is a species of fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
which causes powdery mildew
Powdery mildew is a fungal disease that affects a wide range of plants. Powdery mildew diseases are caused by many different species of fungi in the order Erysiphales. It is one of the easier diseases to spot, as its symptoms are quite distinctive. Infected plants display white powdery spots on the...
An oak is a tree or shrub in the genus Quercus , of which about 600 species exist. "Oak" may also appear in the names of species in related genera, notably Lithocarpus...
Discovery and taxonomy
Today oak powdery mildew is one of the most common diseases in European forests but it is thought to not always have been present in these forests. The first reports of the disease were made in Paris and other regions of France, Spain, Luxembourg and the Netherlands in 1907. By 1908 the disease had spread into many other European countries, becoming an epidemic. By 1909 it had reached Russia and Turkey, followed by Brazil in 1912 and soon after was distributed around the world. The sudden outbreak of the disease puzzled mycologists as they were unsure about the origin of the pathogen. Its anamorph was easily distinguished from Phyllactinia guttata
Phyllactinia guttata is a species of fungus in the Erysiphaceae family; the anamorph of this species is Ovulariopsis moricola. A plant pathogen distributed in temperate regions, P...
which had previously been reported to cause powdery mildew on oaks in Europe at low intensity. It did however share morphological similarities with Oïdium quercinum
, Calocladia penicillata
and Microsphaera penicillata
Microsphaera penicillata is a plant pathogen that causes powdery mildew on sycamore.- External links :* *...
which had previously been reported to cause powdery mildew on oaks in Europe. The sudden appearance and high incidence of the disease made it unlikely that it was caused by any of these species however and instead some authors hypothesised that it was caused by the introduction of a new species from outside Europe. Powdery mildew had already been reported from North America so it was thought that the new disease could have been imported on American oaks into Europe. The absence of the disease on American red oaks growing in France however, made this unlikely. Four years after the initial outbreak, a teleomorph was found in south-east France which Arnaud and Fox identified as an American species, Microsphaera quercina
(now included in Microsphaera alni
). A detailed morphological study by Griffon and Maublanc in 1912 suggested that the species was different from all previously described species and proposed the name of Microsphaera alphitoides
, referring to the flour
Flour is a powder which is made by grinding cereal grains, other seeds or roots . It is the main ingredient of bread, which is a staple food for many cultures, making the availability of adequate supplies of flour a major economic and political issue at various times throughout history...
y appearance of the abundant white sporulation. This identification remained controversial until the 1940s. The origin of the disease was still uncertain however, in 1927, Raymond suggested that the disease may have been caused by a fungus identified in 1877 in Portugal that had been imported from Portuguese colonies. In 1980, Boesewinkel demonstrated that the same species was responsible for powdery mildew on Quercus robur
and on a mango
The mango is a fleshy stone fruit belonging to the genus Mangifera, consisting of numerous tropical fruiting trees in the flowering plant family Anacardiaceae. The mango is native to India from where it spread all over the world. It is also the most cultivated fruit of the tropical world. While...
species from New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
, supporting the hypothesis that the fungus had shifted its host.
Recently, molecular biology
Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
has provided new tools for identifying species and elucidating their relatedness. A study of ribosomal DNA
Ribosomal DNA codes for ribosomal RNA. The ribosome is an intracellular macromolecule that produces proteins or polypeptide chains. The ribosome itself consists of a composite of proteins and RNA. As shown in the figure, rDNA consists of a tandem repeat of a unit segment, an operon, composed of...
from 33 samples of oak powdery mildew from Europe showed that E. alphitoides
contains identical sequences to Oïdium mangiferae
Oidium mangiferae is a plant pathogen that infects mango trees causing powdery mildew. Powdery mildew of mango is an Ascomycete pathogen of the Erysiphales family that was initially described by Berthet in 1914, using samples collected from Brazil. O...
, a major disease of mango in several tropical regions and Oïdium heveae
, an economically important pathogen of the para rubber tree
Hevea brasiliensis, the Pará rubber tree, often simply called rubber tree, is a tree belonging to the family Euphorbiaceae, and the most economically important member of the genus Hevea...
. These results support the findings of Boesewinkel and suggest that E. alphitoides
and O. mangiferae
could be conspecific. The origin of the species is still uncertain but Mougou et. al
(2008) reported that E. alphitoides
likely originated from the tropics and expanded its range to oak after being introduced to Europe.
Only young developing leaves are susceptible to colonisation by E. alphitoides
. It only induces necrosis
Necrosis is the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma. This is in contrast to apoptosis, which is a naturally occurring cause of cellular death...
when infection occurs very early during leaf development. Because the pathogen develops late in spring, after the first leaves of oak seedlings have developed, it is more prevalent on the second and third flushes of leaves that develop in July and August. This habit also reduces the severity of the disease on mature trees.
Effects on host
The disease can be very severe on Quercus robur
and Quercus petraea
in Europe, particularly on young trees. In mature trees the disease is generally less damaging, but in combination with other factors such as defoliation by insects can contribute to tree decline. A study of the effects of E. alphitoides
on Quercus robur
found it decreased stomatal conductance by 15–30%, did not affect the leaf mass to area ratio, decreased the nitrogen content of the leaf and increased dark respiration. Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...
was also reduced in infected leaves by about 40–50% in fully infected leaves and those that were heavily infected were shed earlier than those not infected. Despite it affecting the ability of leaves to photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
e, the effect is low and this is thought to explain why the disease has only moderate consequences for tree health despite heavy infections. Because the infection makes the plants less shade tolerant, it may lead to the death of seedlings growing in shade.
- Aesculus hippocastanum (horse chestnut)
- Quercus species
Wisteria is a genus of flowering plants in the pea family, Fabaceae, that includes ten species of woody climbing vines native to the eastern United States and to China, Korea, and Japan. Aquarists refer to the species Hygrophila difformis, in the family Acanthaceae, as Water Wisteria...