Black spot on banksia leaves

[Phil Rabl]

Over the past 15 years or so there has been a few posts seeking advice about black spots on the underside of banksia leaves. I have been dealing with a big infestation recently, and I have trawled through those posts to seek out the best advice. This is what I have found.

In 2009, Kunzea noted that there are different kinds of black spots. (viewtopic.php?p=5256#p5256)

If the spots are like black spot you can get on roses, visible from the upper surface too, they could well be a fungal disease and a systemic fungicide would be in order.

Black spots on the underside of the leaves
Kunzea said in the same post: If the spots are all on the underside of the leaves, it is more likely they are a kind of scale. They can be killed by rubbing off with a fingernail or something else sharp if you are patient enough. You can also get at them with pest oil/white oil.

IMG_3875.jpeg

The top leaf in this photo is from an unknown species of banksia (let me know if you can identify it). The bottom leaf is from my integrifolia (there is a bit of a story about it below). Looking at these black spots with a strong magnifying device, they certainly are a kind of scale.

In a 2021 post, Grant Bowie said you should definitely get rid of them. If they get in large numbers the leaf will be sucked dry and the branch can die back badly. You can cut back hard to get rid of the bulk of it and grow new growth. Keep the tree in full sun to lessen the possibility of infestation. You could spray with a safe insecticide, and this should reduce populations. Here is a link to that discussion thread. (viewtopic.php?p=286471#p286471)

Which banksias are susceptible?
Grant said he has had this problem on 3 out of 4 of his Banksia integrifolia, but not on the marginata or serrata. Others, me included, have also had bad infestations on integrifolia. But I have recently had a minor infestation on a marginata. The unknown banksia in the above photo is also very susceptible. Like Grant, my serrata have never been affected.

Root problems
In his 2009 post, Kunzea responded to an idea that the problem was in the roots by saying it is known that severe infestations can seriously harm a banksia, but you should be careful in diagnosing the problem as dead roots. If the infestation is severe, the leaves will decline and die and if most leaves are affected, then the health of the plant will decline, and the roots may also die. When the roots are dormant, they might look dead, though are still functional.

Is white oil / pest oil effective?
In my experience, yes. In January 2023, I largely defoliated my badly affected B. integrifolia, and took the opportunity to do a major redesign. As far as I could tell, it had no black spots remaining, but by April some had returned (see above photo). They are a bit hard to scratch off – they seem to slightly bury themselves into the leaf. But within days of spraying with pest oil they come away quite easily (mostly). From now on, I will both spray and remove leaves that develop the spots.

[Rory]

Over the past 15 years or so there has been a few posts seeking advice about black spots on the underside of banksia leaves. I have been dealing with a big infestation recently, and I have trawled through those posts to seek out the best advice. This is what I have found.

Damn. Sorry to hear that. Good post.

I have never really had a problem since dealing with it years ago.
I sat down on all my Banksia and scratched them off, and painstakingly with a somewhat blunt knife on the trunks too.
I spray all my banksia occasionally with Yates Pyrethrum long-life as well. The little suckers do come back from time to time, but then I just scratch them off and spray again…. Haven’t had any problems since.

Often when you browse through Banksia stock at nurseries you’ll see their Banksias covered in them.

[Keels]

I've asked this question before and no one can ever actually provide an answer as to what the spots actually are. Just how best to remove.

My post viewtopic.php?t=28473

[Jhalkhoree]

I’ve have been dealing with this same problem on my Banksia integrifolia for a few years and tried everything others have suggested. However nothing seem to have worked. This year someone mentioned that it could be that the roots are too wet and I tilted the pots. This seems to work so far.

Will the black spots return next year? Will have to wait and see.

Jay