Banksia brownii plants treated with a range of potassium phosphonate solutions were examined histologically to determine the mechanism of resistance to invasion by Phytophthora cinnamomi. Reaction zones in plants treated with the chemical compartmentalized P. cinnamomi in both bark and woody tissues, similar to acquired host resistance. Compartmentalization corresponded with the CODIT model of decay in trees. Reaction zones analogous to walls 2 and 3 of CODIT were actively rather than passively formed, and were associated with the ray and axial parenchyma, respectively. While the anatomical evidence of resistance in Australian native flora to this pathogen has been reported previously, this is the first report of enhanced compartmentalization responses of a susceptible native species after treatment with phosphonate.