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.

Leaf Spots

Wheat streak mosaic and barley yellow dwarf are the two most important viruses that have caused damage to wheat in western Canada. Neither virus is transmitted through the soil, crop residues, or wheat seed and both require an insect vector (an organism that spreads the disease) to spread to uninfected plants in the field. Barley yellow dwarf virus is spread by aphids that can be carried on wind currents for hundreds of miles (kilometres). In contrast, the spread of the wheat streak mosaic virus is dependent upon a fragile, wingless mite whose movement can often be measured in inches (cm). Because of their mode of infection, any discussion of these diseases requires an understanding of both the viruses themselves and their insect vectors.

Both wheat streak mosaic virus and barley yellow dwarf virus are transmitted to their vectors when the insects feed on an infected plant. The vector carrier then spreads the virus when it feeds on plants that were previously virus free. The virus can persist in the vectors' body for several weeks but neither wheat streak mosaic virus nor barley yellow dwarf virus is transmitted through the eggs of the insect to the next generation. Therefore, each new generation of the vector must feed on infected plants before they acquire the virus.

The symptoms produced by wheat streak mosaic virus and barley yellow dwarf virus are very similar and both diseases are often confused with damage due to salinity, cool wet soils, drought, herbicide injury, nitrogen and phosphorus deficiencies, hail, and allelopathy. Transmission (demonstration that the virus can be transferred to healthy plants by the appropriate vector or other recognized method) or serological (antigen-antibody reactions) tests carried out in a laboratory under controlled conditions are the only way of positively identifying these viruses. Serological tests are very specific and can often be used to identify the different virus strains. In regions of the North American Great Plains where wheat streak mosaic and barley yellow dwarf viruses are of significant economic importance, Crop Protection or Plant Diagnostic laboratories will usually conduct the appropriate serological tests on a fee per sample basis.


A. BARLEY YELLOW DWARF VIRUS

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a) Causes and Symptoms:

The symptoms of barley yellow dwarf virus are dependent on the stage of development of the wheat plant at the time of infection. Plants that are infected during the seedling stage in the fall usually express the strongest symptoms. These symptoms include a stunting of growth and a yellowing of the leaf that starts at the tip and becomes progressively more severe until the leaf is killed. The veins often remain green longer than the rest of the leaf producing a striped appearance that may be confused with the damage caused by the wheat streak mosaic virus. The leaf tips of some wheat cultivars may turn a red or purple color. Stunting of the root system also makes the plant more susceptible to drought stress and plants with reduced vigor are more likely to suffer winter damage and are prone to infection by other diseases. The heads of plants infected in the seedling stage may not emerge from the boot. Later infections may result in reduced seed size and grain yield. Wheat plants that are infected at the time of heading or later usually show few symptoms and normally only suffer slight yield reductions.

In addition to the symptoms expressed by the winter wheat plants, disease diagnosis should include consideration of seeding date, the presence of aphids, the pattern of damage in the field, and the condition of fields in the surrounding area. While aphids are necessary for the spread of this disease, symptoms usually take three to six weeks to develop allowing the aphids time to move away from the field. Therefore, the absence of aphids should not be used to eliminate barley yellow dwarf virus as the cause of damage. Transmission or serological tests are the only sure way to identify barley yellow dwarf virus.

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b) Losses:

Barley yellow dwarf is the most common virus disease in cereal crops around the world. Annual small cereal grain losses in the USA alone have been estimated to be in excess of $300 million (US$). Damage due to barley yellow dwarf virus is widespread in the American Great Plains region and epidemics were reported in winter wheat fields as far north as Montana in 1964, 1980, and 1981.

More than 100 species of the grass family can act as barley yellow dwarf virus hosts and about 20 different species of aphids are capable of transmitting the virus. Crop reports from regions with warm winter climates indicate that winter wheat fields located near pasture and other areas that have been planted to grasses are the most frequently damaged. Volunteer cereals and grasses in ditches and fence lines can also harbor both the virus and insect vectors. These observations suggest that there is a large potential reservoir for barley yellow dwarf virus and an abundance of vectors capable of transmitting the virus in Saskatchewan. However, while outbreaks of barley yellow dwarf are frequent on spring cereals, most of the infections in the western Canadian prairie region are thought to arise from infected aphids that are blown north from the U.S.A.

In my nearly 25 years of intensive involvement with winter wheat production in Saskatchewan, I have only seen one winter wheat field that was badly damaged by barley yellow dwarf virus. Symptoms suggested that the field was uniformly infected in the early fall (later seeded winter wheat in the area was undamaged). The symptoms in this field were initially confused with those of a nitrogen deficiency and then wheat streak mosaic virus; however, subsequent serological tests identified barley yellow dwarf virus as the culprit.

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c) Control:

With only one episode of note in 25 years, it is likely that attention should focus on the identification, rather than the control, of barley yellow dwarf virus in winter wheat in Saskatchewan.

