VIRAL DISEASES
 

AGROPYRON MOSAIC

Hosts:

Symptoms:

leaf symptoms - agropyron mosaic on wheat

Look-alike symptoms:

Favorable conditions:

Pattern in field:

Infestation levels:

Life cycle:

History in MT and US: Agropyron mosaic has been reported in Colorado (1).

Crop losses:

Controls:

Cultural controls:

Resistant varieties:

Biological controls:

Chemical controls:

References: 1) Brown, W.M., Jr., French, R.C., Hammon, R.W. , and Jensen, S.G. 1994. Abstract, American Phytopathological Society, Pacific Division Meetings, May 26-28, 1994.
 
 
 
 

BARLEY YELLOW DWARF

Pictures of barley yelloe dwarf:

individual symptomatic plants - field shot, Mocassin Experiment Station, 6.25.80

aphid vector - on infected wheat, July 1987

morphology of vector - illustration of virus transmission

circular symptoms - in field
 

Scientific name: The barley yellow dwarf virus (BYDV) actually includes several related viruses grouped into five strains based primarily on the specific aphid species able to transmit a particular strain.

Hosts: Wheat, barley, oats, rye, corn, sorghum, brome, tall fescue and many other annual and perennial grasses (4). BYVD infects almost 100 species of annual and perennial grasses, including the above (7).

Vector: In Montana, BYDV is vectored by three aphid species in: the English grain aphid, the corn leaf aphid, and the birdcherry oat aphid. During some seasons, the greenbug and the rose-grass aphid may also be vectors of BYDV. In North America, there are 23 species of aphids that can transmit BYDV.

Passive migrations of the winged forms of aphid vectors from the south via low level jet winds generally are responsible for BYD epidemics. Spread of BYDV depends entirely on aphid movement. One very active aphid feeding for short periods on different plants is a more important carrier than several stationary aphids (7). BYD is NOT spread via seeds or through soil, nor is it mechanically transmitted by leaf rubbing or injury.

Symptoms of Barley Yellow Dwarf:

leaf symptoms - Winalta winter wheat, Utica, MT, 6.18.80

red leaf symptom - Olaf spring wheat, 7.11.80, field shot

- Symptoms usually appear during the later stages of crop development (2).

- Early symptoms include small yellow-green blotches near the leaf tips (4).

- Progression of symptoms is from leaf tip to base and margin to mid-rib (7).

- In wheat, yellow leaf tips from one-third to one-half the length of the leaf is common. Later entire leaves may turn color (4).

- Yellowing and reddining from the leaf tips occurs. However, the yellowing of wheat is more of a pale yellow that is not as brilliant as it is in barley; and the reddening is not as prominent as in oats (7).

- There may be some yellow streaking on the flag leaf (2).

- Symptoms are more pronounced under cool temperatures (7).

- Leaves may unfurl before they have completely emerged from the leaf sheath (7).

- Plants often are stunted and plants may have excessive or reduced tillering (1, 7).

- Heading or ripening may be delayed (7).

- Sterility may be increased and fewer and lighter-weight kernels may be produced (7).

Look-alike symptoms:

- Wheat streak mosaic: WSM produces a distinct yellow mosaic (streaked) pattern on leaves whereas the discoloration is more uniform yellow or red with BYD.

- Nitrogen deficiency

- Sulfur deficiency

- Cold spring temperatures

- Herbicide injury due to:

a) Acetanilides (Stampede)

b) Aryloxphenoxy-propionic acids/cyclohexanediones (Achieve, Assure, Assure II, Excel Super, Fusilade 2000, Fusilade II, Hoe-Grass, Hoelon, Option II, Poast, Select)

c) Benzonitriles (Buctril, Pardner, )

d) Imidazolinones (Assert, Pursuit, Scepter)

e) Sulfonylurea symptoms on leaf tips (Accent, Ally, Amber, Beacon, Classic, Express, Glean, Muster, Pinnacle, Refine)

d) as-Triazines (Lexone, Sencor)

e) s-Triazines (Aatrex, Atrazine, Bladex) (4).

