Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Datasheet

Blumeria graminis
(powdery mildew of grasses and cereals)

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Datasheet

Blumeria graminis (powdery mildew of grasses and cereals)

Summary

  • Last modified
  • 28 March 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Natural Enemy
  • Preferred Scientific Name
  • Blumeria graminis
  • Preferred Common Name
  • powdery mildew of grasses and cereals
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Fungi
  •     Phylum: Ascomycota
  •       Subphylum: Pezizomycotina
  •         Class: Leotiomycetes

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Pictures

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PictureTitleCaptionCopyright
Powdery mildew on wheat leaves in the field.
TitleSymptoms
CaptionPowdery mildew on wheat leaves in the field.
CopyrightThorsten Kraska, University of Bonn, Germany
Powdery mildew on wheat leaves in the field.
SymptomsPowdery mildew on wheat leaves in the field. Thorsten Kraska, University of Bonn, Germany
TitleSymptoms on leaf
Caption
Copyright©J.M. Waller/CABI BioScience
Symptoms on leaf©J.M. Waller/CABI BioScience
B. graminis pustules on barley. These pustules usually appear as white powdery patches.
TitlePustules on barley
CaptionB. graminis pustules on barley. These pustules usually appear as white powdery patches.
CopyrightMartin Wolfe
B. graminis pustules on barley. These pustules usually appear as white powdery patches.
Pustules on barleyB. graminis pustules on barley. These pustules usually appear as white powdery patches.Martin Wolfe

Identity

Top of page

Preferred Scientific Name

  • Blumeria graminis (DC.) Speer

Preferred Common Name

  • powdery mildew of grasses and cereals

Other Scientific Names

  • Erysiphe graminis DC.
  • Oidium monilioides (Nees) Link

International Common Names

  • Spanish: cenicilla de los cereales; oidio de los cereales
  • French: blanc des cereales; blanc des graminees; oidium des cereales

Local Common Names

  • Germany: Mehltau: Getreide

EPPO code

  • ERYSGR (Blumeria graminis)

Taxonomic Tree

Top of page
  • Domain: Eukaryota
  •     Kingdom: Fungi
  •         Phylum: Ascomycota
  •             Subphylum: Pezizomycotina
  •                 Class: Leotiomycetes
  •                     Order: Erysiphales
  •                         Family: Erysiphaceae
  •                             Genus: Blumeria
  •                                 Species: Blumeria graminis

Notes on Taxonomy and Nomenclature

Top of page This fungus is the only species of the genus Blumeria but it has previously been treated as a species of Erysiphe. According to Braun (1987), it differs from all species of Erysiphe because its anamorph possesses unique features, for example, digitate haustoria, secondary mycelium with bristle-like hyphae and bulbous swellings of the conidiophores, and because of the structure of the ascocarps. Braun (1987) considers that, because of these differences, there should be a separation at generic level. Molecular sequence analyses proved the separate position of the powdery mildew on Poaceae and showed that Blumeria takes a basal position in the phylogenetic trees of the Erysiphales. Hence, Blumeria is only distantly related to Erysiphe and all other genera of the powdery mildew fungi (Saenz and Taylor, 1999; Mori et al., 2000; Braun et al., 2002).

Description

Top of page Primary mycelium branched, hyaline and uninucleate; hyphal cells (35-)40(-55) x 3-6 µm; appressoria nipple-shaped, solitary or in opposite pairs, 3.5-7 µm diam.; secondary mycelium consisting of bristle-like hyphae which are straight to falcate, thick-walled, 200-400 x 4-7 µm, hyaline, later ochraceous to rusty or reddish brown; conidiophores arising from the mycelium at a right angle with the host surface, erect, 60-90 x 4-7 µm, foot-cells 20-40 x 5-7 µm, with a basal bulbose swelling, ca 10-15 µm wide, followed by shorter cells, ca 12.5-25 µm long; conidia in chains, mostly uninucleate, occasionally binucleate, hyaline, ellipsoid-ovoid, limoniform, (20-)25-35(-45) x (8)12-16(-20) µm, germinating by a simple germ tube, terminal to lateral, straight to somewhat flexuous, ca 12-50 x 2.5-4 µm, after germination germ tube forming an appressorium in juxtaposition with the host cuticle, a hyphal peg penetrating the cuticle and the subcuticular wall and forming an haustorium in the epidermal cell; haustoria ellipsoid with long, finger-shaped appendages radiating from both ends (the fungus advances no further in plant tissue and the remainder of the mycelium, as well as the ascomata, are entirely extramatrical i.e. on the exterior of the host substratum). Ascomata (chasmothecia) immersed in the mycelial felt, at first globose, becoming strongly depressed, often cupulate, 110-280 µm diam., peridial cells obscure, irregularly polygonal, 8-20 µm diam., with simple, rarely branched appendages, few to numerous, in the lower half, usually shorter than the ascomatal diam., mycelioid, septate, hyaline to pigmented, thin-walled, smooth, asci 6-30, saccate-ovate to subcylindrical, short-stalked, 50-110 x 20-45 µm; usually 8-spored, sometimes 4-spored, but ascospores rarely fully developed; ellipsoid-ovoid, subhyaline to pale brown, 20-24 x 10-14 µm.