The development and movement of the aphids that spread barley yellow dwarf virus is favored by warm temperatures. In regions of the North American Great Plains where barley yellow dwarf is a problem, the most effective method of controlling this disease has been to delay seeding until after the peak period for insect movement has passed.

Pesticides can be used to control aphids, but aphid migrations depend on the weather making their movements hard to predict.

There is genetic variability for barley yellow dwarf virus resistance, but the winter wheat breeding programs that have developed the cultivars presently grown in western Canada have not made selection for this character a priority.

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B. WHEAT STREAK MOSAIC VIRUS

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a) Causes and Symptoms:

The first symptoms of wheat streak mosaic are usually observed in the spring along the sides of fields adjacent to fields that were sown to wheat the previous year or in areas of the field with volunteer wheat plants. Initial symptoms on the leaves are yellow streaks or yellow-green mosaic patterns (blotches, dashes, or stripes) that run parallel to the veins. The leaf tips of infected plants usually turn yellow first. The leaf veins remain green creating a yellow and green striped appearance in severely infected plants. Plants affected in the seedling stage (Zadoks stage 10 to 19) will often be badly stunted and produce a large number of non-uniform tillers. These plants usually produce few or no heads and often die before they head. Seed set is usually poor if the infection occurs between tillering (Zadoks stage 20) and jointing (Zadoks stage 30). Infection at later stages may result in low grain test (bushel) weight.

Under field conditions, the wheat curl mite (Aceria tulipae), a small white cigar or rice shaped mite that can be seen under 10x magnification, and wheat streak mosaic virus are both required for the normal spread of this disease. Both the mite and the virus require live, green plants in order to survive.

Wheat curl mites do not have wings and they depend on the wind to move from field to field. The mites have a very short generation time (8 to 10 days) and their numbers can increase very quickly under favorable conditions (hot, dry weather). Mite populations as large as 150,000 per volunteer wheat plant have been reported in Montana. However, because of their fragile nature, the mites will quickly dry out if they are removed from the protected environment created by the plant.

Wheat streak mosaic virus is spread when mites that have fed on infected plants move to and feed on uninfected plants. However, because the mites are very fragile and depend upon the wind to carry them to uninfected plants, the distance that the virus is moved is usually quite short.

When the mites carry wheat streak mosaic virus from an infected field into an uninfected field, there is usually a definite infection trail that can be traced back to the initial source of the virus. When the disease is spread from field to field, the most severe crop damage is normally adjacent to the field that originally carried the virus and the mites. The spread into newly infected fields often shows up as a pattern which is similar to that of a snow drift and, even in extreme cases, crop damage is usually concentrated in the first hundred yards along the edge of the field.

Volunteer spring and winter wheat that is not controlled prior to seeding is the most common reason for the spread of wheat streak mosaic virus. In these instances, damage is often severe only in a one to two foot area around the volunteer wheat plants and the extent of damage is dependent upon the frequency of volunteer plants. For example, early sown spring wheat trials in the centre of wheat streak mosaic infected winter wheat trials on research farms often show few symptoms of the disease and suffer little apparent yield loss even though infected plants are only a few yards away.

Because both the wheat stem curl mite and the virus can survive only on live green plants and the mite does not move great distances, the most severe wheat streak mosaic outbreaks occur where a "green bridge" exists between successive wheat crops that are planted in the same or adjacent fields. This green bridge is found most often in the following situations:

a) Underseeding - When this production system is employed, winter wheat is sown with a spring crop in the spring. The spring crop is harvested in the fall, leaving the stubble for snow trapping, and the underseeded winter wheat remains to produce the next crop.

b) Winter wheat is sown next to a wheat streak mosaic infected spring wheat crop before the spring wheat crop has matured. The mites then carry the virus from the infected spring wheat crop into the winter wheat crop infecting it and providing a virus reservoir from which adjacent spring wheat fields can be infected the following year.

c) Winter wheat sown in the spring for pasture is not destroyed before 1) winter wheat is sown in the fall, or 2) spring wheat is sown the following spring in the same or adjacent fields.

d) Volunteer wheat is not controlled before either spring or winter wheat is seeded. The volunteer wheat then provides the bridge for both the mite and the virus to infect the newly planted crop.

e) Wheat crops are damaged by hail just before maturity. Hail damaged wheat crops are usually listed as the most common cause of volunteer wheat green bridges that result in severe wheat streak mosaic outbreaks.

Wheat streak mosaic virus is not spread by wheat seed or dead plants.

Wheat streak mosaic virus can be spread any time cell sap is moved from an infected to an uninfected plant. Therefore, it could theoretically be spread by something as simple as a rotary mower that is used in an uninfected field immediately after mowing in an infected field. However, when introduced in this manner, the virus does not have a method of spreading from plant to plant unless the wheat stem curl mite is also present.

