Foliar Blight
Two fungi (Phacidiopycnis washingtonensis and Phomopsis vaccinii) are associated with foliar blight and predominate in different years, likely depending on the climate and whether there has been a cold winter. These fungal species have varying optimal temperatures for growth.
Phacidiopycnis washingtonensis can infect fully expanded and mature leaves of Pacific madrone and the emerging new foliage in the spring. It can also survive in dead, attached leaves and spread into the shoot. In severe cases, the terminal bud is killed and the branch will not refoliate. Interestingly, this species can also infect apples and causes rot in cold storage, possibly indicating that it grows best at cooler temperatures.
The other fungus, Phomopsis vaccinii, is found on many other hosts including blueberries.
Literature
Find more details about the fungi mentioned above and how they were discovered in the publication below:
Elliott, M., Chastagner, G. A., Coats, K. P. Sikdar, P., and Xiao, C.L. (2014). First Report of a New Leaf Blight Caused by Phacidiopycnis washingtonensis on Pacific Madrone (Arbutus menziesii) in Western Washington and Oregon. Plant Disease 98:1741. https://doi.org/10.1094/pdis-05-14-0462-pdn
Management
Both of these pathogens are difficult to manage and more research is needed to understand their ecology and life histories. Both species may be present as endophytes, waiting until conditions are right to grow and cause symptoms. Foliar blight is worsened in conditions of high humidity and the most severe symptoms tend to be in the lower part of the canopy. In urban environments, having good air circulation and reflected heat off of a surface such as a wall or road will help to reduce the damage caused by these fungi.
Branch Dieback vs Blight
Branch dieback is often caused by Botryosphaeria species where leaf blight may be caused by a community of microbes, such as the two fungal species discussed above. Studies about the cause of leaf Leaves can regrow after they are killed off by leaf blight, but leaves that die because of branch dieback will not recover. Generally, the foliage is brown from leaf blight and silver from branch dieback.
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