Green Horizons

The close of the summer season in Missouri often confounds the diagnosis of oak wilt with other things that cause similar symptoms. Red oaks (most notably pin oaks) that are browning from leaf tips inward with the classic green margin might cause one to think oak wilt. But, given that extended summer heat seems to deactivate the fungus in the above ground portions of the tree and also that in late summer it is hard to grow the fungus in the lab (which is needed for positive identification) the question arises: Is it oak wilt or something else?

In general, oak wilt will cause pin, shingle, and other red oaks to drop leaves in late spring and total tree mortality will soon follow in as little as six weeks and certainly by the end of the following spring. Trees affected by bacterial leaf scorch will retain browned leaves at season’s end and it may take up to five growing seasons to kill the tree. Abiotic scorch comes on later in the summer to early fall with uniform browning of the entire canopy. Scorch will be visible on a number of trees rather than one or two, and the severity will depend on seasonal drought conditions. This type of scorch is not lethal and the trees should flush as normal next spring.