Pathogen 

• Tilletia controversa

Hosts

The disease affects wheat, barley, rye and triticale.

Symptoms

This disease is not yet known in the UK but has become of interest in recent years because of reports of long term survival of common bunt (T. tritici) in soil. As dwarf bunt can survive for long periods in soil the possibility of hybrids between T. controversa and T. tritici has been widely debated. Dwarf bunt occurs in Canada, North and South America and many parts of Europe and Asia.

Microscopic examination of teliospore morphology is the primary means of distinguishing species of Tilletia. However, because there is a wide range of teliospore characteristics in Tilletia spp. so spore morphology alone is not a suitable method of identification.

The symptoms of dwarf bunt are very similar to bunt except that affected plants are stunted to between half and quarter their normal height. Infected ears tend to have a more ragged appearance than with bunt. Yellowing and flecking of leaves of affected plants is also occasionally found.

Life cycle

The disease cycle of T. controversa matches that of T. tritici very closely. Infected plants appear uninfected until ear emergence. When the ears emerge the seeds are seen to have been replaced by "bunt balls" which break open during harvest, contaminating healthy grain. When contaminated grain is sown the following season the spores on the outside of the grain germinate, eventually reaching the growing point of the plant. It is at this stage that T. controversa differs from T. tritici. With bunt, the fungus normally infects the plant via the coleoptile as the seedling is emerging. However, with T. controversa, there is a long incubation period and a requirement for cool temperatures before the spores will germinate. Thus, as a result, fungus frequently infects plants much later in development.

At harvest, "bunt balls" contaminate the soil as well as healthy seed but with dwarf bunt this is very significant as the fungus can survive in soil for many years. Free spores are reported to remain viable in soil for at least 3 years and "bunt balls" can survive for up to 10 years.

Importance

Presently, the disease is not known in the UK. However, like bunt, the disease could be very damaging although seed treatments commonly used in the UK would prevent seed-borne infection.