Lab diagnosis: It is difficult to consistently detect all five strains of BYDV. This sometimes leads to a false negative diagnosis when the virus really is present. Serological, biochemical, and biotechnological methods are used to rapidly detect all five strains (7).

Favorable conditions:

1) Heavy influx of BYD virus-carrying aphids into the field.

2) Moist, warm (not hot) air temperatures that favor aphid movement and reproduction as well as plant growth (7).

3) Early fall planting can increase infection levels (1).

4) When winter wheat is seeded before spring wheat is harvested, this favors aphid movement and disease spread (5).

5) Fall infections in winter wheat cause the most damage but this usually is not visible until spring (4).

6) Some virus strains are more aggressive than others.

7) Nitrogen stress and dry conditions complicate infections.

8) Symptoms show up well with temperatures of 60-75 F (5). Symptoms may be masked by warmer temperatures.

- Heavy rains may increase aphid spread, but reduce overall aphid populations (7).

Pattern in field:

- Symptomatic plants appear first along the field margins.

- Later they show up as isolated plants in the field. The disease then may spread outward from those infection centers to larger, often circular, patches in the field (2).

- The patches are typically 1 to 20 feet in diameter (4).

- In general, the most severe symptoms occur in the center of the patches with symptoms becoming increasingly less distinct towards the periphery (7).

Infestation levels: BYD is most damaging when plants are infected early in their lifecycle. This can result in severe stunting, excessive or reduced tillering, bright-yellowing or reddening of older leaves, delayed heading or ripening, increased sterility, and fewer and lighter-weight kernels (7). Plants that are infected after the seedling stage generally have less severe symptoms. Only only the upper leaves, or the flag leaves show discoloration. No discoloration may be apparent in mildly-infected wheat (7). Fall infections in winter wheat cause the most damage but this usually is not visible until spring. Spring infections result in less yield loss from the BYDV, but significant damage from aphid-feeding can occur (4). Plants don't recover once infected, but high temperatures can mask symptoms (4).

Life cycle: Aphids arrive in Montana in late May or June and begin to feed. Peak populations may occur in late summer. The aphids feed by sucking juices out of the plants. The aphids can only acquire the virus by feeding on infected plants. The virus does not multiply inside the aphid and the aphid does not transfer the virus to its young. When aphid populations build up in a given area, winged forms develop and may fly to uninfected areas moving the virus with them. Warm air temperatures favor aphid movement and therefore, disease spread. Winter wheat often is infected when it is seeded before spring wheat is harvested. The aphids on the spring wheat move to the winter wheat transferring the virus. The virus then overwinters on the newly seeded crop (5).

History in MT and US: BYD is the most important virus disease of cereal grains worldwide and in ND (2,3). In Montana, the disease has been found in all parts of the state, although it is more common east of the Continental Divide. The disease was first noticed in Montana in 1959. However, it did not become a widespread problem until 1980-1981 when it was epidemic in central Montana winter wheat (5). Occurances have been sporadic since then. When infection occurs early in the year, major yield losses may occur. However, the disease usually reaches Montana in later crop stages so serious outbreaks are sporadic (2).

Crop losses:

- Severe yield losses can occur when seedlings become infected soon after planting (1,4). In ND, incidences of 10-25% are common (3).

- In 1980, losses in the Judith Basin were estimated at 1.8 million dollars due to BYD (loss of 10 bu/acre for 55,000 acres). In 1981, yield losses ranged from 2 to 20 bu/acre (5).

- If infection is uniformly severe, 40% yield loss is probably the worst case scenario in Kansas (6).

Controls: Cultural methods are the best means of controlling BYD.

Cultural controls:

1) Late fall planting of winter wheat and early planting of spring wheat can reduce infections since the aphids usually are less active at these times. Fall planting should be flexible depending on local weather conditions. However, the planting dates listed below are good guidelines:

REGION OF MT FALL DATE SPRING DATE

North- September 5 by late April

Central- September 15 by late April

Southeast- September 20-22 by late April

Effect of planting date on BYD incidence and yield in winter wheat in Kansas (Southeastern Kansas, 1990, Ken Kelly) (6).

