Dwarf bunt of wheat

Dwarf bunt is a result of a soilborne pathogen (Tilletia controversa) and the disease is also known as dwarf smut. It typically occurs with long periods of snow cover, and only impacts winter wheat. Symptoms prior to heading (FGS 10.5) include yellow leaf flecking at late seedling stage, stunting, and excessive tillering. Once the head emerges, the fungus transforms kernels into spherical bunt balls. Infected heads may maintain green color longer, and spikes and awns will be noticeably splayed. Bunt balls are filled with dark colored spores having a fishy odor. The best time to scout is towards the end of the season during ripening.

Wheat plant stunting symptomatic of dwarf bunt. Image: J. Marshall. 
Once the head emerges, the fungus transforms kernels into spherical bunt balls. Image: M. Burrows. 

Soilborne spores are the inoculum source for dwarf bunt, lasting up to ten years in the soil. Fungal germination takes several weeks and is favored by moist conditions with soil temperatures between 37-46°F. Regions with high snowfall, 60-90 days of continuous snow cover on unfrozen soil, and/or areas at relatively high altitudes are more prone to infection.

Bunt balls will be present with dwarf bunt, which can help distinguish the disease from abiotic disorders such as nutrient deficiencies, chemical injury, and soil compaction. The characteristic fishy odor is evident in infected fields.

Highly resistant varieties are available in areas prone to dwarf bunt, and are required under organic production. Early planting as compared with late planting reduces the disease because infection is worse when plants are small going into winter. If disease pressure is severe and your climate allows, plant spring wheat instead of winter wheat. Fungicide seed treatments can nearly eliminate infection, but since Tilletia controversa doesn’t invade the plant until after it emerges, many seed treatments are not effective. Difenoconazole is the only chemical that is registered for dwarf bunt control and is very effective.

Gallery

Gallery images: M. Burrows and J. Marshall.