See also Kapoor (1967) and Braun (1987, 1995).

Distribution

Top of page Although powdery mildew of cereals and grasses has a genuinely worldwide distribution, there appear to be fewer reports of it in South America than elsewhere. Powdery mildew is distributed widely in Europe and North America, and in China.

Distribution Table

Top of page

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanPresentCABI/EPPO, 2004
ArmeniaPresentCABI/EPPO, 2004
AzerbaijanPresentCABI/EPPO, 2004
ChinaPresentKong and Dong, 1997; CABI/EPPO, 2004
-AnhuiPresentCABI/EPPO, 2004
-BeijingPresentDuan et al., 1999
-ChongqingPresentCABI/EPPO, 2004
-FujianPresentCABI/EPPO, 2004
-GansuPresentCABI/EPPO, 2004
-GuangdongPresentCABI/EPPO, 2004
-GuangxiPresentCABI/EPPO, 2004
-GuizhouPresentCABI/EPPO, 2004
-HebeiPresentGao, 1997; CABI/EPPO, 2004
-HenanPresentWang et al., 1992; CABI/EPPO, 2004
-HubeiPresentCABI/EPPO, 2004
-JiangsuPresentCABI/EPPO, 2004
-JiangxiPresentCABI/EPPO, 2004
-JilinPresentCABI/EPPO, 2004
-LiaoningPresentCABI/EPPO, 2004
-Nei MengguPresentCABI/EPPO, 2004
-NingxiaPresentCABI/EPPO, 2004
-QinghaiPresentCABI/EPPO, 2004
-ShaanxiPresentCABI/EPPO, 2004
-ShandongPresentCABI/EPPO, 2004
-ShanxiPresentSheng et al., 1993; CABI/EPPO, 2004
-SichuanPresentCABI/EPPO, 2004
-TibetPresentCABI/EPPO, 2004
-XinjiangPresentCABI/EPPO, 2004; Zhang et al., 2014
-YunnanPresentCABI/EPPO, 2004
-ZhejiangPresentCABI/EPPO, 2004
Georgia (Republic of)PresentCABI/EPPO, 2004
IndiaPresentBasandrai et al., 1996; CABI/EPPO, 2004
-BiharPresentCABI/EPPO, 2004
-ChhattisgarhPresentCABI/EPPO, 2004
-DelhiPresentCABI/EPPO, 2004
-HaryanaPresentCABI/EPPO, 2004
-Himachal PradeshPresentSharma et al., 1996; CABI/EPPO, 2004
-Indian PunjabPresentCABI/EPPO, 2004
-Jammu and KashmirPresentCABI/EPPO, 2004
-Madhya PradeshPresentCABI/EPPO, 2004
-MaharashtraPresentCABI/EPPO, 2004
-RajasthanPresentCABI/EPPO, 2004
-SikkimPresentSrivastava, 1996; CABI/EPPO, 2004
-Tamil NaduPresentCABI/EPPO, 2004
-Uttar PradeshPresentCABI/EPPO, 2004
-UttarakhandPresentCABI/EPPO, 2004
IranPresentCABI/EPPO, 2004; Abkhoo, 2015
IraqPresentCABI/EPPO, 2004
IsraelPresentCABI/EPPO, 2004
JapanPresentMorikawa, 1995; CABI/EPPO, 2004
-HokkaidoPresentCABI/EPPO, 2004
-HonshuPresentCABI/EPPO, 2004
-KyushuPresentCABI/EPPO, 2004
JordanPresentCABI/EPPO, 2004
KazakhstanPresentCABI/EPPO, 2004
Korea, Republic ofPresentLee and Kim, 1990; CABI/EPPO, 2004
KyrgyzstanPresentCABI/EPPO, 2004
LebanonPresentAmano, 1986
MongoliaPresentCABI/EPPO, 2004
NepalPresentGhimire and Pradhanang, 1996; CABI/EPPO, 2004
PakistanPresentCABI/EPPO, 2004
Saudi ArabiaPresentEl-Meleig & Al-Rokibah, 1996; CABI/EPPO, 2004
SyriaPresentCABI/EPPO, 2004
TaiwanPresentTanda and Su, 1995; CABI/EPPO, 2004
ThailandPresentAmano, 1986
TurkeyPresentCABI/EPPO, 2004
TurkmenistanPresentCABI/EPPO, 2004
UzbekistanPresentCABI/EPPO, 2004
YemenPresentCABI/EPPO, 2004

