Dwarf Bunt of Wheat
Field Facts written by DuPont Pioneer Agronomy Sciences
Dwarf bunt (Tilletia controversa) is a fungal disease of winter wheat that infects plants and developing grain. Dwarf bunt gets its name because infected plants are stunted or dwarfed compared to normal plants. Other plant symptoms include excessive tillering, and replacement of kernels with spore masses or "bunt balls", which have dark, black spores. A foul, fishy odor emitted by these spores is also characteristic of the disease. In fact, provincial cereals specialist Peter Johnson was able to predict dwarf bunt occurrence in 2011 by the "fishy" odor detected in tree lines around wheat fields in February.
In Ontario, dwarf bunt occurs primarily in counties bordering Georgian Bay and Lake Huron where snow cover is deep and persistent in late winter and early spring. Dwarf bunt is not an issue every year, as conditions for the disease development may not be present.
The dwarf bunt fungus survives and disperses by producing teliospores, which can remain viable in the soil for up to 7 years. Teliospores at or near the soil surface are the source of most infections. These spores germinate after several weeks of continuous cool, humid conditions and eventually produce hyphae that infect wheat seedlings. Infection is favored by continuous and heavy snow cover on unfrozen ground, and normally occurs between December and February. The fungus grows systemically in the plant and eventually replaces the kernels with bunt balls (see picture below). Teliospores released from bunt balls during harvest infest the seed and soil, completing the life cycle.
- Avoid planting winter wheat in fields known to have dwarf bunt (the fungus stays in the soil for up to 7 years). Early deep seeding may reduce disease severity.
- Do not keep seed to plant from fields infected with dwarf bunt. Purchasing certified seed ensures your seed source is free from dwarf bunt.
- Seed should be treated with a fungicide, Dividend®, which provides 98-99% control of dwarf bunt. As shown in the table below, Dividend controls dwarf bunt equally at both the low rate (325 ml) and high rate (650 ml). Ensuring good seed coverage is important.
Table 1. Effect of seed treatments on the control of dwarf bunt, emergence and yield of winter wheat under Tilletia controversa inoculated field conditions in Bornholm and Palmerston, Ontario, in 2002 - 2005.
Source: Adapted from Xue et al. 2007.
Harvest and Storage for Dwarf Bunt Infected Fields
(from Tenuta and Johnson, 2011)
The following harvest and storage management tips for dwarf bunt infected fields can help increase sample quality:
- Cut high with the combine - Wheat infected with dwarf bunt will be substantially shorter than healthy plants. Raising the header will reduce the amount of bunt balls harvested.
- Harvest below 15% - Bunt balls and spores that are dry tend to be sent out of the combine easier. The wetter the grain, the more likely bunt spores will adhere to it. Removing wet spore balls through the combine is very difficult since they are very heavy.
- Combine wind-blast set at maximum - Turning the wind-blast settings up will remove a large portion of the bunt balls. Minimal good grain will be lost at maximum wind.
- Harvest fence rows and bush areas separately - Infection is most severe where snow was the deepest and stayed the longest. Harvesting those areas separately from the rest of the field should minimize the number of bunt balls in the sample.
- Clean grain before storage - It is important to remove as many bunt balls as possible from the sample before storage. Bunt balls will rupture during grain handling or removal from the bin. Bunt balls are similar in size to wild buckwheat seed. Therefore, screens that remove wild buckwheat should remove many of the bunt balls in the sample.
- Put grain into the bin with full aeration - It will take an extended period of aeration to remove the odor from the sample.
- Finally, store infected wheat separately!
In the 2012 Winter Wheat Crop and Beyond
Growers are advised to plant certified seed to maximize their chances of growing a high producing disease-free crop. Certified seed is inspected to ensure it meets criteria and treated in a regulated and inspected facility. Avoid planting wheat in fields known to have dwarf bunt issues.
Ockey, S.A and S. V. Thomson. Dwarf Bunt of Wheat.
OMAFRA. Field Crop Protection Guide 2011-2012.
Tenuta and Johnson. 2011. Management Tips for Dwarf Bunt.
Xue et al. 2007. Evaluation of winter wheat genotypes and seed treatments for control of dwarf bunt in Ontario. Canadian Journal of Plant Pathology. Volume 29: 243-250.