Phytophthora alni
Encyclopedia
Phytophthora alni is an oomycete plant pathogen that causes causes lethal root and collar rot in alder
s. It is widespread across Europe and has recently been found in North America. This species is believed to have originated relatively recently.
may reproduce by both sexual
and asexual
methods. P. alni is homothallic
meaning that both structures for sexual reproduction (antheridia
and oogonia) appear in a single culture. The antheridia are amphigynous, except in some cultures of P. alni. multiformis where paragynous antheridia may occur. Unlike most species of Phytophthora
, which are diploid, P. alni alni is near tetraploid
and unable to complete meiosis
beyond metaphase I. In culture, many oogonia prematurely abort or appear abnormal and only one third of the oospores
that appear normal are reported to be viable. As a result it is believed to spread predominantly via asexual means, namely zoospore
s which are produced in a specialised structure known as the sporangium
. Water temperature has been shown to affect sporulation, with warmer water increasing sporangia production. Temperatures of 8°C and below prevent production of sporangia.
The only trees described as affected in the wild are alder trees; however, greenhouse inoculation trials have suggested that walnut (Juglans regia), chestnut (Castanea sativa) and wild cherry (Prunus avium
) trees may also be susceptible. Risk factors for infection include: water temperature (disease risk is higher in warmer waters), soil type (disease risk is higher in fine textured soil, especially clay loams) and water course type (disease risk is higher in slow flowing water courses).
Infected trees have abnormally small, yellow and sparse leaves which frequently fall prematurely. The crowns of trees which have been infected for many years have many dead twigs and branches. Tarry or rusty spots may appear at the base of the trunk which are indicative of the death of the phloem
caused by the P. alni invasion. The course of the disease is varied, with many trees dying rapidly once symptoms appear, however, others may deteriorate slowly over many years.
P.alni is regarded as a serious threat to riparian woodland. In 2007 Oregon
listed P.alni as one of its "100 most dangerous invasive species to keep out".
About the Phytophthora disease of alder - UK Forestry Commission
Risk maps for P.alni infection in the USA
Alder
Alder is the common name of a genus of flowering plants belonging to the birch family . The genus comprises about 30 species of monoecious trees and shrubs, few reaching large size, distributed throughout the North Temperate Zone and in the Americas along the Andes southwards to...
s. It is widespread across Europe and has recently been found in North America. This species is believed to have originated relatively recently.
Classification
P. alni was discovered in 1993 in Southern Britain. It has subsequently been reported in many European countries including the Netherlands, Germany, France, Sweden, Belgium, Austria and Hungary and has recently been found in Alaska. P. alni consists of three subspecies, the most commonly isolated and most virulent subspecies P. alni alni (also referred to as the 'standard form') and two less commonly isolated subspecies P. alni uniformis (also called the Swedish variant) and P.alni multiformis (which contains the Dutch, German and UK variants). A recent genetic analysis has suggested that P. alni alni. was generated on many separate occasions by the hybridization of either P. alni uniformis with P. alni multiformis or their ancestors. The same analysis suggests that P. alni uniformis may have P. cambivora as an ancestor. P. alni multiformis may have been generated by autopolyploidization (the spontaneous duplication of chromosome number within a species) or by a hybridisation of two unknown species a long time ago in evolutionary terms (ancient reticulation).Reproduction
Members of the genus PhytophthoraPhytophthora
Phytophthora is a genus of plant-damaging Oomycetes , whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The genus was first described by Heinrich Anton de Bary in 1875...
may reproduce by both sexual
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
and asexual
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
methods. P. alni is homothallic
Homothallic
Homothallic refers to the possession, within a single organism, of the resources to reproduce sexually.It can be contrasted to heterothallic.It is often used to categorize fungi. In yeast, heterothallic cells have mating types a and α...
meaning that both structures for sexual reproduction (antheridia
Antheridium
An antheridium or antherida is a haploid structure or organ producing and containing male gametes . It is present in the gametophyte phase of lower plants like mosses and ferns, and also in the primitive vascular psilotophytes...
and oogonia) appear in a single culture. The antheridia are amphigynous, except in some cultures of P. alni. multiformis where paragynous antheridia may occur. Unlike most species of Phytophthora
Phytophthora
Phytophthora is a genus of plant-damaging Oomycetes , whose member species are capable of causing enormous economic losses on crops worldwide, as well as environmental damage in natural ecosystems. The genus was first described by Heinrich Anton de Bary in 1875...
, which are diploid, P. alni alni is near tetraploid
Polyploidy
Polyploid is a term used to describe cells and organisms containing more than two paired sets of chromosomes. Most eukaryotic species are diploid, meaning they have two sets of chromosomes — one set inherited from each parent. However polyploidy is found in some organisms and is especially common...
and unable to complete meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
beyond metaphase I. In culture, many oogonia prematurely abort or appear abnormal and only one third of the oospores
Oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae and fungi. Also the result of plasmogamy/karyogamy in oomycetes, which in turn leads to the development of hyphae, then mycelium....
that appear normal are reported to be viable. As a result it is believed to spread predominantly via asexual means, namely zoospore
Zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some algae, bacteria and fungi to propagate themselves.-Flagella:...
s which are produced in a specialised structure known as the sporangium
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
. Water temperature has been shown to affect sporulation, with warmer water increasing sporangia production. Temperatures of 8°C and below prevent production of sporangia.
Infection
Affected alder species include:- Alnus cordata - Italian alder
- Alnus glutinosa - Common alder (most susceptible)
- Alnus incana - Grey alder (most resistant)
- Alnus viridis - Green alder
The only trees described as affected in the wild are alder trees; however, greenhouse inoculation trials have suggested that walnut (Juglans regia), chestnut (Castanea sativa) and wild cherry (Prunus avium
Prunus avium
Prunus avium, commonly called wild cherry, sweet cherry, bird cherry, or gean, is a species of cherry, native to Europe, west Turkey, northwest Africa, and western Asia, from the British Isles south to Morocco and Tunisia, north to the Trondheimsfjord region in Norway and east to the Caucasus, and...
) trees may also be susceptible. Risk factors for infection include: water temperature (disease risk is higher in warmer waters), soil type (disease risk is higher in fine textured soil, especially clay loams) and water course type (disease risk is higher in slow flowing water courses).
Infected trees have abnormally small, yellow and sparse leaves which frequently fall prematurely. The crowns of trees which have been infected for many years have many dead twigs and branches. Tarry or rusty spots may appear at the base of the trunk which are indicative of the death of the phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
caused by the P. alni invasion. The course of the disease is varied, with many trees dying rapidly once symptoms appear, however, others may deteriorate slowly over many years.
P.alni is regarded as a serious threat to riparian woodland. In 2007 Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
listed P.alni as one of its "100 most dangerous invasive species to keep out".
External links
About the Phytophthora disease of alder - UK Forestry Commission
Risk maps for P.alni infection in the USA