Africa

AlgeriaPresentCABI/EPPO, 2004
AngolaPresentAmano, 1986
EgyptPresentEl-Sayed et al., 1995; CABI/EPPO, 2004
EthiopiaPresentCABI/EPPO, 2004
KenyaPresentCABI/EPPO, 2004
LibyaPresentCABI/EPPO, 2004
MalawiPresentCABI/EPPO, 2004
MoroccoPresentArifi, 1995; CABI/EPPO, 2004
MozambiquePresentCABI/EPPO, 2004
RwandaPresentCABI/EPPO, 2004
South AfricaPresentCABI/EPPO, 2004
Spain
-Canary IslandsPresentAmano, 1986
SudanPresentCABI/EPPO, 2004
TanzaniaPresentCABI/EPPO, 2004
TunisiaPresentCherif et al., 1994; Yahyaoui et al., 1997; CABI/EPPO, 2004
Western SaharaPresentAmano, 1986
ZambiaPresentCABI/EPPO, 2004
ZimbabwePresentMtisi and Mashiringwani, 1995; CABI/EPPO, 2004

North America

BermudaPresentAmano, 1986
CanadaWidespreadCABI/EPPO, 2004
-AlbertaPresentCABI/EPPO, 2004
-British ColumbiaPresentCABI/EPPO, 2004
-ManitobaPresentCABI/EPPO, 2004
-New BrunswickPresentCABI/EPPO, 2004
-Newfoundland and LabradorPresentCABI/EPPO, 2004
-Northwest TerritoriesPresentCABI/EPPO, 2004
-Nova ScotiaPresentAl-Mughrabi and Gray, 1996; CABI/EPPO, 2004
-NunavutPresentCABI/EPPO, 2004
-OntarioPresentKasha, 1996; CABI/EPPO, 2004
-Prince Edward IslandPresentCABI/EPPO, 2004
-QuebecPresentCABI/EPPO, 2004
-SaskatchewanPresentCABI/EPPO, 2004
-Yukon TerritoryPresentCABI/EPPO, 2004
GreenlandPresentCABI/EPPO, 2004
MexicoPresentCABI/EPPO, 2004
USAWidespreadCABI/EPPO, 2004
-AlaskaPresentCABI/EPPO, 2004
-ArizonaPresentCABI/EPPO, 2004
-ArkansasPresentCABI/EPPO, 2004
-CaliforniaPresentCABI/EPPO, 2004
-ColoradoPresentCABI/EPPO, 2004
-ConnecticutPresentCABI/EPPO, 2004
-DelawarePresentCABI/EPPO, 2004
-District of ColumbiaPresentCABI/EPPO, 2004
-FloridaPresentCABI/EPPO, 2004
-GeorgiaPresentJohnson et al., 1996; CABI/EPPO, 2004
-IdahoPresentCABI/EPPO, 2004
-IllinoisPresentCABI/EPPO, 2004
-IndianaPresentCABI/EPPO, 2004
-IowaPresentCABI/EPPO, 2004
-KansasPresentSears et al., 1997; CABI/EPPO, 2004
-KentuckyPresentPearce et al., 1996; CABI/EPPO, 2004
-MainePresentCABI/EPPO, 2004
-MarylandPresentCABI/EPPO, 2004
-MichiganPresentCABI/EPPO, 2004
-MinnesotaPresentCABI/EPPO, 2004
-MississippiPresentCABI/EPPO, 2004
-MissouriPresentCABI/EPPO, 2004
-MontanaPresentCABI/EPPO, 2004
-NebraskaPresentCABI/EPPO, 2004
-NevadaPresentCABI/EPPO, 2004
-New HampshirePresentCABI/EPPO, 2004
-New JerseyPresentCABI/EPPO, 2004
-New MexicoPresentCABI/EPPO, 2004
-New YorkPresentCABI/EPPO, 2004
-North CarolinaPresentCABI/EPPO, 2004
-North DakotaPresentCABI/EPPO, 2004
-OhioPresentGooding et al., 1997; CABI/EPPO, 2004
-OklahomaPresentCABI/EPPO, 2004
-OregonPresentCABI/EPPO, 2004
-PennsylvaniaPresentCABI/EPPO, 2004
-Rhode IslandPresentCABI/EPPO, 2004
-South CarolinaPresentCABI/EPPO, 2004
-South DakotaPresentCABI/EPPO, 2004
-TennesseePresentCABI/EPPO, 2004
-TexasPresentCABI/EPPO, 2004
-UtahPresentCABI/EPPO, 2004
-VermontPresentCABI/EPPO, 2004
-VirginiaPresentCABI/EPPO, 2004
-WashingtonPresentCABI/EPPO, 2004
-West VirginiaPresentCABI/EPPO, 2004
-WyomingPresentCABI/EPPO, 2004

