Cause Several different fungal cankers on hawthorn have been sent into the OSU Plant Disease Clinic where the diagnoses have included Cytospora spp., Phoma sp., Phomopsis sp., Sphaeropsis sp., and the oomycete Phytophthora syringae. Cytospora canker was diagnosed most often and this fungus is a vigorous wound invader (including wounds from sunscald and winter injury). It can ramify throughout the bark and cambium and to a lesser extent the tree's structural wood. Infection by the fire blight bacteria can also result in branch cankers.
Symptoms All of these diseases are noticed either in the spring when buds fail to sprout or in summer when dead limbs become apparent following heat or water stress. Symptoms include branch dieback as well as cankers and vascular necrosis. Leaves above stem infections droop and discolor and may remain attached, sometimes through the winter. These flag shoots are caused by stem invasions that girdle or nearly girdle the stem immediately below the flag. Cankers are sunken due to dead tissue in the middle and living tissue at distinct margins that continue to grow. Some of these fungal canker surfaces develop scattered raised pinhead-size pimples (pycnidia) that push through the bark.
Cultural control
- Remove dead branches, cutting well below any visible discoloration in the bark. Infected material should be removed and destroyed if possible.
- Take special care to avoid wounding trees during the growing season.
- Avoid excessive nitrogen fertilization, nitrogen applications late in the growing season, or late-season cultivation or irrigation.
- Paint the southwest side of tree trunks with white latex paint to avoid winter injury.
- Pruning wound treatments with sealants or paints have been effective when used within 24 hours of the pruning cut. These products include B-Lock, Spur Shield, or Vitiseal. See labels for details.