The Disease Triangle
Plant pathologists describe disease using a simple model: a susceptible host, a capable pathogen and a favourable environment must all be present at once. Remove any one side and disease cannot develop, or develops weakly. This is why two gardens a street apart can have very different experiences with the same rose variety.
For gardeners, the environment side is usually the easiest to change. Humidity around leaves, how long foliage stays wet, crowding, poor airflow and plant stress all tilt the balance. Before asking "what is attacking my plant?", it is worth asking "what made this plant easy to attack?"
Signs Versus Symptoms
A symptom is the plant's response: yellowing, wilting, spots, galls, dieback. A sign is evidence of the organism itself: fungal spores, mycelium, bacterial ooze, insects, eggs, droppings, webbing or slime trails. Symptoms are shared by many causes. Signs are far more specific, so finding one is worth the extra minutes with a hand lens.
- Powdery mildew forms a white, dusty coating, often worse when roots are dry and air is humid.
- Rose black spot, caused by Diplocarpon rosae, produces dark spots with feathery edges and early leaf fall after wet weather.
- Late blight (Phytophthora infestans), the pathogen behind the Irish famine of the 1840s, causes rapidly spreading brown patches on potato and tomato foliage in warm, wet spells.
- Honey fungus (Armillaria species) may show creamy-white sheets of fungal growth under the bark at the base of woody plants, and dark bootlace-like strands in the soil.
Reading Pest Evidence
Pests usually leave clearer evidence than pathogens, but damage often outlasts the culprit. Notched leaf edges on rhododendrons and euonymus point toward adult vine weevils, which feed at night; the more damaging stage is the C-shaped, creamy larva eating roots in containers. Sticky leaves and black sooty mould suggest sap-feeders such as aphids, scale insects or whitefly above. Fine speckling and webbing suggest spider mites, which thrive in hot, dry conditions and indoors. Silvery streaks can indicate thrips. Ragged holes with silvery trails point to slugs and snails, usually after mild, damp nights.
| What you see | Possible causes to investigate | How to gather more evidence |
|---|---|---|
| Sticky leaves, black film | Aphids, scale, whitefly, mealybugs | Check leaves and stems directly above the sticky area |
| Wilting of one stem, brown streaks inside | Vascular wilt such as Verticillium or Fusarium | Cut a dying stem lengthways and look for staining |
| Sudden collapse of potted plant, roots missing | Vine weevil larvae, root rot | Tip out the root ball and inspect the compost |
| Spots that cross veins with a yellow halo | Fungal or bacterial leaf spot | Note whether spots appear after wet spells or overhead watering |
| Mottled or mosaic leaf colour, stunted growth | Virus infection, herbicide damage, nutrient issue | Look for aphid activity and recent spraying nearby |
Finding an insect on a damaged plant does not prove it caused the damage. Many visitors are passers-by, predators or opportunists feeding on a plant already in trouble.Reasoning caution
Proportionate Response: Integrated Pest Management
Integrated Pest Management, promoted by programmes such as the University of California Statewide IPM Program, ranks responses from least to most disruptive: prevention through plant choice and conditions, monitoring, physical removal, encouraging natural enemies, then targeted treatment only where justified. A tolerance threshold matters. Light leaf spotting late in the season on an established shrub may need nothing more than clearing fallen leaves.
When a Report Is Required
Some problems are regulated because of their wider impact. In Great Britain, ash dieback (Hymenoscyphus fraxineus) and suspected Xylella fastidiosa are examples of tree and plant health threats that authorities monitor, and suspected sightings can be reported via the Forestry Commission's TreeAlert or the government's plant health services. In the United States, state departments of agriculture and Cooperative Extension offices handle similar reports. Do not move, sell or compost suspect material until you have checked current local guidance.
Next Checks
Bag a sample correctly
For a diagnostic clinic, take a sample showing the transition between healthy and affected tissue, seal it in a bag with dry paper, and include notes and photos of the whole plant.
Check at night for hidden feeders
Slugs, snails and adult vine weevils feed after dark. A torch inspection an hour after sunset often settles the question.
Assess airflow and spacing
If leaf diseases recur, measure the spacing between plants and note how long leaves stay wet after watering. See Restore Better Conditions.