Central America and Caribbean

GuatemalaPresentAmano, 1986
NicaraguaPresentAmano, 1986
Puerto RicoPresentAmano, 1986

South America

ArgentinaPresentMolten et al., 1996; CABI/EPPO, 2004
BrazilPresentFelicio et al., 1996; CABI/EPPO, 2004
-Mato Grosso do SulPresentCABI/EPPO, 2004
-Minas GeraisPresentCABI/EPPO, 2004
-ParanaPresentCABI/EPPO, 2004
-Rio Grande do SulPresentCABI/EPPO, 2004
-Sao PauloPresentCABI/EPPO, 2004
ChilePresentMellado et al., 1994; CABI/EPPO, 2004
ColombiaPresentAmano, 1986
EcuadorPresentAmano, 1986
ParaguayPresentCABI/EPPO, 2004
PeruPresentCABI/EPPO, 2004
UruguayPresentCABI/EPPO, 2004
VenezuelaPresentSchotman, 1989; CABI/EPPO, 2004

Europe

AlbaniaPresentCABI/EPPO, 2004
AustriaPresentSykora et al., 1995; CABI/EPPO, 2004
BelarusPresentCABI/EPPO, 2004
BelgiumPresentHerman and Couvreur, 1994; CABI/EPPO, 2004
Bosnia-HercegovinaPresentCABI/EPPO, 2004
BulgariaPresentDobrev and Tufa, 1995; CABI/EPPO, 2004
CroatiaPresentKoric, 1994; CABI/EPPO, 2004
CyprusPresentKari, 1996; CABI/EPPO, 2004
Czech RepublicPresentDreiseitl & Jarecka, 1997; Hanisova and Hanis, 1997; CABI/EPPO, 2004
Czechoslovakia (former)Present
DenmarkPresentJorgensen et al., 1995; CABI/EPPO, 2004
EstoniaPresentPriiliin et al., 1996; CABI/EPPO, 2004
Faroe IslandsPresentCABI/EPPO, 2004
FinlandPresentPeusha et al., 1996; CABI/EPPO, 2004
Former USSRPresent
FrancePresentSteden et al., 1997; CABI/EPPO, 2004
GermanyPresentSachs, 1995; CABI/EPPO, 2004
GreecePresentCABI/EPPO, 2004
HungaryPresentSykora et al., 1995; CABI/EPPO, 2004
IcelandPresentCABI/EPPO, 2004
IrelandPresentCABI/EPPO, 2004
ItalyPresentPasquini et al., 1996; CABI/EPPO, 2004
LatviaPresentCABI/EPPO, 2004
LithuaniaPresentRuzgas and Lintkevicius, 1996; CABI/EPPO, 2004
MacedoniaPresentCABI/EPPO, 2004
MaltaPresentCABI/EPPO, 2004
MoldovaPresentCABI/EPPO, 2004
NetherlandsPresentTimmer, 1996; CABI/EPPO, 2004
NorwayPresentAssveen and Gunnarstorp, 1996; CABI/EPPO, 2004
PolandPresentGolebrink, 1995; CABI/EPPO, 2004
PortugalPresentCABI/EPPO, 2004
RomaniaPresentCABI/EPPO, 2004
Russian FederationPresentTerekhov et al., 1997; CABI/EPPO, 2004
-Central RussiaWidespreadCABI/EPPO, 2004
-Eastern SiberiaWidespreadCABI/EPPO, 2004
-Northern RussiaWidespreadCABI/EPPO, 2004
-Russian Far EastWidespreadCABI/EPPO, 2004
-SiberiaPresent
-Southern RussiaWidespreadCABI/EPPO, 2004
-Western SiberiaWidespreadCABI/EPPO, 2004
SlovakiaPresentSekerkova, 1996; CABI/EPPO, 2004
SloveniaPresentCABI/EPPO, 2004
SpainPresentMarin et al., 1994; CABI/EPPO, 2004
Svalbard and Jan MayenPresentAmano, 1986
SwedenPresentLindblad, 1994; CABI/EPPO, 2004
SwitzerlandPresentAnon, 1994; CABI/EPPO, 2004
UKPresentHigginbotham et al., 1996; O'Hara and Brown, 1996; CABI/EPPO, 2004
-Northern IrelandPresentMercer & Ruddock, 2003
UkrainePresentLisovoy and Parphenyuk, 1997; CABI/EPPO, 2004
Yugoslavia (former)PresentStojanovic et al., 1995; CABI/EPPO, 2004
Yugoslavia (Serbia and Montenegro)PresentCABI/EPPO, 2004

