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Plantwise Technical Factsheet

wheat bunt (Tilletia tritici)

Plantwise images
Host plants / species affected
Agropyron (wheatgrass)
Bromus (bromegrasses)
Elymus (wildrye)
Festuca (fescues)
Hordeum (barleys)
Lolium (ryegrasses)
Poa (meadow grass)
Poaceae (grasses)
Secale cereale (rye)
Triticale
Triticum (wheat)
Triticum aestivum (wheat)
Triticum dicoccum
Triticum turgidum (durum wheat)
List of symptoms/signs
Inflorescence  -  black fungal spores
Inflorescence  -  discoloration panicle
Seeds  -  galls
Whole plant  -  dwarfing
Symptoms
Affected plants are slightly to moderately stunted. The heads are slender and remain green longer than healthy heads; the glumes have a tendency to gape open, exposing the bunt balls. However, affected heads can often be difficult to distinguish from healthy heads.

Usually, all ovaries in a head are affected, but not all heads on a plant may be infected. The entire kernel is usually replaced by bunt balls. Occasionally, the kernel is only partially affected.

Bunt balls are about the same shape and size as normal kernels but are grey-brown in colour. When crushed, they release a black, powdery mass of spores with a characteristic fishy smell. Many bunt balls are crushed at harvest and the grain can be discoloured grey by masses of bunt spores. This grain will smell of rotten fish. When infection is high, a dark smoke-like cloud can be released from the harvester.

See Wiese (1987) and Goates (1996) for further information.
Prevention and control

Introduction

Control is essential where T. tritici is endemic. Resistance is available but its use is made difficult by pathogenic variation in the fungus. Chemical control is readily achieved and relatively inexpensive.

Seed Treatments

The Austrian Health Certification Scheme regards rates of Tilletia laevis/tritici spores up to 50 per grain as satisfactory for sowing, whereas rates up to 100 per grain can be sown if seed treatment is applied (Neergaard, 1977). Spores on the seed are readily controlled by a range of contact and systemic fungicides, which may also prevent infection of seedlings. Treatment of all seed wheat will reduce common bunt to trace levels.

Registered seed treatments in Australia in 1996 included: flutriafol/cypermethrin, triadimenol/cypermethrin, tebuconazole/cypermethrin, carboxin/cypermethrin and carboxin (Wallwork et al., 1995).

Benzimidazole fungicides
Benomyl, carbendazim, and thiophamine (Bourdin et al., 1974).

Triazole fungicides
Difenoconazole (Bassi et al., 1994)
Metconazole (Sampson et al., 1992)
Triadimenol (Tragner-Born and Boom, 1978)
Triticonazole (Mugnier et al., 1993)

Other organic chemical fungicides
Carboxin (Ilyukhin and Dzhiembaev, 1974); efficacy falls when lower rates are applied (Gaudet et al., 1992)
Carbendazim (Singh et al., 1977)
Fenpiclonil (Koch and Leadbeater, 1992; Rieu et al., 1993)
Myclobutanil (Efthimiadis, 1988)

Inorganic fungicides
Copper carbonate dusts were the first widely used dry seed treatment, and during the 1920s reduced bunt from a major to a minor problem in wheat (Neergaard, 1977). There seems to be some renewed interest in their use in the United Kingdom: copper oxychloride was used for control (Yarham, 1994).

Hydrogen peroxide vapour is useful for killing spores admixed with seed but does not kill spores within sori (Smilanick et al., 1994).