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b) Losses:

While wheat streak mosaic is widely distributed in Europe and North America, it has caused the greatest crop losses on the central Great Plains of North America. Significant damage is usually restricted to wheat, but barley, rye, oat, wild oat, corn, and many annual and perennial grasses such as downy brome, wild rye, green foxtail, and crested wheatgrass are also hosts for this virus.

The degree of damage from wheat streak mosaic virus depends on the stage of plant development, time of infection, weather conditions, general health of the crop, crop kind and variety, and the strain of virus. Yield losses are usually most severe when plants are infected in the seedling stage. Plants that are infected at later growth stages may not show typical wheat streak mosaic symptoms. However, they can maintain a virus reservoir for the infection of later developing wheat plants. This is an especially important consideration when spring and winter wheat are grown in adjacent fields or a field is contaminated with volunteer wheat.

Damage by wheat streak mosaic virus has been reported throughout the Great Plains region of North America. In Montana it caused significant winter wheat crop loss in 1964 and an estimated $12.7 million (US) damage in 1993. Consequently, wheat streak mosaic is a disease that must be taken seriously. However, it is important that suspected outbreaks are accurately diagnosed.

Over 90 percent of the fields with wheat streak mosaic symptoms that I have been called on to investigate in Saskatchewan have turned out to be suffering from some other major problem. In these instances, misdiagnosis has often unnecessarily tarnished the image of winter wheat and prevented the proper corrective action from being taken so that crop losses could be minimized. I have also heard many rumors of other fields that have been damaged by wheat streak mosaic virus. However, in my 20 plus years of wandering through winter wheat fields and compiling survey results, I have only seen three confirmed instances of major yield reductions due to this disease. In all three instances, the disease problem was caused by volunteer wheat that had not been properly controlled. In contrast, problems with wheat streak mosaic virus have been more common in and around research stations where there are a large number of small plots of spring and winter wheat grown under a variety of management systems.

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c) Control:

Genes that provide resistance to wheat streak mosaic virus are available in the wheat gene pool. However they have not yet been incorporated into winter wheat varieties adapted to the western Canadian environment.

There are no pesticides recommended to control the wheat curl mite. However, it is very fragile and can only survive for a short time without live, green plant tissue to feed on. Consequently, the most effective means of controlling this disease is to eliminate the "green bridge" required for the wheat curl mite to transfer the virus between successive wheat crops. A one week break in the green bridge is normally all the time that is required to control the spread of wheat streak mosaic virus.

1. Make sure volunteer wheat is destroyed at least one week before spring or winter wheat is planted. Note: Volunteer wheat does not die-off quickly enough to provide the necessary break in the green bridge in no-till production systems that control weeds using glyphosate (Roundup, Laredo, Wrangler, Victor) sprayed immediately before or after seeding.

2. Do not plant winter wheat next to a spring wheat crop until at least one week after the spring wheat crop has ripened completely.

3. Do not plant winter or spring wheat next to spring or summer sown winter wheat that has been sown for pasture until at least one week after the winter wheat pasture has been killed off. Make sure that there are no wheat plants left alive when a winter wheat pasture is destroyed.

4. Do not underseed winter wheat with a spring crop in the spring.

5. Do not seed wheat on fields where infected spring or winter wheat plants have just been destroyed. When the plants that the mites are feeding on die, the mites are encouraged to move to an exposed area on the plant from where they can be carried to adjacent fields by the wind. Consequently, adjacent wheat fields become more vulnerable to infection when wheat streak mosaic virus infected fields are destroyed during the growing season.

6. Have suspected wheat streak mosaic virus outbreaks properly diagnosed. Increase the length of the "green break" to at least two weeks if a wheat streak mosaic outbreak is confirmed in fields adjacent to those where you plan to seed winter wheat.

7. Mite reproduction and movement is highest when temperatures are warm. Therefore, delaying winter wheat seeding until the end of the recommended planting period, when fall air temperatures start to cool down, often helps prevent the spread of wheat streak mosaic virus.

[ Images | Causes and Symptons | Losses | Control ]

 

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Figure 5. Barley yellow dwarf virus causes a stunting of plant growth and a yellowing of the leaf that starts at the tip. Note: Water droplets on the leaves are due to a recent rain. (Image Size = 86k)
Figure 6.The leaves of some cultivars may turn red or purple color when infected by barley yellow dwarf virus. (Image Size = 81k)
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Figure 7. The initial leaf symptons of wheat streak virus infection are yellow streaks or yellow-green mosaic patterns that run paralel to the veins. As the disease progresses, the leaf veins remain green creating a yellow-green striped appearance. (Image Size = 49k)
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Figure 8. Plants infected with wheat streak mosaic virus infection in the seedling stage are often badly stunted. Note healthy, uninfected plants growing next to the plant damaged by wheat streak mosaic. (Image Size = 133k)