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Tilletia controversa
(dwarf bunt of wheat)

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Datasheet

Tilletia controversa (dwarf bunt of wheat)

Summary

  • Last modified
  • 30 January 2019
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Tilletia controversa
  • Preferred Common Name
  • dwarf bunt of wheat
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Fungi
  •     Phylum: Basidiomycota
  •       Subphylum: Ustilaginomycotina
  •         Class: Ustilaginomycetes

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Pictures

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PictureTitleCaptionCopyright
Tilletia controversa (dwarf bunt of wheat); infected wheat spike, with glumes removed to expose fungal sori (bunt balls). USA.
TitleSymptoms
CaptionTilletia controversa (dwarf bunt of wheat); infected wheat spike, with glumes removed to expose fungal sori (bunt balls). USA.
Copyright©Peggy Greb/USDA Agricultural Research Service/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); infected wheat spike, with glumes removed to expose fungal sori (bunt balls). USA.
SymptomsTilletia controversa (dwarf bunt of wheat); infected wheat spike, with glumes removed to expose fungal sori (bunt balls). USA.©Peggy Greb/USDA Agricultural Research Service/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
TitleSymptoms
CaptionTilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
Copyright©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
SymptomsTilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
TitleSymptoms
CaptionTilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
Copyright©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
SymptomsTilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
TitleSymptoms
CaptionTilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
Copyright©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
SymptomsTilletia controversa (dwarf bunt of wheat); symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); close view of symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
TitleSymptoms
CaptionTilletia controversa (dwarf bunt of wheat); close view of symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
Copyright©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); close view of symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.
SymptomsTilletia controversa (dwarf bunt of wheat); close view of symptoms. Dwarf bunt in breeders' plots (no seed treatments for 15 years or more). Bozeman, Montana, USA.©Mary Burrows/Montana State University/Bugwood.org - CC BY 3.0 US
Tilletia controversa (dwarf bunt of wheat); teliospores of dwarf bunt of wheat.
TitleTeliospores
CaptionTilletia controversa (dwarf bunt of wheat); teliospores of dwarf bunt of wheat.
Copyright©Jihad Orabi
Tilletia controversa (dwarf bunt of wheat); teliospores of dwarf bunt of wheat.
TeliosporesTilletia controversa (dwarf bunt of wheat); teliospores of dwarf bunt of wheat.©Jihad Orabi
Tilletia controversa (dwarf bunt of wheat); greatly enlarged teliospores of dwarf bunt of wheat.
TitleTeliospores
CaptionTilletia controversa (dwarf bunt of wheat); greatly enlarged teliospores of dwarf bunt of wheat.
Copyright©Jihad Orabi
Tilletia controversa (dwarf bunt of wheat); greatly enlarged teliospores of dwarf bunt of wheat.
TeliosporesTilletia controversa (dwarf bunt of wheat); greatly enlarged teliospores of dwarf bunt of wheat.©Jihad Orabi

Identity

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Preferred Scientific Name

  • Tilletia controversa J. G. Kühn

Preferred Common Name

  • dwarf bunt of wheat

Other Scientific Names

  • Tilletia brevifaciens G. W. Fisch.
  • Tilletia tritici-anifican F. Wagner (nom.inval.)

International Common Names

  • English: dwarf: rye bunt; dwarf: wheat bunt
  • Spanish: caries enana del trigo
  • French: carie naine du blé; carie naine du seigle

Local Common Names

  • Germany: Zwerg-: Roggen Brand; Zwerg-: Weizen Brand; Zwergstein-: Roggen Brand; Zwergstein-: Weizen Brand; Zwergsteinbrand

EPPO code

  • TILLCO (Tilletia controversa)

Taxonomic Tree

Top of page
  • Domain: Eukaryota
  •     Kingdom: Fungi
  •         Phylum: Basidiomycota
  •             Subphylum: Ustilaginomycotina
  •                 Class: Ustilaginomycetes
  •                     Subclass: Exobasidiomycetidae
  •                         Order: Tilletiales
  •                             Family: Tilletiaceae
  •                                 Genus: Tilletia
  •                                     Species: Tilletia controversa

Notes on Taxonomy and Nomenclature

Top of page The spelling 'contraversa' is sometimes found in literature, but is not correct. When Kühn initially described this disease in the journal Hedwigia in 1974, he called it Tilletia 'contraversa'. However, following this publication, the disease was registered as Tilletia controversa. Although Kühn and others continued to refer to T. contraversa, the name remained erroneously(?) registered as T. controversa. Johnsson (1991b) states that 'contraversa' is therefore the proper name.

