHS Level 2 Diploma in the Principles and Practices of Horticulture: Qualification Specification. London: Royal Horticultural Society. (RHS)

Royal Horticultural Society (RHS) (2026) RHS Level 2 Qualifications. London: Royal Horticultural Society. (RHS)

Royal Horticultural Society (RHS) (2026) Preventing pest and disease problems. London: Royal Horticultural Society. (RHS)

Royal Horticultural Society (RHS) (2026) RHS Plant Health Principles. London: Royal Horticultural Society. (RHS)

City & Guilds (2018) Level 2 Award/Certificate/Diploma in Practical Horticulture Skills (7573-02): Qualification Handbook. London: City & Guilds. (BestLearn VLE)