Cereals: Dwarf Bunt and Common Bunt (Wheat)

Excerpt from Agronomy Guide for Field Crops (Chapter 6)

Order OMAFRA Publication 811: Agronomy Guide for Field Crop

Table of Contents

1. Incidence
2. Appearance
3. Disease Cycle
4. Management Strategies
   • Management Tips
5. Updates on Cereals: Dwarf Bunt and Common Bunt (Wheat)
6. Related links...

Incidence

Common bunt (stinking or covered smut) occurs anywhere in Canada where both spring and winter wheat is grown. This is in contrast to dwarf bunt where the disease has only been reported in two provinces - British Columbia and Ontario. In Ontario, dwarf bunt primarily occurs in the counties bordering Georgian Bay and Lake Huron where snow cover is deep and persistent in late winter and early spring. In severe years, some fields have had over 50% bunt.

Appearance

In Canada, there are three fungal species that can cause bunt in winter wheat. The first two are Tilletia tritici and Tilletia laevis, which cause common bunt or covered smut. The third is Tilletia controversa, which causes dwarf bunt in winter wheat. The main symptom of all three of these pathogens is the production of "bunt balls," which replace healthy kernels. These bunt balls contain masses of black powdery fungal spores called teliospores. When infected grain is harvested or crushed, these bunt balls rupture easily, releasing their spore contents, resulting in contamination of the grain. Besides the bunt balls, one of the most obvious signs of these diseases is the pungent, fishy odour of the spores. The odour is important, since the disease has quarantine significance: many importing countries have zero tolerance for bunt-contaminated wheat shipments. Often the spore cloud and the distinctive odour are the first signs that a crop may have the disease.

Common bunt and dwarf bunt are hard to distinguish between and often require microscopic examination. One difference is that the bunt balls of common bunt are similar in shape and size to the kernels they have replaced. Whereas, for dwarf bunt, the bunt balls are smaller and tend to be rounder. Plants infected with dwarf bunt are dramatically shorter (half as tall as healthy plants) whereas plants infected with common bunt suffer only a slight reduction in height. A fourth bunt fungus causes Karnal bunt or partial bunt. Fortunately, this disease has not been found and does not occur in Canada.

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Disease Cycle

Dwarf bunt and common bunt can infect winter wheat plants either through the seed (seed-borne) or from the soil (soil-borne). Although common bunt can be soil-borne, the fungus appears to be primarily a seed-borne disease and can be effectively controlled with currently registered seed treatments. Dwarf bunt is harder to control, since spores of the fungus can survive for 10 years or more in the soil.

Management Strategies

Start by planting seed that is free of bunt spores. Do not keep seed if you have any bunt present. Some registered seed treatments are more effective than others. Refer to OMAFRA Publication 812, Field Crop Protection Guide, for more details. (Order OMAFRA Publication 812)

Management Tips

1. 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.
2. 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.
3. 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.
4. Harvest fence rows and bush areas separately.
   Infection will be most severe where snow was the deepest and stayed the longest. Harvesting the majority of the field separately should minimize the number of bunt balls in the sample.
5. Clean grain before storage.
   It is important to remove as many bunt balls from the sample before storage as possible. Bunt balls will rupture during handling or removing 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.
6. Put grain into the bin with full aeration.
   It will take an extended period of aeration to remove the odour from the sample.

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Updates on Cereals: Dwarf Bunt and Common Bunt (Wheat)

• Field Crop Protection Guide - Publication 812
  • Cereals: Wheat: Dwarf Bunt
  • Cereals: Wheat: Common Bunt
  • Cereals: Barley: Covered Smut (Common Bunt)
  • Cereals: Oats: Covered Smut (Common Bunt), Loose Smut

Related links...

• Diseases
• Cereals

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