Rust spots on Acacia phlebophylla

[fyzygy]

What is/causes this? Emergent phyllodes were spotless and bright green a week or two ago. Too much water/nutrient? (We've had an unseasonal amount of rain recently.)

[Freakosystem]

I can see discolouration and necrosis but no pustules. Can you confirm the presence of pustules? And describe them if present?

[fyzygy]

I don't think there are pustules. Some of the older spots have a very light texture to them, but in general they're smooth to the touch and flush with the phyllode surface. The rust spots appear only on this season's (new) growth, and only on this one specimen. 

[Freakosystem]

I don't think they're rust spots but rather necrotic patches, likely caused by a sap sucking insect. Rusts are fungal pathogens and it's unlikely they would target only one branch, lack pustules and be non-gregarious in nature.

 

If I'm correct, the patches will not change in size as the damage is done. The red patches may completely die off but won't get larger. If it is a fungal symptom, the patches will grow over time.

[fyzygy]

the spots seem to grow a little darker on maturity, possibly with a light crust or scab. but they don't seem to get any larger. emergent growth on another branch doesn't have the spots. the garden seems quite biodiverse lately, lots of different insects about. so maybe a beetle could be the culprit.  there were ladybirds on those phyllodes a week or two prior to the emergence of the spots. first sign (?) was more of a rust-coloured blotch on a single phyllode, larger and more irregular than subsequent spots. 

 

1 hour ago, Freakosystem said:

If I'm correct, the patches will not change in size as the damage is done. The red patches may completely die off but won't get larger. If it is a fungal symptom, the patches will grow over time.

Freakosystem, your input is much appreciated!