This is an unusual root disease in several ways:
- It is non-native
- It kills and rots the phloem and cambium, not the wood
- It is very host-specific, infecting only Port-Orford cedar.
Port-Orford cedar has a very limited distribution on the Oregon/California border. It is important also because the wood is extremely valuable. The disease appeared in a nursery in 1923, got into ornamental plantings, eventually reached native range in 1952. Now moving within that range.
Fungus infects fine roots like good cortical rotter, then moves up to bigger roots and even a bit up the stem. Phloem and cambium turn from creamy to dark cinnamon brown. Crowns fade through yellow to red to brown. Trees die quickly, in a few weeks for small trees and a few years for large trees.
Phytophthora lateralis, typical Oomycete. Probably introduced from Eurasia.
The fungus is soilborne and waterborne. Propagules include mycelium in organic matter, sporangia, zoospores, oospores, or chlamydospores. Sporangia germinate to produce hyphae or zoospores (Δ). Zoospores are short-lived and can swim only a few centimeters, but they can be transported in streamwater and attack roots adjacent to streams. The pathogen survives periods unsuitable for growth as mycelium or as oospores or chlamydospores. Chlamydospores may survive up to 7 years in dead organic matter.
Disease can enter new areas when soil is carried on vehicles, animals. Then, movement locally is clearly associated with drainages, downhill. Along streams, likelihood of infection increases with tree size and proximity to the stream (Δ). Larger trees present a larger target and those closest to the stream are most likely to be exposed to inoculum.
Impacts of this disease have been great. It has destroyed the ornamental cedar industry, estimated at up to 1 million dollars annually. Port-Orford cedar is extremely valuable, and timber losses have been estimated at 250 million to nearly 1 billion dollars. Ecosystem effects are also considerable.
At this point, there are not many options for management.
Exclusion: Focus production of Port Orford cedar in areas where the disease is absent and it may reasonably be excluded. Examples are hilltops and flats, where infection is unlikely to come down with water drainage. Various operational details are conducted with this in mind: road construction, hydrology, landings, etc. Vehicles are cleaned before entering the exclusion zone. Restrict entry into areas with Port Orford cedar where practical.
Eradication: If hosts are absent for 3-5 years, the fungus will apparently die out. Kill trees in high hazard sites: along roadbeds, along streams, in potholes, wet areas. Keep these areas free of cedar. Keep spacing of cedar such that the fungus cannot grow from tree to tree.