Planting date Incidence (% symptomatic) Yield (bu/A)

Sept. 28 37% 26.3

Oct. 13 6% 31.8

Oct. 27 1% 37.1

Nov. 9 0% 43.0
 

2) Control small grain volunteer to eliminate aphid vectors from spreading BYD via the "green bridge". Two weeks without green material in the field before the next crop emerges should be sufficient (5).

Resistant varieties: Available varieties have little BYD resistance (4).

Biological controls: Lady bugs and parasitic wasps are present in most Montana fields and these help to control aphid populations (5).

Chemical controls: Insecticidal control of BYD aphid vectors in the fall may reduce the incidence of infections. However, if aphids escape the treatments or migrate from untreated areas, insecticides are of little use except in reducing secondary spread within a field (7, 5). Below are results of Kansas State University trials conducted at Manhattan in 1993 by Bill Bockus (6). Wheat was planted early to ensure heavy BYD infection. Furadan was used in some plots to control aphids.

INCIDENCE OF BYD SYMPTOMS YIELDS

TREATED PLOTS 8.5% 49.1 bu/A

UNTREATED PLOTS 68.3% 19.3 bu/A

References: 1) Lipps, P.E. 1988. Wheat disease control in Ohio. Dept. of Plant Path. Ohio State Univ. 2) NDSU Pest Report, "Barley yellow dwarf", July 9, 1993, No. 10. 3) NDSU Pest Report, "Wheat diseases to the south", June 3, 1994, No. 5. 4) Guide to Herbicide Injury Symptoms in Small Grains. Second Edition. 1992. Agri-Growth Research, Inc., Hollandale, MN, pp. 75-76. 5) "Barley yellow dwarf and wheat streak mosaic in small grains" 1988. Montguide, Montana State University Extension Service publication MT8802A. 6) Plant Disease Alert, "Barley Yellow Dwarf hits southeast", May 2, 1994. 7) Watkins, J.E., and Lane, L.C. Barley yellow dwarf disease of barley, oats, and wheat. NebGuide, Cooperative Extension Service, University of Nebraska-Lincoln, Bulletin G89-906, May, 1989.
 
 
 
 

FLAME CHLOROSIS
 

Hosts: Barley, wheat, oats (2), green foxtail (Setaria viridis), and barnyard grass (Echinochloa crusgalli) (3) Barley seems to be most susceptible. Research has shown the virus that attacks barley to be very similar but not identical to the one which attackes wheat and oats. So the virus has the potential to mutate and infect other crops (2).

Disease transmission: Flame chlorosis is transmitted by zoospores from Olpidium brassicae which invade the plant root carrying the virus particles with them. The virus is not transmitted through seed. The disease has been transmitted only by planting seed or seedlings in soil where plants with flame chlorosis had been planted earlier (1). Scientist believe the disease is spread primarily by blowing soil (2).

Symptoms:

leaf symptoms - #1

leaf symptoms - #2

- leaf chlorosis in a "flame"-like pattern (1)

- irregular, yellow-to-whitish striping in a flame-like pattern on leaves that is distinctive from other diseases or conditons (2)

- severe stunting (1)

- failure to produce fertile heads (1).

- Symptoms are prominant from early to mid-June , from the four-leaf stage to just before the boot stage (2).

Lab diagnosis: The virus consists of double-stranded RNA's ranging in size from 900 to 2800 base pairs and distinct from any known cereal virus. No inlusions of particles have been observed, and instead of the characteristic pinwheel-inclusions, there was massive vesiculation, and peripheral distortion of chloroplast and mitochondria (1). To detect the virus, use a cDNA probe wherein the cDNA clones were made using T7 promoter with run-off transcripts (4). A non isotopic (digoxigenin-based) dot-blot hybridization assay for FC-specific RNA has been used successfully to confirm the virus (3).

Look-alike symptoms:

Favorable conditions:

Pattern in field: In Manitoba, infected plants were in circular patches in two adjoining fields, and scattered diseased plants were identified in several nearby fields (1).

