How do I deal with mildew, black spot and rust?

Powdery mildew, black spot and rust are the three fungal problems most gardeners meet. They look different but the conditions that favour them and the measures that reduce them are largely the same.

What you'll need

• Secateurs
• Bags for disposal
• Mulch (optional)

Step by step

1. Identify which you have.Powdery mildew is a white dusty coating. Black spot is dark blotches with yellowing around them, mainly on roses. Rust is orange or brown pustules, usually on leaf undersides.
2. Remove and bag infected leaves.Do not compost them. Home compost rarely reaches a temperature that kills fungal spores, so composting reintroduces the problem next year.
3. Clear fallen leaves from beneath affected plants.Black spot in particular overwinters on fallen leaves and reinfects from the ground up each spring. Autumn clearance genuinely reduces it.
4. Improve airflow by thinning and spacing.Prune to open the centre of shrubs, and space plants properly. Still humid air around foliage is what all three need.
5. Water at the base, never over the leaves.Wet foliage is the condition fungal spores germinate in. Water in the morning so any splashes dry quickly.
6. Mulch to stop soil splashing onto leaves.Rain splashing soil upwards carries spores onto lower leaves. A mulch layer breaks that route.
7. Feed and water properly.Stressed plants are far more susceptible. Drought stress in particular makes powdery mildew much worse.
8. Choose resistant varieties next time.Many roses and vegetables are bred for disease resistance. On a plant that suffers every year, replacement with a resistant variety is more effective than any treatment.

Tips

• Powdery mildew late in the season on courgettes and squash is largely unavoidable and mostly cosmetic. Remove the worst leaves and keep going.
• Clean secateurs between plants when disease is present. Pruning tools spread infection efficiently.
• Fungicides exist but are a supplement to good practice rather than a substitute. Airflow and hygiene do most of the work.

Common mistakes

• Composting infected leaves — Home compost does not get hot enough to kill the spores, so you spread the problem back across the garden next season.
• Watering over the foliage — Wet leaves are exactly what fungal spores need to germinate. Watering at the base removes the main route of infection.
• Leaving fallen leaves under an affected plant — Black spot overwinters on them and reinfects from the ground each spring. Clearing them in autumn breaks the cycle.

Questions people ask