Hi David,

It does sound like your Acacia cognata has received too much moisture. They love to be in a well drained spot and the plant will soon let you know when its roots are too wet. At this stage, you could apply a product called AntiRot Phosacid Systemic Fungicide. It works as a preventative as well as a curative fungicide and has the ability to boost the plants defence mechanisms giving it the ability to prevent disease taking hold. It is applied as a foliar spray and should be applied every 4 - 6 weeks until conditions improve. You may need to lessen the amount of water that you are giving the plant and ensure that the area is well drained.