If you grow dahlias long enough, you will encounter pests and diseases. That’s not a failure — it’s the joy of gardening.
I grow dahlias on a small flower farm on the NSW South Coast, in an area with high rainfall, high humidity and warm spring–summer temperatures. In 2025 alone, we’ve received 1,199.4mm of rain!! Which means pest and disease pressure here is very real. Add coastal humidity, warm nights and dense growth, and you’ve got ideal conditions for fungal issues and sap‑sucking insects.
Over the years, I’ve had to learn (sometimes the hard way) how to manage dahlias within these conditions — not fight them. This post isn’t about eradication. It’s about observation, prevention and balance.
Below are the three dahlia pests and diseases I manage most closely each season, how they show up on my farm, and what’s actually worked for me.
Powdery Mildew (Fungal Disease)
Powdery mildew is, without question, the most frustrating disease I deal with every single season.
Warm days, cool nights and high humidity create ideal conditions for spores to thrive — and despite what many people think, leaves don’t need to be wet for mildew to appear. Poor airflow alone is often enough.
What damage does it cause?
- White or grey powdery patches on leaves and stems
- Distorted or stunted new growth
- Reduced flowering and overall vigour
- Leaf drop and dieback if left unmanaged
On dahlias, it often starts low in the canopy and works its way up.
Preferred conditions
- Warm temperatures with cool nights
- High humidity
- Dense planting and still air
Living on the coast means these conditions are common — which is why prevention is important for me.
How I manage powdery mildew on our farm
Physical / cultural
- Strip lower leaves early in the season to improve airflow
- Remove infected leaves immediately and dispose of them in the bin — never the compost
- Avoid overhead watering late in the day
Preventative
- Maintain strong soil health to support resilient plants
- Begin preventative eco‑fungicide sprays early and continue through the season
- Encourage beneficial insects — including a native fungus-eating ladybird (see pic above) that feeds on powdery mildew!
Powdery mildew is not something I expect to eliminate — only manage.
Two‑Spotted Mites (Sap‑Sucking Arachnid)
Two‑spotted mites are tiny, sap‑sucking pests that can devastate dahlias incredibly quickly. My second season growing dahlias was a crash course in what happens when early warning signs are ignored.
At first, my plants just looked a little off — slightly mottled leaves, reduced vigour. I brushed it off. Within weeks, mite numbers had exploded. By then, control wasn’t realistic.
I made the difficult decision to cut back every dahlia, destroy the plant material and reset completely.
That experience taught me two lasting lessons:
- Early signs matter
- Natural predators must be introduced before there’s an infestation and must be released several times throughout the growing season to help stay on top of numbers
What damage do mites cause?
- Fine speckling or mottling on leaves (see pic above for mottling)
- Leaves turning pale, silvery or rusty
- Fine webbing on leaf undersides and joints
- Leaf drop, distorted growth and reduced photosynthesis
Both larvae and adults feed on plant tissue.
Preferred conditions
- Hot, dry weather
- Low humidity
- Environments lacking natural predators
Although mites favour dry conditions, stressed plants can be attacked anywhere.
How I manage two‑spotted mites on our farm
Physical / cultural
- Regular inspection of leaf undersides
- Immediate removal of heavily infested foliage
- Consistent watering to avoid plant stress
Natural / biological
- Early and ongoing introduction of predatory mites
- Encouraging biodiversity through companion planting, birds and beneficial insects
Chemical
- Avoided wherever possible due to resistance and impact on beneficial insects
This pest completely changed how I approach monitoring — now I act early.
Thrips (Sap‑Sucking Insect)
Thrips are slender, fast‑moving insects that feed on dahlia leaves, buds and flowers. Because they’re small and quick, they’re often overlooked — but their damage is very distinctive once you know what to look for.
What damage do thrips cause?
- Silvery or streaked leaf surfaces
- Distorted buds and flowers
- Stunted growth
- Black specks of poop on the underside of leaves
Thrips can also act as vectors (transmit) for plant viruses.
What do they look like?
Thrips are narrow, cigar‑shaped insects. Adults are winged and will often swarm when flowers are disturbed. Nymphs are smaller and wingless versions of the adult.
Damage occurs during both the nymph and adult stages.
Preferred conditions
- Mild to warm weather
- Still air and protected growing environments
They spread easily via wind and the movement of infected plants.
How I manage thrips on our farm
Physical / cultural
- Prune and remove badly infested plant parts
- Increase airflow around plants
- Use yellow sticky traps to monitor population levels
Natural / biological
- Encourage beneficial insects and predators
- Maintain diverse plantings rather than monocultures (we use flowering annuals and herbs)
Chemical
- Used only as a last resort and never when pollinators are active
Thrips are also strongly attracted to light-coloured flowers, which can sometimes explain why infestations feel sudden or severe.
If you’re scratching your head wondering where they’re all coming from, take a look beyond your garden beds — your lawn can be harbouring them too. Clover is a big one. I actually love clover in the lawn (more on that another time), but thrips love it just as much.
On our farm, I manage this by regularly mowing the lawn to keep clover flowers cut back, which helps reduce thrips numbers before they migrate into the dahlias. It’s a good reminder that sometimes it’s not what’s growing in your garden that’s causing the issue — but what’s growing right next to it.
Thrips are rarely eliminated entirely — the goal is always population management.
The Biggest Mistake I See Beginners Make
One of the most common mistakes I see is spraying everything in the hope of eradicating pests and disease.
The reality is this: you will always have pests and disease.
The goal isn’t eradication — it’s management.
What I Want You to Take Away
Take a breath.
Observe before reacting.
Encourage predators.
Improve airflow.
Adjust how you water.
Care for your soil.
When your soil is healthy, your plants are healthier — and healthier plants cope far better with pests and disease.