Oceania

AustraliaPresentWhisson, 1996; CABI/EPPO, 2004
-New South WalesPresentCABI/EPPO, 2004
-QueenslandPresentCABI/EPPO, 2004
-South AustraliaPresentCABI/EPPO, 2004
-TasmaniaPresentCABI/EPPO, 2004
-VictoriaPresentCABI/EPPO, 2004
-Western AustraliaPresentCABI/EPPO, 2004
New ZealandPresentCromey & Hansen, 1992; CABI/EPPO, 2004

Risk of Introduction

Top of page There are no known quarantine regulations on B. graminis perhaps because of its widespread distribution and airborne dissemination.

Hosts/Species Affected

Top of page B. graminis is found on numerous species comprising more than 100 genera of Poaceae, with the exception of the Maydeae, Andropogoneae, Zoysieae, Paniceae and Oryzeae (Dickson 1956; Amano, 1986).

Growth Stages

Top of page Flowering stage, Fruiting stage, Seedling stage, Vegetative growing stage

Symptoms

Top of page Powdery mildew appears in the form of white, later grey-tan areas on all aerial parts of cereals and grasses i.e. leaves, stems and ears, although leaves are most commonly infected. Initial symptoms are easily overlooked and take the form of chlorotic flecks on plant tissue. This is quickly followed by the development of white patches which produce masses of conidia (asexual spores) and assume a powdery appearance. If the plant is shaken even gently, clouds of conidia are released from the patches. Ascomata (fruit bodies forming sexual spores) may or may not form, but when they do, they occur late in the season and can be found embedded in the mildew colonies as tiny, dark-coloured dots.

List of Symptoms/Signs

Top of page
SignLife StagesType
Inflorescence / lesions on glumes
Leaves / abnormal colours
Leaves / fungal growth
Roots / reduced root system
Stems / mycelium present

Biology and Ecology

Top of page B. graminis overwinters mainly as a mycelial mat on leaves of grasses and autumn-sown cereals. Although the ascomata produced during the late summer are fairly resistant to cold and drying out, they appear to be of secondary importance in overwintering and as a source of inoculum in the spring. This is because in temperate regions, fresh host plant material is nearly always available over the winter period. Nevertheless, in humid weather, ascomata release ascospores which can start infections on autumn-sown crops in the autumn and perhaps also in the spring. As temperatures rise in the spring, dormant mycelium commences growth and conidia are produced rapidly. Conidia usually germinate over a range of temperatures from about 3 to 31°C, although 15°C is probably optimal for germination, together with a relative humidity about 95%. Conidial germination is inhibited by free water. Under favourable conditions, fresh conidia can be found in about 7 days and are dispersed within the crop and further afield in the wind. Crucially, therefore, epidemics of powdery mildew will tend to occur during conditions of alternating wet and dry weather, with some wind to ensure dispersal on the conidia.

