Sudden oak death

What is sudden oak death?

Sudden oak death is caused by Phytophthora ramorum, a water mold. Water molds act like fungi but are a separate group of organisms. APHIS says it can infect more than 100 plant species. It has killed millions of trees, mostly tanoak and coast live oak, in coastal California and southern Oregon.

On oaks, it causes cankers on the trunk that bleed dark ooze. The cankers can girdle and kill the tree. On many shrubs, such as rhododendron, viburnum, camellia, lilac and pieris, it causes leaf spots and shoot dieback instead of death. These shrubs are a big concern because they are sold and shipped by nurseries and can carry the disease to new areas.

Eastern oaks are at risk too. Oklahoma State calls northern red oak and pin oak highly susceptible, and a 2004 Penn State article says inoculated red and pin oak seedlings could be infected. Pennsylvania's first confirmed case was a bonsai camellia shipped to a southeastern Pennsylvania home in 2004. It was kept indoors, and officials said there was virtually no chance for it to escape. A Penn State plant disease lab scientist noted the pathogen grows well at about 68°F and needs free water to cause disease.

Plants it infects: tanoak, coast live oak, California black oak, northern red oak, pin oak, rhododendron, viburnum, camellia, common lilac, pieris, mountain laurel, European beech.

Symptoms and signs

• Bleeding cankers on oak trunks with amber, reddish or black ooze that dries to a stain
• Discolored dead tissue under the bark, often bordered by black lines
• Light brown to black spots and blotches on leaves of shrubs like rhododendron
• Shoot tips that wilt into a shepherd's crook and die back
• Twig dieback on shrubs and small trees
• On trees, a crown that turns from green to pale yellow to brown within a few weeks

Sudden oak death identification: what to look for

• Symptoms: On oaks and tanoak, stem cankers bleed black, reddish or amber ooze that stains the bark. Under the bark, dead tissue is usually bordered by black zone lines (Oklahoma State).
• Symptoms: On shrubs, light brown to black leaf spots and blotches, and shoot tips that wilt into a shepherd's crook and die back.
• Damage: Trees may live one to several years after cankers form. Once the crown starts dying, leaves go from green to pale yellow to brown within a few weeks.
• Spores: Spores spread from plant to plant in rain splash and wind. The pathogen also moves in infected plants, contaminated soil and water.

Disease cycle

The water mold forms spore structures (sporangia) on infected host plants. Rain splash and wind carry spores to nearby plants, and it needs free water to cause disease. It travels farther in infected nursery plants, in soil on tires, tools and shoes, and in water. On oaks the trunk cankers do not appear to spread into the roots. A Penn State scientist noted it grows well at about 68°F.

When to look: Look during and after wet spring weather, and inspect any new nursery plants when they arrive.

Sudden oak death look-alikes

A few things get mixed up with it:

• Other Phytophthora cankers: Oklahoma State says other Phytophthora species can cause the same cankers and black zone lines. A lab test is needed.
• Common leaf spots, sunburn and leaf scorch: Can look like the leaf blotches on rhododendron and other shrubs. Only a lab can tell.

How to manage sudden oak death

Cultural control

• Inspect new nursery stock on delivery, or before you buy if possible (UW-Madison).
• Keep new plants apart from older plants as long as possible.
• Do not move or remove plants that look sick or are dying, except to collect a lab sample.
• Clean pruning and garden tools between plants. UW-Madison describes a bleach soak.
• Avoid overhead watering. APHIS says the pathogen thrives in wet conditions.

Chemical control

APHIS says there is no cure, and prevention is the best defense. The sources opened for this page do not describe a fungicide program for home landscapes. UW-Madison says a plant that tests positive will be removed and destroyed to prevent spread. If any product is suggested, use only products labeled for this use; the label is the law, and tree work should go to an ISA Certified Arborist with a pesticide applicator license (see pesticide applicator license guide).

Use only products labeled for this disease and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Buy rhododendrons, camellias, viburnums and other hosts from nurseries that follow state and federal inspection rules.
• Do not bring host plants or soil from the quarantined areas in California or Oregon.
• Watch new plantings closely, especially in wet weather.
• Keep irrigation off foliage.

When to call a pro

If you suspect sudden oak death, do not remove or move the plant. Report it first (see below). For an oak with a bleeding canker, get an ISA Certified Arborist to help collect a proper sample and assess the tree.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

Questions about sudden oak death

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