Infestation levels:

Life cycle: The virus consists of double stranded RNA.

History in MT and US:

- Flame chlorosis has not been identified in Montana (1).

- Flame chlorosis was identified in western Manitoba, Canada in June 1985 by virologist Steve Haler. Infections have been reported as far north as Dauphin, Manitoba and as far south as the Red River Valley just 10 miles north of the North Dakota border.

Crop losses: Infected plants usually do not produce seed. Yield losses have ranged from minmal to 80 percent on barley (1). Extra tillering partly compensates for yield losses when infection is less than 5%. Above this level, losses vary depending on growing conditions (2).

Controls: Crop rotations have provided the best control of flame chlorosis (2).

Cultural controls: Rotating to non-cereal crops such as canola or flax is thought to be the best line of defense. Substituting wheat for barley in rotations also has been successful (2).

Resistant varieties:

Biological controls:

Chemical controls:

References: 1) Haber, S et al. 1990. "Flame chlorosis: a new soil-transmitted, virus-like disease of barley in Manitoba, Canada. Phytopathogy, Vol. 129, pp. 245-256. 2) Dietz, J. and Henkes, R. Spring 1991. The story behind flame chlorosis. The Furrow. Vol. 96, Issue 4, pp. 7-8. 3) Haber, S. and Harder, D.E. 1992. Green foxtail (Setaria viridis) and barnyard grass (Echinochloa crusgalli), new host of the virus-like agent causing flame chlorosis in cereals. Canadian Journal of Plant Pathology. Vol. 14, pp. 278-280. 4) Steve Haber, phone conversation with Tom Carroll, 4/22/91.
 
 
 
 

HIGH PLAINS DISEASE

Cause: The causal agent is not yet known. However, we do know that it is virus-like and it has a 32 kilo Dalton protein (1).

Hosts: Corn, barley, wheat, and yellow foxtail (1)

Vector: Wheat curl mite (Aceria tulipae)

Symptoms: Field symptoms include very severe stunting and a distinct mosaic and yellowing pattern on the leaves. In advanced stages, marginal leaf reddening or scalding may occur. On sweet corn, the mosaic is very yellow and occurs in bands running the length of the leaf blade (1).

Lab diagnosis: Dr. Stanley Jensen (USDA Virologist, Univ. of Nebraska, Lincoln) initially characterized the virus-like agent chemically. He has now developed a serological test (1).

Look-alike symptoms:

-Wheat streak mosaic (very difficult to differentiate the two diseases based on field symptoms alone)

-Wheat spot mosaic (another wheat curl mite-vectored disease of unknown cause) (1)

Favorable conditions:

Pattern in field:

Infestation levels:

Life cycle: The disease is not spread through soil or by seeds (1).

History in MT and US: High plains disease has been found in Texas and Colorado. As of Jan. 1995, it has not been found in Montana.

Crop losses: At the present time, it does not look as though high plains disease will be significantly limiting to production in areas where it has been found (1).

Controls:

Cultural controls: Controls similar to those for wheat streak mosaic and barley yellow dwarf should be sufficient for control of high plains disease (1).

Resistant varieties: High Plains disease is variety specific. The varieties that are susceptible seem to be severely affected (1). In Texas, the disease was found on Abline wheat; in Colorado, it was on TAM 107 volunteer wheat (1).

References: 1) Brown, W. High plains disease (HPD) A new disease of corn and small grains in Colorado, news release, December, 19, 1994.
 
 
 

OAT BLUE DWARF VIRUS

Hosts: Oats, wheat, barley, flax (1)

Symptoms: The disease is most common in oats, but may infect wheat, barley, and flax. Infected wheat heads are pale green and lighter in color than the rest of the crop and the heads do not fill normally. Occasionally, but not always, the flag leaf associated with these heads has streaking or mottling (1).

Lab diagnosis: Send symptomatic wheat heads to Dr. Mike Edwards, USDA (1).

Look-alike symptoms: Aster yellows

Favorable conditions:

Pattern in field: Infected plants are scattered throughout the field (1).