Mildew is encouraged by very early autumn sowing, especially in barley. In the autumn, heavily infected plants may be less resistant to winter frosts and plants may die. Cereals grown late in the autumn and spring may also be more prone to attack. Powdery mildew is also encouraged by excessive use of nitrogen fertilizer and can be particularly severe in dense crops grown in a sheltered, humid environment.

For further information, see Spencer (1978), Parry (1990) and Manners (1993).

Seedborne Aspects

Top of page

Seed treatment

Seed treatment with difenoconazole, followed by flutriafol, triticonazole and triadimenol was shown to achieve the highest fungal protection in wheat (Reis et al. 2008).

Plant Trade

Top of page
Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Flowers/Inflorescences/Cones/Calyx hyphae; spores Yes Yes Pest or symptoms usually visible to the naked eye
Leaves hyphae; spores Yes Yes Pest or symptoms usually visible to the naked eye
Stems (above ground)/Shoots/Trunks/Branches hyphae; spores Yes Yes Pest or symptoms usually visible to the naked eye
Plant parts not known to carry the pest in trade/transport
Bark
Bulbs/Tubers/Corms/Rhizomes
Fruits (inc. pods)
Growing medium accompanying plants
Roots
Seedlings/Micropropagated plants
True seeds (inc. grain)
Wood

Impact

Top of page Powdery mildew is one of the most common and destructive diseases of cereals. Actual losses depend on the time of disease epidemic onset and its severity, and can at the extreme reach up to 60% (James, 1991; Oerke, 1994). Although it occurs in most, if not all, parts of the world where cereals are grown, it is not considered to be a major problem in every region. This disease can be very destructive to cereals, but generally seems to cause most damage in temperate latitudes, where economically important cereals are more frequently cultivated. However, powdery mildew can also be a limiting factor for cereal production in subtropical and tropical areas. Yield losses by powdery mildews are generally complex to estimate and depend on several factors such as climate, year, cropping system, cereal species and cultivar.

In some parts of Europe, powdery mildews are a limiting factor to the yield of cereals. In the UK, the Agricultural Development and Advisory Service (ADAS, 1980) estimated that increases in yield of winter barley achieved by controlling B. graminis infection in 1978 and 1979 ranged from 0.11 to 0.65 t/ha. Although Cook and King (1984) ranked B. graminis infection as the principal pathogen of spring barley in England and Wales, yield losses vary on a yearly basis. Cock (1975) estimated that losses in spring barley in 1974 and 1975 were 1.3% and 0.5%, respectively. From 1975 to 1978, powdery mildew was the most frequent foliar pathogen in barley and caused yield losses ranging from 2.8 (James et al., 1991) to 8.7% (King, 1977b). In the period 1976-1986, yield losses of 4-9% were recorded for powdery mildew in spring barley in England and Wales (Polley et al., 1993). Field tests in England have suggested that the effects of B. graminis f.sp. hordei infection on grain-filling in spring barley may be determined partly by temperature during the grain-filling period. Fungicides that controlled mildew increased the total grain yield much more in a warm environment (58.2%) than in a cool environment (17.7%). These results illustrated the potential risks involved in using data obtained under one set of circumstances to predict what will happen in another, especially when environments differ so greatly (Jenkyn, 1984).

Estimated yield losses for wheat grown in England and Wales in the period from 1970 to 1975 were 2.8% (King, 1977a), but from 1981 to 1988 yield losses of only 0.5% were estimated (James et al., 1991). In organic wheat, mildew led to yield losses of around 7% in 1991 (Yarham and Turner, 1992). Yield decreases of up to 30% (-1.61 t/ha) were correlated with the severity of mildew on leaf 2 between the watery and milky ripe (GS 71-75) development stages of wheat (Hardwick et al., 1994).

In Scotland, powdery mildew was by far the most important disease from 1970 to 1973, being responsible for average yield losses of 7.0% in barley (James et al., 1991). In wheat, studies carried out between 1970 and 1974 revealed losses due to powdery mildew of 1.7% (James et al., 1991).