In a review of current literature, Mathre (1996) found evidence that three bunt pathogens, T. controversa, T. caries and T. foetida, are variants of a single species.

Description

Top of page Sori occur in the ovaries, usually infecting all of them; mostly globose to broadly ellipsoid, covered by the pericarp; normally pulverulent when mature, but may be hard when immature; dark reddish-brown to almost black.

Teliospores are yellow-brown to red-brown (mature spores mostly much darker), globose or subglobose, mostly 19-24 µm (17-32 µm) diameter, mature spores are typically surrounded by a hyaline gelatinous sheath 1.5-5.5 µm thick. In median view, the exospore is reticulate, with relatively large, regular, polygonal areolae, 1.5-3 µm high and 3.5 µm diameter; areolae are occasionally irregular to subcerebriform.

Sterile cells are fewer and generally smaller than the spores, regularly globose, with smooth walls, hyaline or faintly greenish or brownish, sometimes encased in a hyaline, gelatinous sheath 2-4 µm thick; mostly 11-16 µm (9-22 µm) in diameter, including the sheath. For more information, see Duran and Fischer (1961).

Distribution

Top of page See also CABI/EPPO (1998, No. 245).

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

AfghanistanPresentIMI, 1992; CABI/EPPO, 2012; EPPO, 2014
ArmeniaPresentCABI/EPPO, 2012; EPPO, 2014
AzerbaijanAbsent, unreliable recordCABI/EPPO, 2012; EPPO, 2014
Georgia (Republic of)PresentCABI/EPPO, 2012; EPPO, 2014
IranPresentCABI/EPPO, 2012; EPPO, 2014
IraqPresentCABI/EPPO, 2012; EPPO, 2014
JapanPresentCABI/EPPO, 2012; EPPO, 2014
KazakhstanPresentCABI/EPPO, 2012; EPPO, 2014
KyrgyzstanPresentCABI/EPPO, 2012; EPPO, 2014
SyriaPresentCABI/EPPO, 2012; EPPO, 2014
TajikistanRestricted distributionCABI/EPPO, 2012; EPPO, 2014
TurkeyRestricted distributionCABI/EPPO, 2012; EPPO, 2014
TurkmenistanPresentCABI/EPPO, 2012; EPPO, 2014
UzbekistanPresentCABI/EPPO, 2012; EPPO, 2014

Africa

AlgeriaPresentCABI/EPPO, 2012; EPPO, 2014
LibyaPresentCABI/EPPO, 2012; EPPO, 2014
MoroccoAbsent, unreliable recordCABI/EPPO, 2012; EPPO, 2014
TunisiaPresentCABI/EPPO, 2012; EPPO, 2014

North America

CanadaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
-AlbertaAbsent, invalid recordEPPO, 2014
-British ColumbiaPresentCABI/EPPO, 2012; EPPO, 2014
-OntarioPresentCABI/EPPO, 2012; EPPO, 2014
USARestricted distributionCABI/EPPO, 2012; EPPO, 2014
-CaliforniaAbsent, invalid recordCABI/EPPO, 2012; EPPO, 2014
-ColoradoPresentCABI/EPPO, 2012; EPPO, 2014
-IdahoPresentCABI/EPPO, 2012; EPPO, 2014
-IndianaPresentCABI/EPPO, 2012; EPPO, 2014
-KansasPresentCABI/EPPO, 2012
-MichiganPresentCABI/EPPO, 2012
-MontanaPresentCABI/EPPO, 2012; EPPO, 2014
-New YorkPresentCABI/EPPO, 2012; EPPO, 2014
-OregonPresentCABI/EPPO, 2012; EPPO, 2014
-UtahPresentCABI/EPPO, 2012; EPPO, 2014
-WashingtonPresentCABI/EPPO, 2012; EPPO, 2014
-WyomingPresentCABI/EPPO, 2012; EPPO, 2014