Infestation levels:

Life cycle: The virus is transmitted by aster leafhoppers (Macrosteles fascifrons). The leafhopper transmits the virus at a low frequency. The leafhoppers are migratory and do not overwinter in ND or MT (1).

History in MT and US: In 1994, the aster leafhopper vectors were common in ND from late June into mid-August. High populations may have triggered the outbreak of the disease (1).

Crop losses:

Cultural controls:

Resistant varieties:

Biological controls:

Chemical controls:

References: 1) McMullen, M. 1994. "Cause of pale green wheat heads determined". NDSU Crop & Pest Report, Sept. 2, 1994, Issue 17.
 
 
 
 

SOIL-BORNE WHEAT MOSAIC

Pictures of s-b wheat mosaic:

Kansas mosaic - #1

Kansas mosaic - #2

Scientific name: Furovirus group - Virus

Hosts: SBM is found almost exclusively on winter wheat. This is due to the habits and environmental requirements of the vector (2) Rye, barely, and some annual weedy bromes are hosts, but are are not important in virus survival or spread) (1).

Vector: The soil-inhabiting fungus Polymyxa graminis is the vector that transmits SBMMV. The virus particles are either inside the special spores of the fungus (known as zoospores and resting spores) or attached to the surface of these spores. The fungus is an obligate parasite that invades roots. When adequate moisture and a host are available, the fungal spores germinate, swim to a wheat rot, and invade a root and spreads in the plant diverting energy from growth (1). The fungus has a resting spore stage wich persists in soil for many years. Apparently, the virus also is able to survive this time in the fungal vector. This probably is why crop rotation has not been effective in controlling SBM.

Symptoms: Due to the number of virus strains that exist, symptoms can range from none at all to mild green to yellow mottling and striping. Also, symptoms can there can be moderate to severe stunting and a rosette appearance in some varieties (3).

1) Affected plants are stunted and bronzy-yellow.

2) Leaves are mottled with a yellow background and darker green islands

3) Mosaic symptoms gradually fade away, but stunting often persists until maturity.

4) Symptoms fade quickly with warmer temperatures, but persist into April and May if temperatures are cooler than normal. (1).

Look-alike symptoms:

Favorable conditions:

1) disease is most serious following wet weather since the swimming spore stage requires free moisture. Symptoms expression becomes more pronounced at temperatures below 68 F (3). The virus and its fungal vector are spread from field to field in soil on equipment and probably also with wind-blown dust particles. The virus is not seed-borne.

Pattern in field: In the early spring, irregularly-shaped areas of yellow wheat appear. Once these areas appear, there is no further spread to unaffected wheat plants (1,3). The pattern may be more pronounced in low areas or it may be generally distributed across the field.

Infestation levels: The longer mosaic and stunting persist, the greater the yield loss (1).

Life cycle: Plants are infected in the fall and occasionally show symtptoms then. However, the yellow areas usually are not obvious until early spring (1). Soil-borne wheat mosaic virus particles are stiff, hollow rods of two lengths - 1) 95 to 160 nm (43-63 millionths of an inch) and 2) 20 nm (8 millions of an inch) wide. The virus is quite variable and the length of the predominant short rods often changes as the plants grow. Symptoms change as the predominant rod length changes, but the relation of rod length to yield loss is not known (3). The virus can remain infectious in dried leaves for several years (3). SBWMV is not seed-borne (3).

History in MT and US: SBM is not present in Montana. SBM occurs thourghout the central and eastern states and adjacent eastern Canada, but thus far has not been recorded west of the Rocky Mountains (2). SBM is one of the most serious wheat diseases in eastern-central Kansas (1) and other parts of the midwest. SBWMV was first reported in Illinois in 1919. Since then it has been reported from most of the eastern soft red and hard red winter wheat areas of the US (3).

Crop losses: SBM oten causes losses up to 50% in affected fields planted to susceptible varieties. The disease caused an estimated average annual loss of 2.9% in Kansas between 1979 and 1984 (1). Losses vary from year to year because of the varieties of wheat grown, continuous cropping, strains of the virus, and environmental conditions favoring disease development (3).