The principal disease of barley in central Europe is powdery mildew (Oerke et al., 1994). Field trials with winter barley in Germany between 1975 and 1981 revealed yield losses of 11% due to powdery mildew (Kolbe, 1982). Despite the use of crop protection measures, Lutze et al. (1982) calculated that the average yield loss due to powdery mildew over 6 years was 5.3% (a range of 3.3 to 6.2% was estimated) in barley cultivated in the east of Germany. Based on a yield level of 7.5 t/ha, heavy infections by powdery mildew resulted in losses of 4.8% in wheat during 1969-79 in Germany (Anderl et al., 1984). In the Czech Republic, yield losses of 17% were reported in wheat growing under a severe infection pressure of powdery mildew (Benada and Vanova, 1984). The crop loss in wheat in the Netherlands despite control practices against powdery mildew in 1980 was 1% (Daamen, 1981). Analysis of epidemics showed mildew damage at growth stages GS 32-83 (second node to early dough) to be described by the simple function D = -0.0013 t/ha per colony-day of mildew per leaf, at yield levels of 7-9 t/ha and independently of wheat cultivar, year or nitrogen supply (Daamen, 1989). In Romania, crop losses due to B. graminis f.sp. tritici varied from 1 to more than 20% depending on the region in which it occurred (Ciurdarescu et al., 1987).

Powdery mildew is a destructive disease of wheat in the mid-Atlantic states of the USA in most years. For example, yield losses in Virginia ranging from 12 to 20% have been observed in susceptible cultivars (Griffey et al., 1993). In North Carolina, yield reductions of ca 17% were reported in the susceptible cultivar Saluda when disease severity reached 10% on the flag leaf by heading stage. It has been concluded that powdery mildew can limit yield in modern soft red winter wheat cultivars, although current levels of resistance in certain cultivars are sufficient to prevent large yield reductions (Leath and Bowen, 1989). In Kentucky, USA, an average yield loss of 20% was associated with powdery mildew over two experimental years in soft red winter wheat (Pearce et al., 1996).

Among the pathogens of wheat in Argentina, powdery mildew practically occurs every year in the wheat cropping area, particularly in the first growth stages and less frequently in later stages. In general, it is not economically important in the crop area of La Pampa Humeda, in contrast to areas with higher temperatures such as Santa Fe, Entre Rios and Chaco, where the disease also occurs at later growth stages and can therefore cause significant yield losses. However, no precise data on losses by powdery mildew is available for wheat in Argentina (Molteni, 1996). In field experiments carried out in the south of Brazil during 1981, powdery mildew reduced the yield of a wheat susceptible cultivar by 8% (Luz, 1984). In 1986, experimental results showed that yield losses attributed to the lack of fungicide control of powdery mildew varied from 20-23% (in susceptible cultivars) to 55% (in a highly susceptible wheat cultivar), when compared with conditions where the control was situated (Linhares, 1988).

Based on figures from the Chinese crop protection authorities, Teng (1986) quoted losses nationwide due to powdery mildew in wheat as 3.4%. According to the average wheat yield in China, the permissible loss was 1.7% (Lu and Gong, 1986).

In New Zealand, Cromey et al. (1992) reported losses in winter wheat of up to 63% following wheat diseases including powdery mildew infection. Infection of up to 70% leaf area damage resulted in a 40-60% reduction in grain yield in barley grown in Australia (Chan et al., 1990).

Diagnosis

Top of page Because, under favourable conditions, powdery mildew can infect leaves and commence sporulating within 7-8 days, and is easily identified, specific diagnostic methods are usually not required. B. graminis is an obligate biotroph and cannot be cultured axenically.

Detection and Inspection

Top of page Powdery mildew is easily detected in the crop since the white, fluffy colonies are easily seen on the foliage.

Prevention and Control

Top of page Cultural Control and Sanitary Methods

Since volunteer cereals act as overwintering sources of inoculum and stubble and crop debris may be infested with cleistothecia, eradication of volunteers and disposing of stubble and debris are important aspects of cultural control. Isolation of autumn-sown and spring-sown cereals (i.e. not growing them too close together) will reduce the risk of infection of the autumn-sown crop spreading to the spring-sown crop. In addition, because nitrogen fertilizer promotes lush crop growth and encourages mildew development, excessive use of nitrogen should be avoided.

