Deciduous dilemma Branch dieback and cankers of deciduous trees

Deciduous dilemma

Branch dieback and cankers of deciduous trees

by Jean Williams-Woodward, Extension Plant Pathologist, University of Georgia

Redbud showing foliage symptoms of vascular streak dieback.

Tree nursery growers and landscapers have seen a lot of branch dieback on deciduous trees in recent years. Several years ago, I conducted a survey of tree nurseries to determine what was causing the dieback, especially on red maples. Bot canker was identified as the primary cause of black lesions that lead to sunken cankers on the trunk and branches of primarily red maples, but it was also seen on tulip poplar, redbud, sweet gum, and others. Infection killed the tops of trees, which resulted in excessive tree culling within nurseries. It is very likely that branch dieback seen in landscape trees, especially those recently planted, may also be due to Bot canker. The fungus, Botryosphaeria spp., is a relatively weak pathogen that can live as an endophyte within trees. Any stress that reduces water pressure within the tree such as drought, root or trunk damage will cause the fungus to grow, colonize tissues, and eventually cause a canker. New infections of injured or open wood can also occur from water-splashed and wind-blown spores produced on the dead branches. However, it is because of the endophytic behavior of Botryosphaeria that fungicide control is so difficult. Nurseries have greatly reduced the incidence of Bot canker by applying fungicides (I recommend propiconazole because it is very systemic) with the adjuvant Pentra-Bark during the fall to winter months. Pentra-Bark helps move the fungicide into and through thin tree bark. It should be used after leaf drop in the fall and prior to leaf emergence in the spring because it will burn leaves. There are other similar adjuvants to Pentra-Bark on the market; they work okay, too. During the growing season Captan, chlorothalonil, or strobilurin fungicides (azoxystrobin, pyraclostrobin, trifloxystrobin, etc.) can be used. With the fungicide program, one grower told me that they have reduced their red maple losses from about 60-70% to around 10%. It is uncertain whether landscape trees would benefit from fungicide applications. The one question that I am repeatedly asked is what is killing the dogwoods. The cause for dieback in dogwoods (Cornus florida) has been harder to pin down. I did not recover a consistent pathogen from dying branches in my survey. Sometimes, I recovered Botryosphaeria or the fungus, Phomopsis (which often follows freeze injury). Sometimes, damage was due to twig borers. The dieback is so common and severe that many nurseries have stopped growing flowering dogwood and are only growing Japanese or ‘Kousa’ dogwood (C. kousa) cultivars. Most often in landscape trees, the dieback seen in dogwoods is due to years of stress such as poor planting site location, mechanical damage to the roots or trunk, dogwood tree borer, and environmental stresses. Probably the issue causing the greatest concern recently is the dieback seen on redbuds. In my survey, I did find Bot canker on some redbud; however, often no pathogen was recovered, which was puzzling at the time. What is now believed to be causing the dieback in redbuds is what we are calling Vascular Streak Dieback (VSD). I am collaborating with pathologists in Tennessee, North Carolina, and Indiana on a yet to be fully described suspected fungal pathogen that is most closely related to a thread-blight pathogen of cacao from southeast Asia. The fungus, Rhizoctonia theobromae, was found in association with killed redbud and maple branches. The fungus cannot be cultured and because of this we have not been able to prove pathogenicity. The fungus has also been identified from red maple, redbud, dogwood, wax myrtle, spicebush, and magnolia. The fungus has not been confirmed in Georgia, although I have seen similar foliage symptoms on redbud as seen in neighboring states. For more information on Vascular Streak Dieback, Janna Beckerman wrote about it in the July 26 Purdue University Landscape Report (https:// www.purduelandscapereport.org/article/vascular-streakdieback-of-redbud-what-plant-pathologists-know-sofar/).