South America

ArgentinaPresentCABI/EPPO, 2012; EPPO, 2014
UruguayAbsent, invalid recordCABI/EPPO, 2012; EPPO, 2014

Europe

AlbaniaPresentCABI/EPPO, 2012; EPPO, 2014
AustriaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
BulgariaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
CroatiaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
Czech RepublicWidespreadCABI/EPPO, 2012; EPPO, 2014
DenmarkAbsent, no pest recordCABI/EPPO, 2012; EPPO, 2014; DCA - Nationalt Center for Fødevarer og Jordbrug, Denmark, 2018
FranceAbsent, formerly presentCABI/EPPO, 2012; EPPO, 2014
GermanyRestricted distributionCABI/EPPO, 2012; EPPO, 2014
GreecePresentCABI/EPPO, 2012; EPPO, 2014
HungaryRestricted distributionCABI/EPPO, 2012; EPPO, 2014
ItalyPresentCABI/EPPO, 2012; EPPO, 2014
LatviaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
LithuaniaAbsent, intercepted onlyEPPO, 2014
LuxembourgPresentCABI/EPPO, 2012; EPPO, 2014
MoldovaPresentEPPO, 2014
MontenegroPresentCABI/EPPO, 2012
PolandPresentCABI/EPPO, 2012; EPPO, 2014
PortugalAbsent, intercepted onlyEPPO, 2014
RomaniaPresentCABI/EPPO, 2012; EPPO, 2014
Russian FederationRestricted distributionCABI/EPPO, 2012; EPPO, 2014
-Central RussiaPresentEPPO, 2014
-SiberiaPresentCABI/EPPO, 2012
-Southern RussiaPresentCABI/EPPO, 2012; EPPO, 2014
-Western SiberiaPresentEPPO, 2014
SerbiaAbsent, unreliable recordEPPO, 2014
SlovakiaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
SloveniaRestricted distributionCABI/EPPO, 2012; EPPO, 2014
SpainAbsent, invalid recordCABI/EPPO, 2012; EPPO, 2014
-Spain (mainland)Absent, unreliable recordCABI/EPPO, 2012
SwedenWidespreadCABI/EPPO, 2012; EPPO, 2014
SwitzerlandWidespreadCABI/EPPO, 2012; EPPO, 2014
UkraineRestricted distributionCABI/EPPO, 2012; EPPO, 2014

Oceania

AustraliaAbsent, invalid recordBiosecurity Australia, 2010, personal communication; CABI/EPPO, 2012; EPPO, 2014
-New South WalesAbsent, invalid recordBiosecurity Australia, 2010, personal communication; CABI/EPPO, 2012; EPPO, 2014
-South AustraliaAbsent, invalid recordBiosecurity Australia, 2010, personal communication; CABI/EPPO, 2012; EPPO, 2014
-Western AustraliaAbsent, invalid recordBiosecurity Australia, 2010, personal communication; EPPO, 2014
New ZealandAbsent, unreliable recordCABI/EPPO, 2012; EPPO, 2014

Risk of Introduction

Top of page RISK CRITERIA CATEGORY

ECONOMIC IMPORTANCE Moderate
DISTRIBUTION Worldwide
SEEDBORNE INCIDENCE Low
SEED TRANSMITTED Yes
SEED TREATMENT Yes


Notes on Phytosanitary Risk

None of 552 samples examined in 1983 had dwarf bunt infestation levels <1 g of teliospores/kg seed (equivalent to 20,000 teliospores/seed) necessary to result in bunted spikes when the seed is planted. Therefore, in areas where this disease is not known to occur, there seems to be minimal risk that the importation of US-produced wheat grain with low levels of infestation will result in significant disease development (Grey et al., 1986).

Very probably, T. controversa has reached the limits of its natural distribution in the EPPO region (Wagner, 1966). However, its introduction into areas where it does not occur would certainly create difficulties for the export of grain to other continents, even if direct damage was slight.

