Sudden oak death (Phytophthora ramorum)

Synonyms: Phytophthora ramorum

Common Names: SOD, ramorum blight, ramorum leaf blight, ramorum dieback, sudden larch death

Description:

Sudden oak death is a serious plant disease caused by Phytophthora ramorum, a fungus-like organism called a water mold [1][2]. On oaks and tanoaks it causes deadly "bleeding" cankers on the trunk, and on many shrubs ? like rhododendron, viburnum, and lilac ? it causes leaf blight and twig dieback [1][3]. MISIN tracks it because it has killed huge numbers of trees in California and Oregon, it can attack more than 100 kinds of plants, and it is a federally regulated pathogen that travels on infected nursery plants [1][2].

Identification:

Sudden oak death is a disease, so you identify it by its effects, and those effects come in two very different forms [1]. On oaks and tanoaks, it makes deadly cankers low on the trunk that "bleed" ? a dark reddish-brown or black sap oozes from cracks in the bark and stains the trunk, and the tree's leaves may turn brown rather suddenly [1][3]. On shrubs and smaller plants ? such as rhododendron, viburnum, lilac, camellia, and Pieris (andromeda) ? it instead causes brown or black leaf spots and blotches, often at the leaf tip, along with wilted and dying shoots; this form is called ramorum blight [1][3]. These leafy hosts rarely die but are the main way the disease moves on nursery plants [1].

Signs are easiest to see in cool, wet weather, especially spring, when the water mold is most active; trunk bleeding can be seen year-round [3]. Be especially alert to recently purchased rhododendrons, viburnums, lilacs, or camellias that develop leaf blight, since nursery plants are the main pathway into new areas [1].

At-a-glance cues:

1. Dark reddish-brown or black sap "bleeding" from cankers low on oak or tanoak trunks [1][3]
2. Brown or black leaf spots and blotches, often at the leaf tip, on rhododendron, viburnum, lilac, or camellia [1][3]
3. Wilted, dying shoots and twig dieback on those shrubs [3]
4. Symptoms worst in cool, wet weather, especially spring [3]

Habitat:

Sudden oak death lives in and on plants, so its "habitat" is its enormous host range ? more than 100 kinds of plants [2]. It causes deadly trunk cankers mainly on oaks in the red oak group and on tanoak, while a long list of shrubs and trees ? rhododendron, viburnum, lilac, camellia, Pieris, bay laurel, and many others ? get the leaf-and-shoot blight form [1][3]. The water mold thrives in cool, moist, rainy conditions, so the disease is most damaging in wet coastal climates [3].

The origin of Phytophthora ramorum is unknown; it emerged as a new disease in the 1990s and has killed large numbers of oaks and tanoaks in the coastal forests of California and Oregon [1][2]. In most of the rest of the United States, including the Midwest and East, it is not established in forests and turns up mainly in plant nurseries ? so in the MISIN region it is a watch species, with the nursery trade as the pathway to guard [1][2].

Reproduction:

Sudden oak death spreads as a water mold ? it makes spores that move in water. Over short distances, wind-driven rain and splashing water carry spores from infected leaves and bark to nearby plants [1][3]. The dangerous long-distance jumps, though, are caused by people: the disease travels on infected nursery plants, on cuttings and other plant parts, in contaminated soil and potting mix, and on dirty tools, boots, tires, and equipment [1][3]. This is why a backyard far from any wild outbreak can still get the disease ? on a single infected nursery shrub [1].

There is no "seed-set" window to target as with a weed. The practical steps are about prevention and not spreading it: buy healthy plants, do not move infected plants or soil, and clean tools and footwear (see Management) [1][3]. Because it is a regulated pathogen, a suspected infection should be reported and left in place for officials to sample, not dug up and moved [1].

Impact and Damage:

Sudden oak death is not a danger to people or pets ? it harms plants [1]. For a homeowner, it can kill yard and woodland oaks and disfigure or kill ornamental shrubs like rhododendron and viburnum, and a dead oak becomes a removal cost and a hazard [1]. There is no cure once a plant is infected [1].

In forests, the disease has been devastating where it is established: it has killed very large numbers of tanoaks and oaks in coastal California and Oregon, opening the canopy, increasing dead wood and fire fuel, and reducing the acorns that wildlife depend on [1][2]. Because Phytophthora ramorum attacks so many host plants and spreads readily in the nursery trade, it is treated as a high-risk pathogen nationwide [2]. It is a federally regulated and quarantined pathogen, and the movement of host nursery stock out of regulated areas is restricted to keep it from reaching new forests [1][2].

Similar species:

Oak wilt (caused by the fungus Bretziella fagacearum) is another lethal disease of oaks that can be confused with sudden oak death. Oak wilt usually shows as rapid leaf wilting and browning from the top of the canopy down, rather than the dark "bleeding" cankers low on the trunk that mark sudden oak death.

Bacterial wetwood (slime flux) also causes dark, wet ooze to seep from tree trunks, but it is a long-running, generally non-lethal bacterial seepage ? often with a sour smell ? rather than the lethal, spreading canker of sudden oak death.

Other Phytophthora bleeding cankers (such as those caused by Phytophthora cinnamomi) produce very similar oozing trunk cankers on oaks and other trees, and they can only be told apart from sudden oak death by laboratory testing.

Monitoring and rapid response:

Signs to watch for:

1. Dark reddish-brown or black sap bleeding from cankers low on oak or tanoak trunks [1][3]
2. Brown or black leaf spots and dying shoots on rhododendron, viburnum, lilac, or camellia ? especially on recently bought nursery plants [1][3]
3. Symptoms appearing or worsening in cool, wet weather, especially spring [3]

What you can do:

1. Buy healthy plants from reputable nurseries, and inspect new rhododendrons, viburnums, lilacs, and camellias for leaf blight before planting [1].
2. Do not move plants, cuttings, or soil from areas known to have the disease, and clean soil off tools, boots, and tires after working in wet woods or gardens [1][3].
3. If you suspect sudden oak death, do not dig up or move the plant. Because it is a regulated pathogen, leave it in place, photograph it, and report it (below) so officials can collect a sample and confirm it [1].
4. Land managers and crews should follow clean-equipment practices between sites (clean, drain, dry, and disinfect gear), watch nursery and planting pathways and high-risk hosts, and coordinate with the state plant-health program on any suspected find [1][3].

When to call a professional:

1. Any suspected sudden oak death ? contact your state department of agriculture, plant-health program, or extension office before disturbing the plant, since confirming it requires laboratory testing and it is federally regulated [1][2].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. USDA Animal and Plant Health Inspection Service (APHIS). Phytophthora ramorum (Sudden Oak Death). aphis.usda.gov
2. USDA National Invasive Species Information Center. Sudden Oak Death. invasivespeciesinfo.gov
3. American Phytopathological Society. Sudden Oak Death, Sudden Larch Death, and Ramorum Blight. apsnet.org

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