Sudden oak death (SOD)
Phytophthora ramorum is the oomycete plant pathogen known to cause the disease sudden oak death (SOD). Oomycetes have the same lifestyle as fungus, but are distinctly different. Another Phytophthora species caused the the potato blight that lead to the Great Famine of Ireland in the 19th century.
Above: Bole failure in a coast live oak (Q. agrifolia). P. ramorum spores muster in California bay laurel reservoirs. Bay trees don’t suffer significant disease from ramorum. When rained on, the spores germinate into swimming zoospores that travel to oaks by water splash and wind-driven spray. On the oak, they invade the phloem which is the thin layer just inside the bark. Phloem is rich in sugar and is somewhat edible to humans. Once infected all the way around the circumference, the tree starts to die and becomes vulnerable to true fungi and insects, including cramp balls and ambrosia beetles, which invade the xylem which is the wood part of a tree. The tree can keel over while still having green leaves as in the photo above, or it can be dead-standing.
When a large tree goes over, it can break branches of neighboring trees as shown above.
Above: Courtesy of your federal government, here are microscope photos of the dreadful creatures. “Phytophthora ramorum (ex-type CPHST BL 55G) asexual phase (a–h): (a) sporangia in sporangiophores, (b–d) semipapillated sporangia with short pedicels; (e–h) chlamydospores, (f) terminal, (g, h) intercalary and lateral; photos by Gloria Abad, USDA-APHIS-PPQ.” Oomycetes are neither plant, animal, nor fungus. They’re in the grab-bag, everything-else kingdom called Protista.
Here’s a dead-standing coast live oak (Q. agrifolia) at the washout on Bear Creek Road. The leader stems have broken off, with the primary scaffold branches remaining, so far. Taken with an extreme wide angle lens, the left area of the image is front-lighted while the right area is back-lighted.
Early buds of a California black oak (Q. kelloggii)
Craggy. Click the photo for the definition given in the 1933 Oxford English Dictionary.
