Winter Wheat Production Manual

Written by D. B. Fowler
Crop Development Centre
University of Saskatchewan

© University of Saskatchewan. All rights reserved.  No part of the Winter Wheat Production Manual may be reproduced in any form by any photographic, electronic, mechanical or other means, or used in any information storage and retrieval system without the written permission of the University of Saskatchewan or Ducks Unlimited Canada.

Smuts

Six species of fungi are responsible for five smut diseases of wheat. While all five kinds of smut are important diseases on a world basis, only loose smut and common bunt are found in the wheat producing region of the Canadian prairies.

  • Loose smut - Ustilago tritici
  • Common bunt (stinking smut or covered smut) - Tilletia tritici (T. caries) and Tilletia laevis (T. foetida)
  • Dwarf bunt or TCK - Tilletia controversa
  • Karnal bunt or partial bunt - Tilletia indica (Neovossia indica)
  • Flag smut - Urocystis agropyri (U. tritici).

Loose smut is found in wheat growing areas throughout the world. It is a seed-borne disease that infects the crop during flowering. Loose smut is spread from field to field through infected seed.

Common bunt, which is also known as stinking smut and covered smut, is a worldwide disease of wheat. It is both a soil- and seed-borne disease, but it is mainly spread by spores that attach themselves to the surface of the wheat seed.

Dwarf bunt is restricted to regions where prolonged winter snow cover on unfrozen soil maintains the soil temperature at or slightly above freezing for at least six weeks. These conditions are found in parts of the U.S.A. Pacific northwest, the North American Great Lakes region, and the British Columbia interior.

Karnal bunt was first identified in wheat fields near the city of Karnal, India in 1931. Since then, it has been found in all major wheat growing regions of India, Pakistan, Iraq, and Afghanistan. It is thought that karnal bunt was brought into Mexico in seed imports in the late 1960's. First reports of karnal bunt in the U.S.A. were from Arizona in 1996. Subsequent intensive surveys identified karnal bunt on wheat in Texas, New Mexico, and California. Eradication and quarantine measures were immediately put into place in these regions of the U.S.A. The survival of karnal bunt is determined by a highly restrictive set of environmental factors, and given its rather wide dispersal, it is likely that isolated, non-persistent infestations went undetected in these regions of the U.S.A. in the past. While U.S.A. crop losses due to this disease have been insignificant, it has been a major concern because most countries of the world have import restrictions on wheat from karnal bunt infested regions.

Flag smut has been found in most of the major wheat producing regions of the world. Unlike the other smuts, flag smut attacks the leaves and stems of the plant.

Almost all smuts are seed-borne. As a consequence, seed is the most important means by which smut is introduced into a region. Because they do not occur in Canada, both flag smut and karnal bunt are subject to strict plant quarantine regulations that prohibit the entry of wheat, rye, or triticale seed from any region where these diseases are found.

Most species of smut have distinctive races that vary in their ability to attack different wheat cultivars. Infested seed is the primary means by which new races are spread. Therefore, seed treatment and certification play an especially important role in limiting crop losses due to smut.


A. LOOSE SMUT

Loose smut is a common disease of wheat on the Canadian prairies.

Loose Smut [ Causes and Symptons | Losses | Control ]


a) Causes and Symptoms:

Although infected plants may be slightly stunted in their growth, it is usually only the heads of the plant that exhibit easily recognized symptoms of this disease. The floral parts of infected plants are replaced by masses of dark-brown to olive-black spores by the time the heads of an infected plant emerge. Within a few days of head emergence, the spores are blown away by the wind or washed off by rain leaving a bare rachis (central stalk of the head) with remnants of the glumes (chaff) and a few black spores attached.

Unlike many of the other fungal diseases such as the rusts, loose smut only goes through a single reproductive cycle each crop year. Plants are only susceptible to infections for approximately one week at the time of flowering. Spore releases from infected plants are timed so that the spores are blown into the open flowers of uninfected plants. Spores that land in the flowers of uninfected plants germinate and produce mycelia (thread-like fibres) that invade the ovaries of the flowers. Infected seed matures normally and there are no outwardly visible signs of loose smut infection in the harvested grain. The loose smut, which has penetrated the seed embryo, resumes growth once the seed germinates. It follows along with the growing point of the plant and completely replaces the spikelets of the developing head with a mass of spores. These spores renew the cycle once they are spread to the flowering heads of healthy plants by wind or rain splash.

Loose Smut [ Causes and Symptons | Losses | Control ]


b) Losses:

Yield losses due to loose smut are directly proportional to the percentage of smut infected heads in a crop.

