Tilletia caries

Scientific Name Tilletia caries (DC.) Tul. & C. Tul., Tilletia laevis Kühn in Rabenh.
Synonyms T. tritici (Bjerk.) G. Winter [syn.], T. foetida (Wallr.) Liro [syn.]
Common Names English: Common wheat bunt, complete bunt, stinking smut; Spanish: Carbón hediondo, Carbón volador; German: Weizensteinbrand, Steinbrand; French: carie commune
Description Common bunt of wheat is caused by the closely related basidiomycetes Tilletia caries and T. laevis (Teliomycetes, Ustilaginales). Both fungi have similar life cycles and may occur together in the same infected plant; the two species readily hybridize (T. intermedia Gassner). They occur worldwide, distinguished by spore morphology and to some extent by geographic distribution.

Biology

Damage

Affected plants are usually stunted with slender heads which often remain green longer than healthy heads; the glumes tend to gape open, exposing the bunt balls. Often it is difficult to distinguish diseased from healthy heads. Usually, all ovaries of the ear are affected, but not all heads on a plant may be infected. The entire kernel is replaced by bunt balls full of teliospores, however, some kernel may be only partially affected. Teliospores are black in mass, and light pale-yellow to gray or reddish brown individually. The fragile pericarp of bunt balls remains ruptures only at harvest. Their shape and size are similar to normal kernels, but tend to be gray-brown. During harvest, the black, powdery mass of spores with a characteristic odor of rotten fish due to the production of trimethylamines contaminates healthy kernels and the soil.

Symptoms caused by T. caries are identical to those caused by T. laevis and similar to those of dwarf bunt due to Tilletia controversa. Partially bunted kernels have symptoms similar to those of Karnal bunt caused by T. indica. T. caries and T. controversa have reticulate teliospores, those of T. laevis are smooth. Spores of T. indica are darker with thick, compact projections and are larger than those of T. caries and T. laevis. A morphological identification of single spores is not possible. Occasionally, sori of loose smut caused by Ustilago tritici remain intact, however, they lack the foetid odor of common bunt and ustilospores are smaller than those of T. tritici.

Lifecycle

Common bunt fungi persist as dikaryotic teliospores on seed and in soil. After karyogamy they germinate to form a basidium, on which eight to 16 terminal hyaline haploid basidiospores (n=4) develop. The filiform basidiospores (primary sporidia) are of two mating types (+ and -) and fuse near their middle in compatible pairs to form H-shaped structures and establish a dikaryon. The dikaryon then yields infectious hyphae or hyaline sickle-shaped secondary sporidia wich produce infectious hyphae or more secondary sporidia. The fungus infects the shoots of wheat seedlings before the plants emerge from the soil. Most infection results from seed-borne spores; in areas where spores survives in the soil, infection may result from soil-borne spores. Optimum conditions for spore germination are soil temperatures in the range of 5-15 °C.
The pathogen grows within the terminal meristem, especially the flower primordia of the spike, with hyaline mycelium and completes its life cycle by transforming the ovaries into smut balls instead of kernels. The ovary wall is modified to become the exterior of the kernel-shaped sorus which remains intact until harvest.

Cause

Teliospores of T. caries can survive for several years on contaminated seed and in the soil. Their nucleus is diploid and undergoes meiosis just before germination. Dispersal is mainly by movement of grain and farm machinery contaminated with teliospores.

Occurrence

Additional Crop Information

Common bunt mainly infects Triticum species (T. aestivum, T. dicoccum, T. durum, T. spelta), rye and various grass species may also become infected.

Agricultural Importance

The disease reduces wheat yield and grain quality. With high infections, spores form a dark smoke-like cloud around the harvester. Occasionally bunt results in explosions in combines and elevators because of the extreme combustibility of the oily spores in the presence of sparks from machinery.

Control

Integrated Crop Management

Wheat seed lots may be contaminated with teliospores of T. caries. An increase in bunt incidence in Nordic countries has been attributed to the ban of mercury seed treatments and a decline in the use of other seed treatments.
Fifteen major resistance genes have been identified in wheat cultivars. Near-isogenic lines are used to identify pathogenic races of T. caries and T. laevis. Because of the large number of pathogenic races and the availability of effective fungicides, the role of resistant cultivars to control common bunt is limited.
In organic farming, seed-borne diseases play an important role. Thermal treatment of seed with humid air, hot water and skimmed milk powder are suitable at low infection pressure, but less effective than synthetic fungicides at higher disease pressure.

Chemical Control

Although very effective, organo-mercurial compounds have been discontinued in most countries because of environmental and human health concerns. Tilletia spores on the seed are readily controlled by a range of contact and systemic fungicides (benzimidazoles, phenylpyrroles, triazoles, etc.), which may also prevent infection of seedlings. For example seed treatment products containing triadimenol, tebuconazole, triticonazole, bitertanol and prothioconazole are all well suited to control common wheat bunt very effectively.

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