Phytosanitary measures
Since the main mode of dispersal is by seeds, seed crops of wheat from countries where the fungus occurs should be examined during the growing season and found free from T. controversa (OEPP/EPPO, 1990).

Habitat List

Top of page
CategorySub-CategoryHabitatPresenceStatus
Terrestrial
Terrestrial – ManagedCultivated / agricultural land Principal habitat Harmful (pest or invasive)

Hosts/Species Affected

Top of page Wheat (Triticum spp.) is the principal host for T. controversa. Winter wheat is especially attacked, but, the fungus is not known to occur on spring-sown wheat. It has also been reported on barley in northern Utah, USA (Dewey and Hoffmann, 1975).

Occasionally other Poaceae (68 species) are attacked, such as Aegilops spp., Agropyron spp., Alopecurus myosuroides, Alopecurus agrestis, Arrhenatherum elatius, Bromus spp., Dactylis glomerata, Elymus spp., Festuca spp., Hordeum bulbosum, Hordeum leporinum, Hordeum vulgare, Koeleria macrantha, Lolium spp., Poa spp. and rye (see also Hardison et al., 1959).

Growth Stages

Top of page Flowering stage, Vegetative growing stage

Symptoms

Top of page Identification is difficult, since both the characteristics of the causal organism and symptom expression of the host vary widely; its only consistent characteristics are the long incubation period (at least 21 days) and the low temperature <15°C) required for teliospore germination.

An infected plant shows no obvious sign of disease until the ears emerge, but its stem is usually shorter than in healthy plants and it may have more tillers. Bunted ears are somewhat narrower than healthy ones, but, as ripening proceeds, the glumes are pushed apart laterally so giving them a characteristic dishevelled appearance. Sori are spherical (elliptical in common bunt - T. caries) and contain a blackish powdery mass of teliospores surrounded by a thick grey-brown tegument. For more information, see Grassner and Niemann (1954), Baylis (1958), Purdy et al. (1963), Rapilly et al. (1966).

List of Symptoms/Signs

Top of page
SignLife StagesType
Inflorescence / black fungal spores
Whole plant / dwarfing
Whole plant / unusual odour

Biology and Ecology

Top of page Significant infection by T. controversa does not result from seedborne spores, but originates almost exclusively from soil infestation. T. controversa survives between crops as teliospores in the soil and on seed. Spores can remain viable in the soil for 3-10 years in the absence of wheat and are able to pass through the digestive tract of chickens and cows without losing their viability (Smilanick et al., 1986).They typically germinate following a preconditioning exposure to light and at least 3-5 weeks at about 5°C. The most favourable conditions for infection are temperatures of 0-8°C (maximum 10-12°C), as found under persistent snow cover. Exposure to temperatures of 15°C inhibits spore germination (Tyler, 1958; Hoffmann, 1982). Dwarf bunt tends to be localized at altitudes of 300-1000 m, and years with frequent snow falls are usually associated with serious attacks. Soil compaction and shallow seeding also promote dwarf bunt infection. Most infection occurs in the winter (December to February-April) when plants are forming susceptible stem buds.

In grass crops, infection is apparently confined to the secondary stem buds of grass seeded the previous spring. Lack of infection in grass plants over 1 year old is correlated with the absence of stem buds from many species during the winter/spring infection period and associated with the extensive development of basal leaves which protect the buds (Hordison, 1963).

Infection of winter wheat by T. controversa does not occur during seed germination to seedling emergence, but only after the seedling is well established. Following penetration, mycelium passes into the crown and keeps pace with the growth of the apex until the ear is formed. A smut ball (sorus) containing teliospores then forms in the ovaries.

A number of races differing in pathogenicity exist and continue to be distinguished (at least 15 in the northwestern USA alone; Hoffmann and Metzger, 1976). For more information, see Grassner and Niemann (1954), Baylis (1958), Tyler and Jensen (1958), Purdy et al. (1963).

The pathogen is mainly dispersed on infected wheat and grass seed, However, dispersal with soil or manure might be possible.

