Phytophthora
Phytophthora. Various species in the genus are known primarily for the plants that they kill: sudden oak death, potato blight, cinnamon root rot, etc. In the case of conifer pathogens, though, the species is usually called simply Phytophthora.
Phytophthora is a genus of about 100 species of a group commonly called the "water molds" because they may be distributed by water and were formerly thought to be fungi. It is a representative of kingdom Chromalveolata, phylum Heterokontophyta, class Oomycota. Its closest widely-known relatives are brown algae and diatoms. Unlike these, the oomycetes are primarily a group of terrestrial plant pathogens.
With regard to conifers, some principal species of concern are Phytophthora cinnamomi, P. citricola, and P. ramorum.
Phytophthora is generally invisible to the naked eye and identification is very difficult, often requiring culture and molecular analysis. It can live in plant tissues or in the soil, and can take various different forms, including mycelia and sporangia (the usual active form), zoospores (active reproductive units), and chlamydospores (dormant form, tolerant of harsh environments). It is typically identified according to the symptoms that it produces in infected plants, where the mycelia grow through the plant tissue, absorbing carbohydrates and nutrients, and killing the tissues. This appears as rot. Where the infection is primarily in the roots, it is a root rot, and may kill the plant by depriving it of water and nutrients or by contributing to death by windthrow. Where the infection is in the stem, it may interrupt the flow of nutrients, water and carbohydrates up and down the stem, thereby killing the plant. Symptoms include wilting and yellowing of foliage.
The genus' distribution is approximately worldwide, and most species formerly had relatively restricted ranges. However, importation of infected plants by humans and distribution of chlamydospores by human activity (for instance, on the soles of shoes) has caused many species to be introduced to ecosystems where they are not native and where the native species lack resistance to the invasive Phytophthora. Consequently, the group has increased greatly in importance in recent years and is now one of the principal threats to native vegetation in many parts of the world. It is currently one of the principal factors limiting the survival of Chamaecyparis lawsoni both in its native range and in its use as an ornamental, and is the cause of substantial local declines in other conifers, such as Agathis australis. One of the rarest conifers in the world, Wollemia nobilis, is highly vulnerable to Phytophthora infection and extraordinary precautions are currently required to ensure that the few surviving stands of that species do not become infected. Other conifers known to be vulnerable to Phytophthora include Araucaria heterophylla, Juniperus chinensis, J. communis, J. conferta, J. sabina, Sequoia sempervirens, Sequoiadendron giganteum, Thuja occidentalis, Abies concolor, A. fraseri, A. grandis, A. magnifica, Larix kaempferi, Pinus attenuata, P. echinata, P. radiata, P. strobus, Pseudotsuga menziesii, Taxus baccata, T. brevifolia, Torreya california, and perhaps T. taxifolia (suspected). In most cases, infection is best documented in nursery trees, perhaps because these plants are at the highest risk of exposure to a variety of non-native Phytophthoras. Many conifers seem to be substantially resistant to Phytophthora infection, for instance, it is rarely observed in Thuja plicata; but this may simply be because those conifers have not yet been exposed to a fatally virulent species of Phytophthora.
The almost unspellable and unpronouncable name Phytophthora is derived from the Greek words for "plant destroyer."
Global Invasive Species Database, accessed 2010.04.29.
Phytophthora Database, accessed 2019.03.01.
Fifth IUFRO Working Party Meeting on Phytophthora, http://www.phyto2010.com/programme.html, accessed 2010.04.29, now defunct.
Last Modified 2023-12-16
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