Other
In organic agriculture, seedborne diseases are important due to their strong potential for increase and the limited range of effective seed treatments that are acceptable for this form of agriculture (Borgen et al., 2000). Electron beams (Burth et al., 1991), thermal treatment with humid air (Kristensen and Forsberg, 2000) and direct seed treatments with skimmed milk powder, wheat flour, powdered seaweed (Becker and Weltzien, 1993), hot water, mustard extracts and acetic acid (Borgen et al., 2000) have been studied. Seed dressing based on mustard extracts and seed rinsing plus coating with milk powder were the most effective organic treatments found by Plakolm and Sollinger (2000). Seed treatment with skimmed milk powder and skimmed milk gave similar control to chemical seed treatments in field trials over 4 years in Egypt (El Naimi et al., 2000). Winter et al. (1997) found that skimmed milk powder and warm water treatments were suitable at low infection pressure but less effective than chemical fungicides at higher disease pressure.

The efficacy of control by the antagonistic organisms Bacillus subtilis, Trichoderma harzianum and Pythium oligandrum was influenced by varietal sensitivity, soil temperature and moisture (Filkuka, 2000).

Once harvesting equipment is contaminated with teliospores, up to five emptyings with non-contaminated grain is needed to reach an acceptably low level of contamination. Thus, a preventive strategy is to omit the first five emptyings during harvest before collecting seed grain (Kristensen et al., 1996).

Seed testing was recommended for organic agriculture: when the inoculum level exceeded the threshold level, a warm water treatment (2 h at 45°C) should be used (Ruegger et al., 1998).

Discontinued treatments
Although very effective, organomercurial compounds have been discontinued in most countries because of environmental and human health concerns.

Fenaminosulf is highly effective against bunt (Kuiper, 1974a; Moore and Kuiper, 1974), but its use has been discontinued because of environmental and health concerns.

Mancozeb and maneb are also highly effective (Moore and Kuiper, 1974). However, emergence declines when treated seed is stored for 1 year before sowing; such seed should be sown within 2 months of treatment to avoid phytotoxicity (Kuiper, 1974b). Dithiocarbamates provide good control of bunt (Gilchrist and Christie, 1998) but their effect on seed emergence after prolonged storage of treated seed has not been reported.

Seed treatment strategies in the UK were recently reviewed by Paveley et al. (1997).

Cultural Control

Wheat sown into soil while the temperatures remain above 15°C will escape infection.

Breeding for Resistance

Goates (1996) states that 15 major resistance genes have been identified. These are used in monogenic lines to identify pathogenic races of T. tritici and T. laevis. Because of the large number of pathogenic races and the readily available control with fungicides, there has been only limited use of resistant varieties to control common bunt.

Babayants et al. (1999) reported dominant resistance that was not identical to the known resistance genes and is probably new. Resistance genes are in Hordeum chilense on chromosome 7 and to a lesser extent on chromosome 6 (Rubiales and Martin, 1999).

Selection for resistance in breeding programmes has relied on screening by inoculating seed with teliospores and assessing subsequent bunt incidence in the heads. On the basis of field experiments, Pospisil et al. (2000) recommended that to assess resistance to common bunt, the seed should be inoculated with at least 1 g of spores per kg seed, plots should be replicated four times in different parts of the field, 100-150 ears should be scored, and the assessment repeated in different years.

A PCR marker has been developed for rapid identification of the Bt-10 gene for common bunt resistance (Laroche et al., 2000). A transgene from Ustilago maydis-infecting virus has increased endogenous resistance to bunt in wheat (Clausen et al., 2000).

Impact
Common bunt caused by T. tritici and T. laevis is potentially very important in most wheat-growing areas of the world. However, these two fungi are readily controlled with chemical seed treatments and the disease is now usually rare or minor. Common bunt has a greater effect on the value of wheat because contaminated grain has an objectionable odour and will be rejected by most growers (Brennan and Murray, 1988). Untreated, common bunt can destroy more than 50% of grain, but losses are usually 5-10% (Goates, 1996; Saari et al., 1996). Infection by T. tritici has been shown to increase with depth of sowing, and to be greater on sandy soil than on moderate or heavy soil (Arafa, 1981).

