Phytophthora cinnamomi is one among the most destructive species of Phytophthora associated to the decline of forestry, ornamental and fruit species, as well as of some 900 other woody perennial plant species (Ferraris et al. 2004). Commercial enterprises such as agricultural projects and related plant industries are negatively impacted by the impact of the P. cinnamomi infection. In Italy P. cinnamomi infections are being reported with increasing frequency: first on chestnut coppices and Rhododendron spp. in plant nurseries and more recently on chestnuts (causing ink disease) and Chamaecyparis in nurseries and on avocado, oak, walnut and highbush blueberry (Vaccinium corymbosum) in nurseries and the field. The ability of P. cinnamomi to infect oaks has led to speculation that P. cinnamomi is involved in the severe decline of oaks in the general Mediterranean region (Robin et al. 2001). The pathogen also causes significant damage on the African continent and is highlighted as the most damaging disease in South African avocado orchards, where is causes root rot and heavily reduces crop yields (Bezuidenhout et al. 1987).
Just as concerning is the impact that P. cinnamomi may have on native species. According to Rudman (Undated) 181 plant species have so far been recorded as hosts for P. cinnamomi in Tasmania, Australia. At least 39 of Tasmania's threatened plant species are susceptible to P. cinnamomi and it is possible that native species may be rapidly killed and unable to regenerate in infected areas. As is the case in other areas there is considerable variation in response to infection by P. cinnamomi amongst host species, some showing resistance and some extreme susceptibility.
P. cinnamomi is causing and has the potential to cause significant ecological damage in native North American biomes from California to the Appalachian mountains, impacting on ecosystems as diverse at the Sierra Nevada desert and the Appalachian forests. A scientific study of the recent mortality of Ione manzanita- a rare, endemic, evergreen shrub restricted to Ione formation soils in the foothills of the Sierra Nevada, California, (Arctostaphylos myrtifolia) found that the cause of mortality was due to the P. cinnamomi pathogen. The pathogen which causes wilting, foliage desiccation and root necrosis in native plants is believed to have a significant impact on the conservation of the already threatened A. myrtifolia (Swiecki and Bernhardt 2003). P. cinnamomi is also impacting native Californian species in the woodlands around Lake Hodges, where 27% of coast live oaks (Quercus agrifolia), show disease symptoms and are suspected to be infected with the pathogen (Garbelotto, Hüberli and Shaw 2006). All natural oak woodlands in the western United States are potentially at risk of ecological damage from the pathogen and studies such as the one by Garbelotto, Hüberli and Shaw (2006) may contribute to an understanding disease factors (susceptibility, present of other pests) and may ultimately help to minimise the spread of the disease. In eastern North America, in the Appalachian forests, chestnut forests are struggling to regenerate, a situation partly attributable to the impact of P. cinnamomi. While chestnut blight disease has historically been linked to chestnut mortality, among the chief obstacles facing chestnut restoration are the oomycete pathogens of the genus Phytophthora. Recent plantings of chestnut seedlings in Appalachian forests have experienced high mortality attributable through standard diagnostic practices to Phytophthora, principally P. cinnamomi (Rhoades et al. 2003).
Location Specific Impacts:
Reduction in native biodiversity: The following native plants are affected by Phytophthora cinnamomi: Epacris purpurascens var. purpurascens, Eucalyptus imlayensis, Genoplesium rhyoliticum, Leionema ralstonii, Tasmannia pupurascens, Westringia davidii and Wollemia nobilis. The southern brown bandicoot Isoodon obesulus and the smoky mouse (see Pseudomys fumeus in IUCN Red List of Threatened Species) are affected by the loss of habitat.
Ecosystem change: Phytophthora cinnamomi causes severe disease in the understorey, or the shrub and ground layers of the vegetation in Tasmania. The susceptible species in Tasmania tend to come from the shrub and herbaceous families Dilleniaceae, Epacridaceae, Fabaceae, Proteaceae and Rutaceae. Resistant species generally belong to the grass and sedge families. The presence of P. cinnamomi can therefore lead to an understorey dominated by grasses or sedges.
Economic/Livelihoods: Root rot of avocado caused by P. cinnamomi is one of the most damaging diseases to South Africa's avocado orchards.
Sierra Nevada (United States (USA))
Reduction in native biodiversity: Ione manzanita (Arctostaphylos myrtifolia) is a rare, endemic, evergreen shrub restricted to Ione formation soils (infertile, acidic, sedimentary oxisols) in the foothills of the Sierra Nevada (near Ione). The widely distributed A. viscida (whiteleaf manzanita) intermixes with A. myrtifolia at the margins of Ione formation soils. Swiecki and Bernhardt (2003) examined mortality rates in this mixed vegetation and using scientific testing determined that wilting, foliage desiccation and root necrosis was being induced in the plants by the pathogenic fungus P. cinnamomi. This is the first report of root and crown rot caused by P. cinnamomi on A. myrtifolia and A. viscida. This disease will significantly impact conservation of the already threatened A. myrtifolia (Swiecki and Bernhardt 2003).
California (United States (USA))
Economic/Livelihoods: Phytophthora cinnamomi is the cause of avocado root rot, the main limiting factor in avocado production. In California, it affects between 60 and 75% of orchards, and in 1989 caused an annual loss of approximately $44 million.
Pathogenic: During an intense survey of natural woodlands around Lake Hodges (33°N, 117°W) in June 2001, symptoms typical of root and collar rot caused by Phytophthora spp. were observed on 27% of 474 coast live oaks (Quercus agrifolia) and on none of 86 Engelmann oaks (Q. engelmannii), in spite of intermixing of the two species. Symptoms on coast live oaks included viscous exudates emerging through intact bark matched by underbark dark lesions with irregular margins. Lesions were delineated by a dark line and present on the root collar or the buttress of symptomatic trees. Crowns of trees with lesions ranged from completely healthy to declining or dead. All symptomatic trees were in proximity of the lake or streams. P. cinnamomi was isolated and identified from trees in three distinct sites. Although Q. agrifolia is a known host for P. cinnamomi in California, this is the first report of widespread infestation of P. cinnamomi in natural oak woodlands in the western United States. Survey and inoculation results indicated Q. engelmannii to be less susceptible to infection. Inoculation results confirm previous research that cold temperatures are unfavorable to this pathogen and isolates differed in pathogenicity toward Q. agrifolia. Decline of oaks infected by P. cinnamomi was observed only in conjunction with other factors, in particular with the presence of the oak twig girdler, Agrilus angelicus Horn., an insect favored by stress conditions such as drought. Similar effects have been reported for Mediterranean oaks infected by the same pathogen (Garbelotto, Hüberli and Shaw 2006).