Addition to the EPPO Alert List of sudden oak death: a new Phytophthora disease of oak in California, US
Since 1995, significant mortality has been observed on oak trees in coastal areas of California, US. Large numbers of Lithocarpus densiflorus (tanoaks), Quercus agrifolia (coast live oak) and Q. kellogii (black oaks) have been killed by a disease complex which was called sudden oak death. So far, the disease has not been observed on other Californian oak species such as: Q. lobata (valley oak), Q. douglasii (blue oak) and Q. wislizenii (interior live oak). Tree mortality has been noted in various forest types, as well as on trees growing at the urban-wildland interface. All age classes of trees are affected from saplings to mature trees over 1 meter diameter. Symptoms vary slightly between tree species. In L. densiflorus, wilted shoots are usually observed as the first symptoms. Older leaves become pale green and 2 to 3 weeks later the foliage turns brown, announcing the death of the tree. On the lower portion of the trunk, a burgundy-red to black sap oozing (bleeding) appears on the bark surface. In Q. agrifolia and Q. kellogii, the earliest symptom is usually the sap oozing. Sunken or flattened cankers are observed beneath the bleeding with a distinctive dark red canker margin in the bark and outer sapwood. Apparently, no root symptoms are observed on diseased trees. In June 2000, a Phytophthora species was isolated from dying trees. Based on morphological characteristics and nucleotide sequence of the internal transcribed spacer region (ITS) of the rDNA, it could not be assigned to any known Phytophthora species (the closest relative being P. lateralis) and hence was considered as a new Phytophthora species. Koch’s postulates were completed by inoculating seedlings and mature trees. However, it is still not clear whether Phytophthora alone can kill mature trees. Other pathogens and pests, as Hypoxylon thouarsianum, and Pseudopityophthorus pubipennis (bark beetle), Monarthrum scutellare and M. dentiger (ambrosia beetles) are commonly present on diseased trees. In addition, it may be possible that unfavourable environmental conditions (e.g. water stress) could also play a role in the disease. So far, this new Phytophthora species is restricted to the following Californian counties: Marin, Monterey, Napa, San Mateo, Santa Cruz, Sonoma (central coastal areas of California). It appears that the pathogen is favoured by cool wet conditions, such as the ones prevailing in the coastal areas of California. In culture, optimum growth was obtained around 20 °C. Like other Phytophthora, the pathogen is likely to be transported in infected plants or plant parts, and soil. As susceptible oak species also grow in Oregon, the Oregon Department of Agriculture and the Oregon Department of Forestry have recommended not to transport oak wood (firewood especially, but also seedlings, logs, bark products and acorns), and soil from infested areas (in particular, vehicles, shoes, bicycles have to be cleaned).
The origin of this new disease remains unknown. It is not known whether it has been introduced or if changes in environmental conditions have favoured the emergence of an already existing pathogen. The mode of transmission of the disease from tree to tree is not known. Research has been initiated on this new Phytophthora to better understand its etiology, biology and epidemiology, as it is felt that it could present a serious threat to oaks in California and in other parts of USA where susceptible oak species are present. European oaks are not known to be susceptible, but any damaging oak disease is a threat to the EPPO region.
Sudden oak death
Why: Sudden oak death came to our attention as significant tree mortality has been observed on several oak species in California (US). Recently, a new Phytophthora species has been found associated with the disease and is considered as the primary causal agent. However, other factors might be involved in the disease (such as secondary pests: Hypoxylon thouarsianum, Pseudopityophthorus pubipennis (bark beetle), Monarthrum scutellare and M. dentiger (ambrosia beetles) and unfavourable environmental conditions).
Where: USA: central coastal areas in California (counties of Marin, Monterey, Napa, San Mateo, Santa Cruz, Sonoma)
On which plants: Lithocarpus densiflorus (tanoaks), Quercus agrifolia (coast live oak) and Q. kellogii (black oaks). These oak species are native to California.
Damage: Symptoms vary slightly between tree species. In L. densiflorus, wilted shoots are usually observed as the first symptoms. Older leaves become pale green and 2 to 3 weeks later the foliage turn brown, announcing the death of the tree. On the lower portion of the trunk, a burgundy-red to black sap oozing (bleeding) appears on the bark surface. In Q. agrifolia and Q. kellogii, the earliest symptom is usually the sap oozing. Sunken or flattened cankers are observed beneath the bleeding with a distinctive dark red canker margin in the bark and outer sapwood.
Transmission: Infection would occur through zoospores, sporangia and chlamydospores. As for other Phytophthora, it is likely that the disease can be transmitted by infected plants and soil. However, it has also been observed that sporangia of the fungus are deciduous which opened the possibility that they could be transported by air currents but this has not been demonstrated. Bark beetles and ambrosia beetles are commonly found on diseased trees but their potential role of vectors has not been studied yet.
Pathway: Plants for planting, wood, bark of L. densiflorus (tanoaks), Q. agrifolia (coast live oak) and Q. kellogii, soil from areas where the disease occurs.
Possible risks: Oaks are important forest and amenity trees in the EPPO region, and in USA significant oak tree mortality is observed. However, there is no data on the susceptibility of European oak species to the disease. From experience with other Phytophthora diseases, control is difficult in practice. As a consequence of tree mortality, it was felt in USA that the disease could also have a negative impact on the biological diversity of forests and lead to environmental problems (enhanced fire risk and damage to water catchments). More data is needed on the identity, biology, host range, geographical distribution and epidemiology of the pathogen.
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EPPO RS 2001/002
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Panel review date: -
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Entry date 2001-01
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Sources
INTERNET
University of California Sudden Oak Death Research Team Updates: http://himalaya.cnr.berkeley.edu/oaks/
University of California Cooperative Extension Sudden Oak Death: http://cemarin.ucdavis.edu/index2.html
Oregon Department of Agriculture Sudden Oak Death Alert: http://www.oda.state.or.us/Information/news/Sudden_Oak_Death.html
NAPPO Pest Alert: http://www.pestalert.org
Observations and comments on oak and tanoak dieback and mortality in California by Tedmund J. Swiecki (2000-10-19): http://www.phytosphere.com/tanoak.html