Seedborne Aspects

Top of page

Incidence

In a survey of 552 samples of wheat of for export from Pacific Northwest ports in 1983, 141 were free from T. controversa. The remaining samples, representing all classes of wheat, contained dwarf bunt teliospores, but none of these seed lots was heavily infested i.e. contained >1 g of teliospores/kg seed (equivalent to 20,000 teliospores/seed) (Grey et al., 1986).

Bechtel et al. (1999) showed that a high percentage of spores can be removed from wheat by mechanical cleaning but that it is not feasible to remove all of them.

Effect on Seed Quality

Teliospores released from sori adhere to the seed resulting in discolouration, due to blackening of the kernels, and a typical fishy odour of trimethylamine. The usefulness of the kernels for milling is reduced. Pirson (1978) estimated that 5 mg of bunt balls could contain as many as 1 million spores.

Pathogen Transmission

Spores adhering to the seed can contaminate uninfested soil and, under suitable conditions, germinate and penetrate the emerging seedling where infection becomes systemic (Neergaard, 1977). Field tests were conducted in five states of the USA over 2 years to determine if seedborne T. controversa teliospores could induce disease. Bunted spikes resulted only when heavily infested seed (>1 g teliospores/kg) was planted in disease-conducive locations (Grey et al., 1986).

The incidence of T. controversa as a function of inoculum density was studied at three disease-conducive locations in Montana, USA, for three seasons (1993-1996) (Goates and Peterson, 1999). In soil-inoculated plots, a minimum of 16 x 10³ teliospores/row was needed to cause trace amounts of disease (0.6% maximum). Only trace amounts or no disease occurred below the 16 x 10<(sup)5> rate. In the seed-inoculated plots, infection was rare and occurred only at inoculation rates of 2 x 10<(sup)5> teliospores/g or higher, with the highest disease incidence being 0.4%.

Seed Treatment

Fungicides

In a test of 22 formulations of fungicidal seed treatments, thiabendazole provided the highest level of control (Hoffman et al., 1983). Mathre et al. (1990) found that infection of healthy seed was reduced when seeds were treated with formulations containing carboxin or thiabendazole. All teliospores of T. controversa were destroyed when 0.13 M NaOCl was applied at 55°C for 30 seconds (Chastain, 1991). Although these seed treatments provide a means to control the spread of the disease to uninfected areas, Fushtey (1961) found that it was not effective in protecting the crop from infection by soilborne inoculum. The first completely effective control of the dwarf bunt fungus in susceptible wheat varieties was achieved with difenoconazole in 1994 (Sitton et al., 1993).

Treatment of winter wheat (Triticum aestivum) seeds with Dividend XL RTA (difenoconazole+metalaxyl-M), Vitaflo-280 (carbathiin+thiram), or bioagent ACM941, a strain of Clonostachys rosea, all significantly reduced seedborne instances of T. controversa. Dividend XL RTA was the most effective treatment, reducing pathogen instance by 98-99% relative to the control (Xue et al., 2007).

Fumigation

It has been suggested that although treatment in a chemosterilizer, that produces hydrogen peroxide vapour by a pulse-injection system, has insufficient activity for quarantine purposes if sori are present, it may be a practical seed-surface disinfestation process for nonhost seeds, such as barley, where contaminating teliospores from grain handling equipment are borne superficially on seed, and sori are rarely or not present (Smilanick et al., 1994).

Seed Health Tests

Wash method (Kietriber 1984).

- A quantity of seed is agitated in water to which a small amount of detergent has been added.
- The resultant suspension is decanted, centrifuged and resuspended.
- The resuspension is then either (a) filtered through a cellulose nitrate membrane which is then air dried, and examined under a microscope; or (b) placed on a haemocytometer and examined under a microscope.
- Spores of T. controversa are identified and counted.

