Cankers occur on a great deal of the woody plant material that comes through the Plant Clinic. They are generally caused by secondary, or stress, pathogens, and are only a part of the problem. Cankers are merely a clue that something more is wrong. Refer to issue no. 16 of this newsletter for basics on cankers.
Readers in the northern part of the state, especially the Chicago area, will have noticed canker disease on Russian-olive. This species suffers from many cankers. The principal ones are fungal and are caused by Phomopsis, Lasiodiplodia (Botryodiplodia), Nectria (Tubercularia), and Phytophthora. In Illinois, the most important of these by far is Phomopsis. Unlike other canker fungi, Phomopsis is an aggressive pathogen that can attack and kill vigorous trees. Trees seriously injured by drought, hail, or ice are subject to decline and more rapid spread of the disease. The earlier drought in the Chicago area most likely predisposed many of these trees to infection.
Phomopsis-infected trees often appear ragged, with several dying or dead twigs and branches. Current-season twigs and small branches often wilt and die, with the dead, withered leaves hanging on for some time. Oval-to-elongate, depressed cankers are most evident on the large branches and trunks. Diseased bark on such cankers varies from orange-brown to dark reddish brown. Ridges often form around the canker margins. Branches girdled by the enlarging and encircling cankers wilt and die. The white sapwood beneath the cankers turns dark brown to black and extends beyond the margins. Minute, slightly raised, rounded pustules of the Phomopsis fungus are embedded in the dead, cankered bark.
Avoid all unnecessary bark wounds because they are the pathogen's main avenue of entry. All seriously infected trees showing dieback should be cut off near the ground and destroyed, preferably by burning. More details on this disease can be found in Report on Plant Diseases No. 606.