|Scientific Name||Tilletia controversa J.G. Kühn,|
|Synonyms||T. brevifaciens Fischer [syn.], T. tritici-repentis (DC Liro) [obs.]|
|Common Names||English: Dwarf bunt; Spanish: Caries enana del trigo; German: Zwergsteinbrand; French: Carie naine du blé|
|Description||Dwarf bunt is caused by the basidiomyceteous fungus Tilletia controversa (Teliomycetes, order Ustilaginales). The spores of the fungus stay alive for at least 10 years in the soil.|
Infected plants exhibit no obvious sign of disease until the ears emerge, but their stems are usually stunted (20-70 % of non-infected) compared to healthy plants and they may produce up to 8 tillers. Identification is difficult, since symptom expression widely varies with the genotypes of host and pathogen as well as with weather conditions. Differentiation from T. caries infection - common bunt - is sometimes difficult.
Bunted ears are somewhat narrower than healthy ones, but, as ripening proceeds, the glumes are pushed apart laterally so ears characteristically appear dishevelled. Sori are spherical - in contrast to elliptical in common bunt due to T. caries - and contain a blackish powdery mass of teliospores surrounded by a thick gray-brown tegument. Teliospores released from sori adhere to the seed resulting in discoloration, due to blackening of the kernels, and a typical fishy odour of trimethylamine. The usefulness of the kernels for milling is reduced. 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 assay.
Germinating spores produce a short promycelium with a terminal whorl of 14-30 basidiospores (primary sporidia). Monocaryotic basidiospores of compatible mating types fuse in pairs by producing lateral branches and appear as H-shaped structures. The fused primary sporidia become dicaryotic and produce secondary sporidia by germination; these dikaryotic secondary sporidia form infectious mycelium.
Following the penetration of well established seedlings, mycelium passes into the crown and keeps pace with the growth of the apical meristem until heading. A smut ball (sorus) containing teliospores then is produced in the ovaries; usually all ovaries are infected. Sori are dark reddish-brown to almost black, globose to broadly ellipsoid, covered by a pericarp. Teliospores are brown with spores becoming darker during maturing, globose or subglobose (20-28 µm in diameter). Mature teliospores are surrounded by a thin hyaline gelatinous sheath. The exospore is reticulate, with 3.5 µm diameter regular, polygonal areolae; occasionally areolae are irregular to subcerebriform. Sterile cells in the sori are fewer and generally smaller (10-18 µm) than the spores, globose with smooth walls, hyaline or faintly greenish or brownish, sometimes encased in a hyaline, gelatinous sheath. A number of physiological races exist differing in pathogenicity.
Germination of spores and infection require a >3-5 weeks period of low-light conditions together with temperatures from 1-8 °C. Exposure to temperatures of >15 °C inhibits spore germination. Only spores on the soil surface germinate and infect the seedlings shortly after emergence. Plants are often penetrated at the snow-soil interface when snow falls on unfrozen ground and remains persistent for several months. Most infection occurs in the winter when plants are forming susceptible stem buds.
Significant infection originates almost exclusively from soil infestation; although seed-transmitted, these spores are only of importance for the introduction into non-infested areas. Teliospores can remain viable in the soil for 3-10 years and are able to pass through the digestive tract of animals without losing viability. Dispersal with soil or manure may be also possible.
Additional Crop Information
T. controversa is able to infect wheat, rye, barley,and several grass species including Aegilops, Agropyron, Alopecurus, Bromus. Elymus, Lolium, Poa, etc..
Dwarf bunt occurs in moderate to cool wheat areas, especially in highland areas (300-1000 m) were a persistent snow cover favours the rather long process of infection. Years with frequent snow falls are usually associated with serious attacks. While T. controversa only affects a small portion of global wheat production area, the presence of this quarantine pathogen has increased the economic importance due to the potential of spreading to uninfected areas.
Integrated Crop Management
Dwarf bunt is an important disease, particularly of winter wheat grown at higher altitudes. Soil compaction and shallow seeding also promote T. controversa infection. The control is difficult as the resistant resting spores remain viable in the soil for a number of years and because most fungicidal seed treatments are not effective. Disease is occasionally severe in susceptible cultivars of winter wheat where wheat is grown under a persistent snow cover. Cultivar resistance is the primary means of control; several resistant wheat cultivars are available.
Applications of fungicides onto soil surface gives effective control but is economically not justified. Systemic fungicides have been shown to provide good control after disease establishment in the plant. Treatments of seed of (partially) resistant cultivars are able to suppress the disease effectively. All teliospores of T. controversa are killed when treated with 0.13 M NaOCl at 55 °C for 30 s. Although these seed treatments are effective in preventing disease spread to uninfected areas, they are not effective in protecting the crop from infection by soil-borne inoculum.
Effective control of the dwarf bunt fungus in susceptible wheat cultivars can be achieved with some azole fungicides via seed treatment. So, for example bitertanol based seed treatment products have shown a good activity against dwarf bunt.
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