PCR - Multiple Tilletia spp. (McDonald et al., 1999)

A simple technique of conducting PCR on single ungerminated teliospores of Tilletia species was developed. Teliospores were manually cracked under a stereo microscope prior to adding to the PCR reaction mixture. Amplification product was obtained using primers for either a portion of the nuclear ribosomal intergenic spacer region or a portion of the mitochondrial DNA. Collections of teliospores from T. indica, T. barclayana, T. controversa, T. tritici, T. laevis and an unidentified Tilletia sp. from Lolium varied in the proportion of spores from which amplification product could be detected, with the success rate ranging from 100 to 10%. This technique avoids the difficulty and time delay in having to germinate teliospores prior to extracting DNA from a mycelial matte and thus will be of great value in the application of PCR methods for regulatory testing and phylogenetic studies of Tilletia species.

Extracting spores T. controversa from bran (Zhang et al., 2002)

Methods of extracting spores of T. controversa from bran were studied. Three methods 'sieving-alpha-amylase degradation', 'sieving-density gradient centrifugation' and 'sieving-alpha-amylase degradation-density gradient centrifugation' were used to extract the spores. The number of T. controversa spores extracted depended on the use of sieving, alpha-amylase and density gradient centrifugation while the purity mainly on the density gradient centrifugation. Between 84 and 94% of bran could be removed by the method of double-layer gauze and five different combinations of sieving treatments. Between 95 and 99% of bran could be removed and 19 to 52% of T. controversa spores could be extracted by the method of sieving-alpha-amylase degradation. Using the sieving-density gradient centrifugation method, only a treatment of 60 meshes + 200 meshes + 300 meshes + 30 µm + 11 µm worked so that 7.2% pure spores were obtained after further density gradient centrifugation, which did not influence the reticulum height, autofluorescence and germination of spores. Using the sieving-alpha-amylase degradation-density gradient centrifugation method, a treatment of 60 meshes + 200 meshes + 300 meshes and 60 meshes + 200 meshes + 300 meshes + 30 µm + 11 µm was effective, and 18.8 and 12.2% of pure spores were obtained after further density gradient centrifugation, which did not influence the reticulum height of spores. The most effective condition for alpha-amylase to degrade insoluble starches was at 71°C and 350 r/min for 3 mins. These methods have been extensively applied in routine quarantine at ports.

Extraction of TCK spores from soil

A sucrose-centrifugation method was developed to extract teliospores of T. indica, T. controversa and T. barclayana from soil (Babadoost and Mathre, 1998).

Notes on methods

Where positive identification cannot be made from spore morphology alone, biological tests (spore germination on inoculation to differential hosts) may be necessary. Spores can be distinguished from those of Tilletia caries by fluorescence microscopy (Stockwell, 1986).

A rapid biochemical test was developed to indirectly assess the viability of teliospores of T. controversa that contaminate wheat grain. Lipase activity was detected consistently in extracts from viable teliospores by a fluorescein diacetate (FDA) assay (Chastain and King, 1990).

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
Stems (above ground)/Shoots/Trunks/Branches hyphae Yes Pest or symptoms usually invisible
True seeds (inc. grain) hyphae; spores Yes Yes Pest or symptoms usually invisible
Plant parts not known to carry the pest in trade/transport
Bark
Bulbs/Tubers/Corms/Rhizomes
Fruits (inc. pods)
Roots
Seedlings/Micropropagated plants
Wood

Impact

Top of page Dwarf bunt is a serious disease, particularly of winter wheat at relatively high altitudes. It is very difficult to control because of the resistant resting spores which remain viable in the soil for a number of years and because most seed-applied fungicides are not effective. In Oregon, USA, in 1952-1953, dwarf bunt destroyed 50-90% of the seed in several 1-year-old fields of Elymus hispidus and Arrhenatherum elatius (Hordison, 1963). Disease is occasionally severe in susceptible cultivars of winter wheat in the western USA where wheat is grown under a persistent snow cover. In 1983, Grey et al. (1986) examined 552 samples of wheat being exported from the Pacific Northwestern USA for T. controversa. Of the samples tested, 411 were found to be infected, but none contained levels greater than 1 g of teliospores/kg of seed. Further experiments found that bunted spikes only occurred at levels of infection higher than the samples described and only in disease-conducive conditions (Grey et al., 1986). They then concluded that the risk of transmission of high levels of disease to new areas by such low levels of infestation was minimal. Since February 1974, the export of wheat from Pacific Northwest ports to China has been halted as China has prohibited the introduction of grain carrying dwarf bunt.

