I have several bull banksias [Banksia grandis] in my yard, and after working out in the garden today I noticed they’ve all got this gritty powdery type of fungus on the leaves.
It looks like grains of salt sitting on the white powder. When you scrape the white bits back there’s brown and yellow patches underneath, like it’s killing the leaves.
There’s an area of bushland nearby with heaps of the banksias and they’re all full of the stuff too. What can I do about it?
Jeff Holmes, Dawesville
Thanks for your call, Jeff. From looking at the photos and your description, it sounds like it could be one of two things: a sap-sucking insect like scale, or a fungus.
Little is known about the pests and diseases affecting banksias, except their notorious enemy, dieback, which is definitely not your issue here.
At first I was more inclined to think it was a fungus because of the “powdery” texture you described, and the round spots of yellow which look similar to other fungus.
However, since you said you can scrape the white parts back, I’m fairly sure it’s a sap-sucker we’re dealing with.
Of course, because sap-sucking insects like scale often secrete a sugary substance, the byproduct of their parasitism, fungal infections can go hand-in-hand (as well as ant infestations).
My suggestion would be to cut off any sections that are very badly infected, disposing of them in the rubbish to avoid further spread through the garden, and then treating the plant.
Treat the remaining areas with a regular spray of white oil, which can be bought from most gardening stores or made up at home using a cup of vegetable oil, a quarter cup of dishwashing liquid to form a concentrate, which should then be diluted into water at a rate of one tablespoon per litre.
This solution should be sprayed twice weekly, and don’t be afraid to apply it heavily, but be sure to avoid spraying it on very hot days: you don’t want to fry the leaves!
Do you have a gardening question for Jess? Call 9550 2409 or send your queries to jess.cockerill@fairfax media.com.au
