Phytophthora cinnamomi (oomycete)
A study (Moreira & Martins, 2005) undertaken during 1995-98, surveyed cork and holm oak stands in four different regions of Portugal (Trás-os-Montes, Alentejo, Ribatejo and Algarve) for the presence of P. cinnamomi. Tree decline severity, sudden death and site characteristics were assessed in varied conditions. Analysis of the survey results indicated: that 56% of surveyed flora were infected with the pathogen; the flora belonged mainly to the following families Ericaceae, Cistaceae and Leguminosae; recovery of the pathogen was more frequent in shallow soils; soils with low fertility and low mineral nutrient levels, particularly phosphorus, seemed to favour infection and sites facing south showed higher occurrence of P. cinnamomi, which was also more frequent in slopes and valleys than on hilltops.
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).
Bergot et al. (2004) states that, "Disease development was shown to be strongly hampered by cold winters (Robin et al. 1992b; Marçais et al. 1996), in agreement with the known sensitivity of P. cinnamomi to frost (Benson, 1982). The hypothesis that lethal frost effects on the pathogen could be a major factor limiting disease range in oaks was put forward by Delatour (1986)."