How to spot them, why they happen, and the simple steps that actually help

When I spotted rust on my Hollyhocks over the weekend, I didn't just tackle to problem I decided to sit down and help you tackle any you might be encountering too

Fungal diseases are one of those things every gardener bumps into sooner or later. They’re part of gardening life, especially in the UK, where our weather swings between damp, mild, and unpredictable. The good news is that most fungal problems look worse than they are, and with a bit of understanding you can manage them well enough to keep your plants healthy and blooming.

Here are the most common fungal diseases you’re likely to see in flowering plants, what causes them, and the practical steps that make the biggest difference.

1. Rust

Rust shows up as yellow spots on the top of leaves and rusty orange pustules underneath. Hollyhocks are famous for it, but many ornamentals can get it.

Why it happens

Rust thrives in warm, damp conditions, spreads on the wind, and overwinters on old leaves.

What to do

• Remove affected leaves early
• Don’t compost them — bin or burn
• Water at the base, not over the foliage
• Improve airflow around plants
• Clear away debris at the end of the season

Rust is almost inevitable on some plants, so the aim is management, not perfection.

2. Powdery Mildew

A very common one — it looks like someone dusted your plants with flour. You’ll see a white, powdery coating on leaves, stems, and buds.

Why it happens

• Dry roots + humid air
• Crowded planting
• Stress from drought or poor nutrition

What to do

• Water deeply at the base
• Space plants for airflow
• Remove badly affected leaves
• Mulch to keep soil moisture even

Most plants bounce back once conditions improve.

3. Botrytis (Grey Mould)

Botrytis looks like soft, collapsing growth covered in fuzzy grey mould. It often appears on damaged or fading flowers.

Why it happens

• Cool, damp weather
• Poor ventilation
• Dead or dying plant material left in place

What to do

• Remove affected flowers and stems
• Improve airflow
• Avoid overhead watering
• Keep the area tidy and free of spent blooms

Botrytis is opportunistic — tidy plants suffer far less.

4. Black Spot (mainly on roses)

Black spot causes round black patches on leaves, followed by yellowing and leaf drop.

Why it happens

• Wet leaves
• Poor airflow
• Spores overwintering on fallen leaves

What to do

• Pick off affected leaves
• Don’t compost them
• Water at the base
• Mulch well in spring to bury overwintered spores
• Choose resistant varieties where possible

It won’t kill a rose, but it can weaken it — good hygiene helps enormously.

5. Downy Mildew

Often confused with powdery mildew, but this one causes yellow patches on top of leaves and greyish mould underneath.

Why it happens

• Prolonged damp
• Cool nights + warm days
• Poor air circulation

What to do

• Remove affected leaves
• Improve airflow
• Water early in the day
• Avoid wetting the foliage

Downy mildew can be persistent, but removing infected growth early helps slow it down.

6. Leaf Spot Diseases

A broad group of fungi that cause brown, black, or purple spots on leaves. Many ornamentals get them, especially in wet summers.

Why it happens

• Splashing water spreads spores
• Overcrowding
• Old infected debris

What to do

• Remove affected leaves
• Water at soil level
• Thin plants for airflow
• Keep the area clean

Most leaf spots are cosmetic rather than catastrophic.

Why fungal diseases happen (the simple version)

Fungi love:

• Moisture
• Still air
• Stressed plants
• Old, infected debris

If you can reduce those four things, you’ll reduce most fungal problems.

What actually helps (and what doesn’t)

Helps:

• Good spacing
• Watering at the base
• Mulching to keep soil moisture even
• Removing infected leaves promptly
• Cleaning tools
• End‑of‑season tidy‑ups
• Choosing resistant varieties

Doesn’t help much:

• Panicking
• Overusing fungicides (they’re often unnecessary and not very effective outdoors)
• Trying to eliminate fungi entirely — it’s impossible

The honest truth

Fungal diseases are part of gardening. They come and go with the weather, and even the most experienced growers deal with them every year. The aim isn’t to create a disease‑free garden, it’s to keep plants healthy enough to grow, flower, and bring you joy.

A little knowledge and a few simple habits go a long way.