The possibility for establishment of T. controversa in winter wheat regions in China was assessed by simulation models and geographical information system (GIS) by Chen et al (2002). The winter wheat regions were divided into four areas, high, medium, low and basically no risk areas. In the high risk area, T. controversa occurs over nine times every eighteen years; the medium risk area, four to eight times every eighteen years; and the low risk area, one to three times every eighteen years. The percentage of areas with high and medium risk is 19.3% of the whole winter wheat regions of the country.

In Italy, epidemic outbreaks of dwarf bunt were reported in 1879, 1919, 1929 and 1946; however, in 1956, the disease was reported not to cause great concern. In Bavaria (Germany), up to 30% losses in wheat yield were recorded on crops grown more than once in a rotation of 5-6 years. In the 1970s, the disease was reported to be of great economic importance in the EPPO region in Austria, Poland and the former USSR and of less economic importance in the other countries in which it is established.

While the dwarf bunt fungus only affects a small portion of the total wheat production worldwide, presence of this disease has heightened economic importance due to the possibility of spread of the disease to uninfected areas (Mathre, 1996).

Diagnosis

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T. controversa was identified in wheat plants using PCR (Kochanová et al., 2004). Both Yuan et al. (2009) and Nian et al. (2009) developed a TaqMan real-time PCR to successfully detect T. controversa in asymptomatic wheat tissue. Gao et al. (2010) designed specific primers using sequence characterised amplified region (SCAR) for use in PCR detection assays.

A different method of diagnosis, using amplified fragment length polymorphism (AFLP), is detailed in Liu et al. (2009). Alternatively, Cai et al. (2009) successfully diagnosed the pathogen during early stages of infestation in wheat using the hyper-branched rolling cycle amplification (HRCA).

Image analysis using disease micrographs has also been used to identify T. controversa (Deng et al., 2012). Overall recognition accuracy using this method was 82.9%.

Detection and Inspection

Top of page In order to detect dwarf smut spores, a sample of seed should be agitated in water, filtered through gauze, centrifuged at 1500 revolutions per minute for 2 minutes and the remaining sediment mounted in Shears solution for microscopic examination at x 100-900 (Schoen, 1974). If spores are mounted in lactophenol, the sheath is hardly visible. Where positive identification cannot be made from spore morphology alone, biological tests (spore germination on inoculation to differential hosts) may be necessary. Spores can be distinguished from those of Tilletia caries by fluorescence microscopy (Stockwell, 1986).

Teliospores are produced in large quantities when Tilletia controversa is grown at 15-18°C on a wheat-based medium. Vegetative hyphae of T. carries and T. controversa were not distinguishable on this media (Trione, 1974). For more information on the culture of T. controversa see Trione et al. (1989) or Trione (1964, 1974).

Prevention and Control

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Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Host-Plant Resistance

Varietal resistance is the primary means of control. Several resistant wheat cultivars are available including Weston, Hansel and Winridge (Taylor et al., 1983; Johnsson, 1988).

Chemical Control

Fungicides applied to the soil surface give control but are uneconomic (Hordison, 1963). Systemic fungicides have been used, for example, etaconazole has been used to provide good control after disease establishment in the plant (Hoffmann, 1971; Hoffman et al., 1983). Experiments in Sweden showed that seed treatments, in combination with the use of resistant cultivars, suppressed disease by up to 99.6% (Johnsson, 1991a). In a test of 22 formulations of fungicidal seed treatments, thiabendazole provided the highest level of control (Hoffman et al., 1983). Mathre et al. (1990) found that the infection of healthy seed was reduced when seeds were treated with formulations containing carboxin or thiabendazole. All teliospores of T. controversa are destroyed when 0.13 M of NaOCl are applied at 55°C for 30 s (Chastain, 1991). Although these seed treatments provide a means to control the spread of the disease to uninfected areas, Fushtey (1961) found that they were not effective in protecting the crop from infection by soil-borne inoculum. The first completely effective control of the dwarf bunt fungus in susceptible wheat varieties was achieved with difenoconazole in 1994 (Sitton et al., 1993).

References

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