I was told by a local arborist that Arbutus ‘Marina’ in the area are all dying of some disease. I have 5 that were adversely affected or died over last winter but at the time I thought this was due to a cold winter. Is there a disease going around, or is it safe to get some new plants of this species and try again?
I know that the City of Seattle has listed Arbutus ‘Marina’ as an alternative to the disease-prone Arbutus menziesii, because it is supposedly less susceptible to the fungal and bacterial problems affecting our native madronas. However I imagine it is not immune, and perhaps what you have observed indicates that conditions for disease development were just right this year. You might want to talk to the city arborist and ask how the city’s plantings of this tree are doing. You could also talk to tree expert Arthur Lee Jacobson to see if he has any thoughts on this.
University of British Columbia Botanical Garden and University of Washington both have information about diseases that affect the genus Arbutus. A commercial operation, San Marcos Growers, comments on disease in Arbutus ‘Marina:’
“In the initial release of Arbutus ‘Marina’ the Saratoga Horticultural Foundation noted that the tree was a fairly pest free and disease resistant; we in fact started calling it the garden tolerant Madrone. Through years of growing and gardening with Arbutus ‘Marina’ we have discovered that it in fact has a few pests that feed upon it and a few diseases that can cause it harm. The new growth is occasionally attacked by aphids, which will cause the associated sooty mold. Ant control seems to be the best preventative for this. On occasion we also see Greenhouse thrip and soft scale. For these pests, the pest pressure on our garden plants have never reached a threshold that required us to treat with a pesticide. We treat our nursery plants as necessary to assure that they are pest free.
For many years we thought that Arbutus ‘Marina’ was resistant to plant diseases but in conditions that promote the disease we have found that Arbutus ‘Marina’ is susceptible to at least 2 plant pathogens. Phytophthora root rot is the most serious of these diseases but when planted correctly in well drained soils and not over irrigated this disease has not appeared to be a problem. In most cases we have seen this problem when the tree is being overwatered or has been planted too deep or in compacted soil. Unfortunately this disease seems to eventually kill the infected plant.
More recently we have seen several established trees losing lower leaves and small twigs inside the canopy. A laboratory analysis has proved this to be caused by the fungus Botryosphaeria. This fungus is opportunist in nature and usually only attacks plants that are under environmental stress of some form. A tree infected with Botryosphaeria may appear vigorous and healthy at its growth tips yet have twig die out within the canopy. The information for controlling this disease on ornamentals is limited but the general consensus is to reduce the stress to the plant and avoid wounding the plant unnecessarily. When pruning infected branches, do so well below all discolored wood and dispose of dead plant material. Clean pruning tools between cuts with a dilute solution of household bleach (1 part bleach to 9 parts water).”