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Plantwise Technical Factsheet

flag smut of wheat (Urocystis agropyri)

Host plants / species affected
Aegilops triuncialis var. triuncialis
Agropyron (wheatgrass)
Agrostis (bentgrasses)
Agrostis capillaris (common bent)
Agrostis stolonifera var. palustris (bent grass)
Alopecurus (foxtails)
Avena sativa (oats)
Bromus (bromegrasses)
Bromus carinatus (California brome grass)
Bromus marginatus (Mountain brome(grass))
Calamagrostis canadensis
Carex (sedges)
Dactylis glomerata (cocksfoot)
Elymus (wildrye)
Elymus repens (quackgrass)
Elymus smithii (Colorado bluestem)
Festuca (fescues)
Festuca rubra (red fescue)
Glyceria (mannagrass)
Holcus (softgrasses)
Hordeum (barleys)
Koeleria
Lolium (ryegrasses)
Lolium multiflorum (Italian ryegrass)
Lolium perenne (perennial ryegrass)
Melica
Phleum (timothies)
Phleum pratense (timothy grass)
Poa (meadow grass)
Poa pratensis (smooth meadow-grass)
Thinopyrum elongatum
Trisetum
Triticum (wheat)
Triticum aestivum (wheat)
Triticum dicoccum
Triticum turgidum (durum wheat)
List of symptoms/signs
Inflorescence  -  discoloration panicle
Leaves  -  abnormal forms
Leaves  -  fungal growth
Leaves  -  leaves rolled or folded
Seeds  -  empty grains
Seeds  -  shrivelled
Stems  -  stunting or rosetting
Stems  -  witches broom
Vegetative organs  -  surface lesions or discoloration
Symptoms
Infected leaves and leaf sheaths exhibit white to yellow streaks that turn grey and then black. Sori then appear in vegetative parts, most commonly in the leaves, as narrow elongated blisters between the leaf veins, some extending almost the entire length of the leaf lamina. Young sori are covered by the host epidermis, which later ruptures to expose the spore mass, splitting the leaves into ribbons (Mordue and Waller, 1981).
 
The leaves may droop and be thin and stunted before sporulation occurs. Black streaks may also occur on the stem, which is stunted and dwarfed at the internodes. Infected wheat plants are thus stunted and spindly, often bearing more tillers than healthy plants, which gives them a grass-like appearance. The inflorescence is stunted, distorted, and frequently sterile, the rachis bearing black streaks at maturity. Spike development usually stops before the head emerges from the leaf whorl, so that infected plants do not produce seeds. In cereals, especially in resistant varieties, only some tillers may show symptoms (Purdy, 1965).
 
Symptoms on turfgrasses are described by Smiley et al. (2005) as stunting, yellowing in cool weather, and stiffening of leaves, with streaks turning from yellow-green to dark grey. After the sori mature and release spores, the leaves turn brown, dry up and die.

Prevention and control

Prevention

U. agropyri is soilborne and externally seedborne. Therefore, in the past, quarantine regulations on the movement of infested seed, chaff and farm machinery from endemic areas were established to restrict its spread (Anon., 1919; 1925; 1931; 1935; 1946; 1953; 1955). However, quarantine restrictions have been gradually lifted or relaxed because U. agropyri has now been reported from many agroclimatic regions worldwide (UK CAB International, 1991). Line (1998) argues that quarantines were, and are, not necessary for the control of flag smut of wheat [Triticum aestivum], because seed treatment, the use of resistant cultivars, and appropriate cultural methods can control the disease. Nevertheless, in Canada, where the fungus does not occur on wheat (Sansford et al., 1999), restrictions on importation of wheat from parts of the USA and from countries worldwide where “wheat strains” of U. agropyri occur are maintained (CFIA, 2008). The USA maintains quarantine measures on various articles of wheat or those made from wheat, such as seeds, plants, straw, etc., which may carry “foreign strains” of the flag smut pathogen that could be introduced (Anon., 2005).
 
Eradication 
 
Sufficiently long cycles of crop rotation may result in eradication of inoculum in the soil, but the smut spore balls are known to persist for several years (Purdy, 1965). Repeated planting of resistant varieties may have the same local effect; Purdy (1965) attributes the decline or disappearance of flag smut on wheat from parts of the USA to the use of resistant varieties.
 
