Powdery mildew is a

fungal A fungus ( : fungi or funguses) is any member of the 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, separately from th ...

disease of

barley Barley (''Hordeum vulgare''), a member of the grass family, is a major cereal grain grown in temperate climates globally. It was one of the first cultivated grains, particularly in Eurasia as early as 10,000 years ago. Globally 70% of barley pr ...

caused by ''

Blumeria graminis ''Blumeria graminis'' (commonly called barley powdery mildew or corn mildew) is a fungus that causes powdery mildew on grasses, including cereals. It is the only species in the genus ''Blumeria''. It has also been called ''Erysiphe graminis'' and ...

'' f. sp. ''hordei''. The disease has a worldwide distribution and is most damaging in cool, wet climates. The host range of the form species ''hordei'' is restricted to barley and other ''

Hordeum ''Hordeum'' is a genus of annual and perennial plants in the grass family. They are native throughout the temperate regions of Africa, Eurasia, and the Americas. One species, ''Hordeum vulgare'' (barley), has become of major commercial importan ...

species''.

Symptoms

At first, powdery

mildew Mildew is a form of fungus. It is distinguished from its closely related counterpart, mould, largely by its colour: moulds appear in shades of black, blue, red, and green, whereas mildew is white. It appears as a thin, superficial growth consi ...

can be observed as small greyish patches of fluffy fungal growth (

mycelium Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrate (biology), substrates. A typical single ...

) on the upper surface of the lower leaves. These spots resemble small cushions of white powder. Leaf tissue on the opposite side of an infected leaf turns pale green to yellow. The fungus only infects the epidermal layer and can be easily scraped off with a fingernail. Infections can also occur on the leaf sheaths and ears. Leaves remain green and active for some time following infection, then gradually become

chlorotic In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

and die off. As the disease progresses, the mycelium often becomes dotted with minute black points (

cleistothecia An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are ...

), which are the sexual fruiting bodies of the fungus. The fungus does not kill the host plant as it requires the hosts' water and nutrients to grow. It settles on foliage and gradually slows the host growth process. Fruits, vegetables and flowers do not reach maturity if they become infected by powdery mildew. Flowers bloom is delayed or aborted. Vegetable and fruit crop yield is significantly lowered or the produce is of reduced quality.

Disease cycle

The fungus overwinters as cleistothecia on straw, and in milder climates, also as mycelium and

conidia A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...

on stubble and straw or volunteer barley and certain grasses. Windborne

ascospores An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or ...

or conidia are the primary inoculum (also known as the ''

propagule In biology, a propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by dispersal. The propagule is usually distinct in form from the parent organism. Propagules are produced by organisms ...

'') and can be dispersed over considerable distances. Infection by conidia requires high humidity, but not free water on the leaf surface.

Sporulation In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

and spore dispersal are favored by drier conditions. Thus the disease does well under alternating wet and dry conditions. Production of conidia declines markedly as the colony ages. Cleistothecia develop on older leaves as the plant matures. Low temperatures, together with the wetting of the cleistothecia for at least 72 hours, induces the maturations of the ascospores. Ascospores are released following rains, but are relatively sparse in comparison to the condia.

References

External links

Extension publications

EPPO - Europe

Europe: Scotland

US: Oregon
{dead link, date=March 2018 , bot=InternetArchiveBot , fix-attempted=yes Fungal plant pathogens and diseases Barley diseases