Brown bumps demystified | In the Green Room

To Jess,

I noticed this weird brown knobbly stuff on the leaves and stems of a wattle plant in the bush near my house.

The more I looked the more of it I noticed on other plants around the place.

I can’t seem to pull them off or anything and I’m worried it may spread to the wattles in my own yard or the rest of my plants.

Do you know what it is, and should I do anything about it?

Samantha, Dudley Park

I was always curious as to what these were, and the explanation is quite an interesting little bit of biology!

Those distinctive, clay-brown lumps are caused by a fungus call Acacia gall rust.

They are found on over 100 different Acacia species, and several Fabacea, too. 

When the rust infects the stem or leaves of the Acacia plant, it produces a chemical which causes the plant to grow these ugly brown knobs known as ‘galls’.

They are actually part of the plant itself, which is why they can’t be scraped off.

The galls, in turn, often provide a place for insects like moths or wasps to distribute their larvae, as they like to eat the surface or inside of the galls.

The bad news is there’s no way to actually treat the galls.

While Acacia gall rust can cause problems for plant health – such as reduced seed production, leaf canopy, and increased sensitivity to drought – it doesn’t usually kill the plants, and I’d say the biggest problem is if you consider the galls to be unsightly.

In that instance, it’s easy enough just to cut them off using a good sharp pair of secateurs. 

Awesome Acacias

Winter is a great time to plant Australian natives in your garden.

There are a range of lovely Acacia species available commercially, but just keep in mind that all are not created equal: a lot of east coast varieties are considered pests over here in WA, so try to look for local natives when you can.

I recommend: Acacia saligna; Acacia truncata; Acacia huegelii; Acacia sessilis; or Acacia rostellifera.  

Do you have a gardening question for Jess? Send your queries to jess.cocker ill@fairfaxmedia.com.au