Modern agriculture has been equipped with the tools required to eliminate yield loses due to loose smut. However, loose smut still persists and crop losses can be expected to increase if efforts to control this disease are relaxed.

Loose smut spores are dispersed before the crop matures and, unlike bunt smuts, seed infection does not affect the feed or food quality of the grain.

Loose Smut [ Causes and Symptons | Losses | Control ]




c) Control:

There are three primary methods available for the control of loose smut.

1. Grow loose smut resistant cultivars. The winter wheat cultivars available for production in western Canada are susceptible to loose smut. Therefore, control of loose smut in this region is presently dependent upon the use of smut-free seed and fungicide seed treatments.

2. Plant smut-free seed. Select seed from fields that were free of loose smut. When own-farm seed stocks are replaced, purchase seed that is certified free of loose smut. Certification is usually based on field inspections that are made prior to harvest to determine the presence of loose smut infected heads. Suspect seed lots can be tested for loose smut by plant disease laboratories.

3. Routinely treat the seed of cultivars that have poor or very poor loose smut resistance with a registered systemic fungicide. Systemic fungicides penetrate the seed coat to provide disease control within the seed and seedling.

Loose Smut [ Causes and Symptons | Losses | Control ]




B. COMMON BUNT (STINKING SMUT OR COVERED SMUT)

Common bunt is both a soil- and seed-borne disease that has caused significant crop damage on the Canadian prairies. Seed-borne spores are the most important means of crop infection in both spring and winter wheat. Compared to spring wheat, fall germinating winter wheat seedlings are more likely to be exposed to cool soil temperatures that favor common bunt infection. Because common bunt spores remain viable in the soil after harvest, winter wheat also has a greater risk of infection from soil-borne spores than does spring wheat.

Common Bunt [ Causes and Symptons | Losses | Control ]

a) Causes and Symptoms:

The release of clouds of dark spores during harvest and the presence of fragile "bunt balls" in newly harvested grain is often the first obvious indication that a wheat crop has been infected by common bunt. The light-brown bunt balls, which tend to be rounder and often larger than wheat kernels, are easily broken open to release masses of dark brown, powdery spores that have a distinctive fishy smell.

Bunt spores that are released during harvest can be spread by the wind across the field and into neighbouring fields. These spores may persist in the soil long enough to infect winter wheat that is seeded in these fields immediately after harvest. However, spores that adhere to the surface of harvested grain are the primary source of infection of both spring and winter wheat.

Moist, cool soil temperatures favor the germination of common bunt spores. Wheat seedlings are infected before they emerge from the soil, but the fungus remains inactive in the growing points of the infected plants until the heads start to form. The developing wheat kernels are then replaced by bunt balls that contain millions of spores.

Common bunt infected wheat plants may be slightly stunted. The heads of infected plants are often a blue-green color and they may be later maturing. The bunt balls are usually larger than the wheat kernels causing infected spikelets to spread open giving smutted heads a flared or splayed appearance as the crop approaches maturity. The awns of awned wheat varieties that are infected with common bunt may be shortened or prematurely lost.

Common Bunt [ Causes and Symptons | Losses | Control ]


b) Losses:

Common bunt causes a loss in grain yield by preventing the development of wheat kernels. Before effective methods of control were available, common bunt often caused significant yield reductions in wheat grown on the Canadian prairies. However, the introduction and widespread use of low cost fungicide treatments has essentially eliminated the threat of significant yield losses due to this disease.

In recent years, contamination of grain has been the main cause of financial losses due to common bunt. Even low levels of contamination with common bunt spores can give wheat a pungent, fishy odour that is a major degrading factor. A strong smutty odour results in degrading of wheat to sample and the need for separate binning which greatly discounts the market value of the grain. Contamination with common bunt only affects the odour, taste, and color of food products. It is not considered a health risk.

Common Bunt [ Causes and Symptons | Losses | Control ]


c) Control:

1. Treat seed with registered fungicides. Common bunt is primarily spread by spores that adhere to the surface of the wheat seed where they can be easily controlled by any one of several seed applied fungicides.

2. Plant seed that is free of bunt spores. Seed should be treated when smut has been observed in a crop.

3. Grow resistant cultivars. Many of the spring wheat cultivars that are registered for production in western Canada are resistant to current races of common bunt. In contrast, most of the winter wheat cultivars available for production in western Canada are susceptible, or at best have intermediate resistance, to common bunt. Therefore, western Canadian winter wheat producers are heavily reliant on the use of disease free seed and seed applied fungicides for the control of common bunt.

Common Bunt [ Causes and Symptons | Losses | Control ]