Pest & Plant Disease Troubleshooting
Common Plant Diseases and How to Treat Them
How to recognise powdery mildew, blight, root rot and other common plant diseases — what causes each one and how to treat it.
Most plant diseases are fungal, spread in damp or crowded conditions, and show up as a change in colour or texture on leaves and stems rather than physical bite marks. Catching them early usually means the plant can be saved.
- Powdery mildew
A fungal disease that coats leaves and stems in a white or grey powdery layer. It thrives in warm, dry air with poor airflow — common on courgettes, roses and cucumbers grown in cramped, still conditions.
- Root rot
Fungal decay of the roots caused by waterlogged, poorly drained soil. Leaves yellow and wilt even though the plant is technically getting enough water, because the damaged roots can no longer take it up.
Recognising the common ones
| Disease | Signs | Common cause | Treatment |
|---|---|---|---|
| Powdery mildew | White powdery coating on leaves | Poor airflow, dry soil, humid air | Improve spacing; remove worst leaves; fungicide if severe |
| Blight | Brown, spreading patches on leaves and stems | Wet weather, splashing water on foliage | Remove affected parts fast; water at the base, not the leaves |
| Root rot | Yellow leaves, wilting, mushy dark roots | Overwatering, poor drainage | Repot into fresh mix with drainage; water less |
| Leaf spot | Small dark or brown circular spots | Fungal or bacterial, spread by water splash | Remove affected leaves; avoid overhead watering |
| Rust | Orange-brown pustules on leaf undersides | Damp conditions, crowded planting | Remove affected leaves; increase airflow |
Frequently asked questions
Can a plant recover from root rot?
Sometimes, if caught early. Remove it from the pot, trim away any dark, mushy roots, and repot into fresh, well-draining mix. If most of the root system is affected, the plant usually won't survive.
Is powdery mildew caused by too much water?
The opposite, usually — it thrives on dry soil combined with humid, still air around the leaves. Watering the soil (not the foliage) and improving airflow around plants helps prevent it.
Should I remove all affected leaves at the first sign of disease?
Yes, for most fungal diseases. Removing affected leaves as soon as you spot them slows the spread far more effectively than waiting to see how bad it gets.