Control
 
Cultural Control
 
Cultural practices and sanitary methods that tend to reduce inoculum, such as rotation with non-hosts, early fallow with thorough working of the soil, and burning stubble, are quite useful. Nevertheless, as Wiese (1987) indicates, spore balls are capable of surviving in soil for at least 3 years, so shorter rotations or fallow periods may only reduce the soilborne inoculum to some extent. Manipulation of planting time and depth, to conserve soil moisture and avoid soil temperatures that are favourable for spore germination and subsequent infection, can also be effective in disease control in specific areas. However, such measures may not be profitable because they may result in lower yields (Purdy, 1965). The disease is favoured when practices such as deep planting, shallow tillage and early-autumn planting are followed in areas where soil moisture is a limiting factor for wheat production (Line, 1998).
 
In Egypt, the 'afir method' of planting wheat is better than the 'herati method' (Jones and El Nasr, 1938) for control of flag smut in endemic areas. In the afir method, seed is planted in dry soils that are later irrigated, so that the inoculum is 'dormant' while seedlings are at a susceptible stage, whereas in "herati", seed is planted in moist soil, where spore inoculum is able to germinate and infect seedlings.
 
In Australia, Greenhalgh and Brown (1984) found that the incidence of flag smut was higher at -126 kPa than at -37 kPa; however, it was generally unaffected by the depth of sowing.
 
Although the effects of fertilizers on flag smut of wheat are complicated (Purdy, 1965), increased levels of nitrogen are known to favour the disease in turfgrasses (Smiley et al., 2005). In India, Kumar and Singh (2004) found that higher levels of nitrogen and phosphorus fertilizer, as well as the addition of poultry manure to the soil, reduced the level of the disease in wheat, although not to the low level obtained with seed treatments.
 
Host-Plant Resistance
 
Among all the cereal smuts, U. agropyri has shown the least tendency towards pathogenic specialization (Fischer and Holton, 1957). According to Hafiz (1986), only six races were reported, in all, from different countries. Some peculiar features in the life cycle of this fungus (e.g. production of fewer sporidia compared to related smut fungi, the absence of secondary sporidia, and sporidial fusions in situ) are probably responsible for limiting its variability (Nelson and Duran, 1984). As a consequence, resistance to flag smut has remained effective in Australia and elsewhere (Platz and Rees, 1980; Goel, 1992) and new races have not been detected (Line, 1998). Purdy (1965) and Line (1998) attribute disappearance of flag smut as a problem on wheat in parts of the USA primarily to the use of resistant varieties.
 
According to Johnson (1984), there is no evidence of genotype-specific pathogenicity of the type indicating a gene-for-gene relationship. Therefore, the incorporation of resistance into commercial cultivars offers prospects for continued effective disease management (Goel, 1992).
 
Chemical Control

 
Effective control of flag smut of wheat originating from both seedborne and soilborne inoculum was achieved through seed treatment with quintozene (Yasu and Yoshino, 1963). With the advent of systemic fungicides, effective disease control without a marked reduction in seedling survival was obtained with benomyl, carboxin and oxycarboxin seed treatments (Metcalfe and Brown, 1969).
 
Dry seed-dressing with non-systemics such as copper carbonate, and systemics such as carboxin, oxycarboxin and pyracarbolid have also been used (Neergaard, 1977). In addition, fenfuram, triadimefon, triadimenol and tebuconazole provide control of U. agropyri in the Indian subcontinent (Goel and Jhooty, 1985a; Tariq et al., 1992).
 
Flag smut of wheat has ceased to be a problem in the regions where seed treatment with systemics is a routine practice for the control of this disease and for loose smut (Ustilago tritici). As Loughman (1989) states concerning flag smut on wheat in Australia, seed treatment may be applied, not so much for the resulting increase in yield (of susceptible varieties), as to prevent an increase in the fungus population in the soil. A reduction in the use of seed treatment with fungicide is considered to be one factor in the increase of flag smut in the Near East and North Africa (Mamluk, 1998). The fungus readily produces significant amounts of long-lived inoculum from a small number of infected plants.

Related treatment support
 
External factsheets
Department of Agriculture and Food Western Australia Farmnotes, Government of Western Australia, 2011, 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
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