Sudden oak death

Phytophthora ramorum

also called Ramorum blight

A water mold that bleeds oaks to death on the west coast and merely spots the rhododendrons that spread it. The nursery trade is how it travels.

Water mold No cure Airborne sporesRain splashSpread in waterSoil-borne Spreads to neighbors

What you'll see

Dark red-brown or black sap bleeding from the trunk of an oak, often several seeps a meter or two up, with no hole or crack behind them.
A whole oak canopy turning brown over a few weeks, usually years after the infection began.
On rhododendron, camellia, viburnum and pieris: brown leaf blotches spreading from the tip or the stalk, and shoot dieback.
On bay laurel and other foliar hosts, brown-tipped leaves with a diffuse yellow edge — these plants are barely harmed and are what spread it.
Bark cut into showing a dark stained margin between healthy and dead tissue.

Why it happens

A water mold with an unusual double life. On oaks and tanoaks it invades the trunk and kills the tree; on a long list of shrubs and small trees it produces only leaf spots and shoot dieback but sporulates heavily, so those plants are the engine of the epidemic. Spores move in rain, mist, streams and mud, and over long distances on infected nursery plants.

Established in coastal California and Oregon, where wet winters, cool fogs and the presence of bay laurel drive it. Elsewhere it appears as nursery outbreaks on rhododendron, camellia and viburnum, and in Britain and Ireland on larch.

Treating it

There is no cure for an infected oak. Have it assessed and removed if it is a hazard.
Do not move soil, wood, leaf litter or plants out of an affected area.
Prune out and bag infected shoots on rhododendrons and camellias, and destroy a heavily infected shrub rather than nursing it.
Clean boots, tools and vehicle tires before leaving an infested site.
Report it. This is a regulated pathogen in most jurisdictions and the reporting is the point.

Keeping it away

Buy nursery plants from suppliers who inspect for it, and inspect rhododendron, camellia, viburnum and pieris yourself before planting.
Remove California bay laurel from within ten meters of a valued oak in the affected zone; the bay is the spore source, not the victim.
Do not water oaks overhead or irrigate near their trunks.
Avoid wounding oak trunks, and do not prune oaks in the wet season.
Keep soil and mulch away from oak root collars.

Who gets it

Tanoak, coast live oak and black oak are killed; rhododendron, camellia, viburnum, pieris, bay laurel and larch carry it with far less damage.