Snapdragon Rust: How to Prevent & Treat Rust on Snapdragons

Snapdragon Rust: How to Prevent It and What to Do If You Spot It

Snapdragons are one of my favourite flowers to grow for cutting, and starting them early can give you beautiful, long florist-worthy stems. But there is one problem worth knowing about: snapdragon rust.

If you’ve ever noticed yellow patches developing on your snapdragon leaves, followed by rusty brown pustules — particularly on the undersides — you may be dealing with Puccinia antirrhini, the fungus responsible for snapdragon rust.

The good news is that there are quite a few things we can do to reduce the risk.

What is snapdragon rust?

Snapdragon rust is a fungal disease caused by Puccinia antirrhini and specifically affects snapdragons (Antirrhinum).

One of the important things to understand is that you don’t necessarily introduce rust through bad gardening practices.

The spores can be carried through the air. So even if your plants were perfectly healthy when you planted them, rust can still arrive.

What we can control is the environment we’re growing our snapdragons in and make conditions less favourable for the disease to establish and spread.

Rust is favoured by abundant dew, cool nights and warmer daytime temperatures.

What does snapdragon rust look like?

Keep an eye out for:

• Small yellow or pale spots on the leaves

• Yellowing or chlorotic patches on the upper surface

• Brown or rusty-coloured pustules, often particularly noticeable underneath the leaves

• Increasing numbers of affected leaves as the infection develops

Rust diseases produce huge numbers of spores, which is why early detection matters. If something looks suspicious, turn the leaf over. The underside can make what’s going on much more obvious.

How to prevent rust on snapdragons

You can’t completely guarantee that your snapdragons will never get rust. The aim is to reduce the conditions that favour infection and slow its spread if it does arrive.

1. Give your snapdragons good airflow

This is a big one. It is very tempting to squeeze another plant into a bed — particularly when you’re growing flowers for cutting and want to maximise every square metre.

But densely packed plants reduce air movement and allow moisture to remain on the foliage for longer.

Give your plants adequate spacing and avoid allowing them to become one enormous impenetrable mass of foliage.

2. Keep the leaves dry

Where possible, water snapdragons at soil level rather than watering over the foliage.

Rust fungi need moisture for infection, so repeatedly wetting the leaves creates more favourable conditions.

Drip irrigation or carefully watering around the base of the plants is preferable to regularly soaking the entire crop.

3. If you have to overhead water, do it early

Sometimes overhead watering is unavoidable.

If that’s the case, water early enough for the foliage to dry quickly rather than leaving wet plants sitting overnight.

4. Ventilate your greenhouse or polytunnel

If you’re growing snapdragons undercover, ventilation is really important.

High humidity and condensation mean foliage can remain wet even when you haven’t watered it.

Open vents and doors when conditions allow and encourage plenty of air movement through the crop.

This becomes particularly worth watching whenever you’ve got established plants sitting undercover during periods when condensation and prolonged leaf wetness are common.

5. Keep checking your plants

Don’t wait until half the crop is covered in rust before doing something about it. Make a habit of looking at the foliage as you’re cutting, watering or tending your plants.

And remember to look underneath the leaves.

Spotting an infection early gives you a much better opportunity to remove affected material before large quantities of spores are produced and dispersed.

What should you do if you find rust?

Remove affected material promptly.

With a small, localised infection, remove infected leaves and get them away from your growing area. If a plant is badly affected, removing the whole plant may be the more sensible option.

Don’t leave infected leaves lying around underneath your plants.

Rust does not survive in the soil itself, but can survive in plant refuse. It can also persist on living snapdragon plants, So good crop hygiene matters.

Can you completely prevent snapdragon rust?

Unfortunately, no.

You can do everything correctly and spores can still arrive. But you can make life considerably harder for it.

Space your plants. Keep the foliage as dry as you reasonably can. Ventilate undercover crops. Water early. Check the undersides of leaves. And act quickly if you spot those characteristic rusty pustules.

Because, as with most problems in the cutting garden, dealing with one suspicious leaf is considerably easier than dealing with an entire bed of them.