Sudden Oak Death

The plant pathogen Phytophthora ramorum, commonly called Sudden Oak Death (or SOD), is a water mold in the Oomycota class. I reported on Sudden Oak Death a couple of years ago as part of a series on oak trees. Most Phytophthora species are soil-dwelling, root pathogens, but P. ramorum is a leaf pathogen. Spores from the leaves and twigs of infected trees spread to an oak through water, wind-driven rain, plant material, or human activity. Once the spores are on the oak, microbes use natural openings to access bark tissues. The microbes kill cells, causing bark cankers, clogged water and transport systems, leaf die-off, and ultimately death of the tree. Take a look at SOD zoospores: http://youtu.be/rTo6UwnZiQg

Fascinating to me is that P. ramorum has a wide host range (including tanoaks, and non-oaks, such as California bay laurel, Rhododendron, and Camellia), but these hosts are not killed by the pathogen. Oaks, on the other hand, are considered terminal hosts; when they encounter the pathogen, they may become infected very quickly in the right conditions. According to the IPM, the best predictor of the disease is the presence of a California bay laurel.

Since there is no cure, prevention is the best way to manage Sudden Oak Death. Purchase non-infested nursery plants from a reputable dealer; remove infected oaks following local ordinances; remove non-oak hosts (such as California bay laurel); and dispose of infected plant debris properly (leave on the property as mulch or compost). The IPM website lists using phosphonate fungicide treatments (Agri-Fos) as a preventative measure. The treatment must be made on healthy trees, and repeated every year or two, so it is recommended for high-value trees.