Host-Plant Resistance

Host-plant resistance is very important in the control of powdery mildew on cereals. A wide range of resistance to powdery mildew is exhibited by varieties of wheat, barley and oats, and in those countries where mildew is widespread, it is prudent to choose a variety with a good disease resistant rating. If the variety grown has poor resistance to powdery mildew, careful crop monitoring will be necessary in order to optimise the timing of fungicide applications.

Spread of mildew from one field to another can be reduced by the correct diversification of varieties. In the UK, the National Institute of Agricultural Botany (NIAB) produces a leaflet on Diversification Schemes which gives information on good combinations of varieties. Growing mixtures of varieties is also an option to reduce mildew (Chin and Wolfe, 1984).

Rubiales et al. (2001) found that chromosomal addition lines of Hordeum vulgare and Hordeum chilense possessed resistance to wheat powdery mildew. This resistance was expressed as a reduction of disease severity and is of broad genetic basis, conferred by gene (s) present on different chromosomes of both H. vulgare and H. chilense. This resistance may be useful in breeding new wheat varieties with resistance to powdery mildew. In addition, a powdery mildew resistance gene, originating from wild emmer wheat (Triticum dicoccoides) accession C20, from Israel, was successfully transferred to hexaploid wheat through crossing and backcrossing (Lin et al., 2002). Genetic analyses revealed that a single dominant gene controls the powdery mildew resistance at the seedling stage. The gene was assigned to chromosome area 5BS and appears to be a new gene, designated PM30.

A range of spring barley mixtures including one set made from cultivars grown in the UK and one from cultivars grown in Poland, were examined, along with their component cultivars, in nine trials at Scottish Crop Research Institute, Dundee, UK, or at the Experimental Plant Breeding Station of the IHAR, Bakow near Kluczbork, Poland, over five seasons. In four trials where inoculum pressure was controlled, mixtures reduced infection more at lower inoculum pressures, but this did not translate into yield benefit. Smaller plots increased mildew in monocultures but not mixtures. Fertilizer levels increased mildew levels but did not affect mixture efficacy. There were large differences between both Polish and UK germplasm, and between Polish and UK trial sites, but the performance of the mixtures compared with their respective monoculture components was similar within both germplasm groups and trial sites. Mixtures reduced lodging and affected plant height and heading date. The advantages of mixtures for improving yield, reducing fungicide applications and improving agronomic characteristics was demonstrated and there seems to be great potential for their further improvement and exploitation.

Chemical Control

Fungicides are widely used in the control of powdery mildew in cereals. Mildew on wheat and barley can be controlled using morpholines (e.g., fenpropidin), triazoles (e.g., tebuconazole and cyproconazole) and the more recent strobilurin fungicides. Differences in the efficacy of fungicides used in mildew control are due, in large part, to the development of isolates of B. graminis which are tolerant to fungicide groups. Tolerance has been detected to the triazoles, for example, flutriafol, propiconazole and triadimefon. However, these are often combined with another active ingredient, commonly a morpholine such as tridemorph, in order to increase efficacy and reduce pressure on the pathogen to produce triazole-tolerant isolates. Resistance to strobilurin fungicides has been detected in isolates from B. graminis f.sp. tritici from commercial crops in England and Germany (Fraaije et al., 2002). These workers developed a quantitative real-time PCR diagnostic procedure for the early detection of resistance genes at low frequency which, if used with risk evaluation, would be invaluable for further resistance risk assessment and validation of anti-resistance strategies.

Mildew infection on oats can also be controlled using morpholines and triazoles, although in some countries such as the UK, the range of fungicides approved for oats is limited compared to other cereals. A new fungicide, Falcon 460 EC containing the new active ingredient spiroxamine, was introduced in Slovenia in 2000 for the control of a range of diseases including powdery mildew on cereals (Kraner, 2001). In addition, simeconazole (2-[4-fluorophenyl)-1-(1H-1,2, 4-triazol-l-yl)3-trimethylsilylpropan-2-01), a novel triazole fungicide with broad spectrum activity was launched as a new seed treatment (Tsuda et al., 2000). It has activity against powdery mildew (B. graminis) and has been shown to increase wheat yields by 10% compared to untreated crops.

Computer based decision support systems (DSS), for example, EPIPRE, have been developed to aid fungicide spray recommendations for the main diseases on wheat including powdery mildew (Forrer, 1992) and a DSS for disease in spring barley is under development

For further information, see Parry (1990).

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