In surveys of West Asia and North Africa, Mamluk and Zahour (1993) found that the average kernel infection per spike indicated a yield loss of 0.925 in relation to disease incidence in naturally infected spikes. Using inoculation tests, the authors reported that T. tritici had a clear host-preference for durum wheat. Field surveys in Turkey in 1972-74, where seed treatment is commonly used against common bunt, revealed that T. tritici and T. laevis affected ca 10% of fields causing 10-15% damage, but sometimes 90% in fields where untreated seed was sown (Parlak, 1981).

Greek field trials between 1985 and 1988 revealed that there was a correlation between bunt infection and yield increase following treatment with myclobutanil (Efthimiadis, 1988). In 1973, fungicide treatments gave good control of T. tritici, resulting in grain yields of 3.15-3.21 t/ha compared with 0.83 t/ha for the infected control and 2.75 t/ha for the uninfected control. In 1974, the same treatments gave grain yields of 3.13-3.51 t/ha compared with 1.77 t for the infected control and 3.26 t/ha for the uninfected control (Skorda, 1974).

In Denmark, infection of wheat with T. tritici was high in seed treatment trials during 1966-70. Seed treatment increased yield by 1.4 to 1.7 t/ha. Field emergence was increased by 1.3% in barley and rye and by 8-13% in wheat (Noeddegaard and Hansen, 1972).

In Poland, 20 varieties of winter wheat were screened for resistance to T. tritici. The percentage of diseased ears ranged from 0.75 to 20. In most cases, grain yield losses were related to the percentage of diseased ears found on experimental plots with susceptible varieties (Kubiak, 1998). A field experiment was conducted in 1996 to determine the effect of new fungicides on winter wheat diseases including T. tritici. The use of the fungicides significantly increased winter wheat yields (Bobes et al., 1971).

Romanian wheat trials for the control of bunt were carried out between 1956 and 1965. In 1965, seed treatment increased yield from 1.69 t grain/ha in the untreated control to 3.96 t/ha. In 1962, seed treatment increased yields from 4.14 to 4.54 t/ha (Bobes et al., 1971).

In southern Italy, fungicide dressings on durum wheat seeds were effective against T. tritici infections. Yields were significantly higher than those of the control with one exception (Cariddi and Lops, 1996).

Common bunt and loose smut (Ustilago nuda) are the most important cereal crop diseases in Kazakhstan. Seed treatment decreased the incidence of T. tritici on wheat from 11.7 to 1.9% and on barley from 61.7 to 22.6%, increasing yield (Merls and Stepanov, 1980).

In experiments in Russia conducted during 1984-87 with seven fungicides applied as seed treatments, control of bunt was achieved, accompanied by increased yield (Mikhailyukov, 1989). In trials with 14 fungicides, infection by T. tritici was decreased by 97-98% and yield was increased by 8-37% (Novokhatka and Stril'nik, 1978). A pre-sowing treatment of wheat seeds by soaking in a fungicidal solution for 24 hours reduced the incidence of T. tritici. Growth vigour, stand density and yield were increased (Arkangel'skaya et al., 1980).

In Australia, the average potential annual loss in wheat was estimated at $A 270 million, based on the fact that contaminated grain has an objectionable odour, although the actual loss is negligible (Brennan and Murray, 1998).
Related treatment support
Plantwise Factsheets for Farmers
Faizi, Z.; Stanbkzai, Z.; CABI, 2012, English language
Faizi, Z.; Stanbkzai, Z.; CABI, 2012, Dari language
 
Pest Management Decision Guides
Ghanizada, G.; CABI, 2014, English language
Ghanizada, G.; CABI, 2014, Dari language
 
External factsheets
Bayer CropScience Crop Compendium, Bayer CropScience, English language
CIMMYT Plant Pest and Disease Factsheets, Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMYT) (International Maize and Wheat Improvement Center), English language
Bayer CropScience Crop Compendium, Bayer CropScience, English language
HGCA On-Farm Information, Home-Grown Cereals Authority (HGCA), English language
HGCA On-Farm Information, Home-Grown Cereals Authority (HGCA), English language
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