Controls:

Cultural controls:

1) There is no practical way to rid the soil of the SBM virus (1).

2) Rotation out of wheat to other crops for several years has not been effective. Probably, this is because the fungal vector is able to survive for many years in the soil (1,3).

3) Adequate nitrogen fertility will help SBM diseased plants recover to some degree, but it will not cure the disease (1).

Resistant varieties: Any field that has ever shown symptoms of SBM should be planted to a resistant variety (Kansas). Resistant varieties are an important means of control in Kansas. Resistance depends on a single major gene with some minor genes, but has been very stable and reliable (1).

Biological controls:

Chemical controls:

References: 1) Wheat Diseases. 1984. Cooperative Extension Service, Kansas State University, Manhattan, pp. 4-5. 2) 2) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, p. 50. 3) Watkins et al. 1985. "Soil-borne wheat mosaic" NebGuide. Cooperative Extension Service, U. of Nebraska, G74-202.
 
 
 
 

WHEAT STREAK MOSAIC

Pictures of WSM:

symptomatic plants - field shot

wheat curl mites - on wheat leaf

wheat curl mite (electron microscope)

virus particles

volunteer wheat growing - from previous year's grain in field

word slide of green bridge

Scientific name: Potyvirus group

Hosts: Wheat is the most favored host by far. However, WSMV can survive and increase in oat, barley, rye , sorghum and pearl millet (10) and in many wild grass genera including Bromus, Hordeum, Lolium, Agropyron, and Poa as well (?).

- In Kansas, WSMV was found in 5 annual grasses, but in no perennial grasses. The annual grasses were: green foxtail (Setaria viridis), giant foxtail (Setaria faberi), prairie cupgrass (Eriochloa contracta), barnyard grass (Echinochloa crusgalli), and common witchgrass (Panicum capillare) (8). WSMV has not been found in any Agropyron tested (crested, intermediate, or western wheatgrass) (11).

Vector: Wheat curl mite (Aceria tulipae)

1) The mite is very small, white, and cigar-shaped. Using a 10x hand lens, the mite often can be found rolled up in young leaves (6).

2) The mite life cycle, from egg through two larval stages to the adult and egg, can be completed in 7 to 10 days. Under ideal conditions, one adult could theoretically produce several million offspring in 60 days (7).

2) Mites are protcted from the harsh environment and predators by causing the wheat leaves to curl inward around them (6).

3) Wheat curl mites survive only on green plants.

4) Mites can acquire WSMV only at the nymphal stage, but then may transmit WSMV at all instar and adult stages (6).

5) Mites feeding on infected plants acquire the WSMV within minutes of beginnig to feed. They retain the ability to transmit the virus to healthy plants for at least a week later (7).

5) Spread of WSMV occurs whe the mites crawl up the stems to the heads are are blown into adjacent areas by wind.

6) It has long been thought that the mites cannot survive long distance air travel because they dry out. Wet, humid weather may allow for longer air travel (6).

Symptoms of wheat streak mosaic:

leaf symptoms

symptoms - field margins on Alberta winter wheat

1) General yellowing and stunting

2) Mottled yellow stripes or streaks running up and down the leaves.

3) Edges of young still-green leaves cup upward and roll toward the center of the leaves. One can often unroll the leaves and observe the larvae-like mites with a hand lens , (x,5).

4) If infection occurs before tillering, growth usually is halted and few or no heads are formed (6).

Lab diagnosis: At Montana State University, WSMV is diagnosed through serologically-specific electron microscopy. This method employs an antiserum that specifically traps WSMV particles on a small grid so that they can be viewed at very high magnifications through an electron microscope.

Look-alike symptoms:

1) Early septoria leaf blotch

2) Winter injury or frost injury (yellow leaf tips)

3) Nitrogen deficiency

4) Sulfur deficiency

5) Barley Yellow Dwarf (5)

6) Common root rot (5)

7) Herbicide injury due to:

Favorable conditions:

- Early fall seeding and late spring seeding

- Poor control of volunteer wheat. The movement towards minimum-till and no-till has been shown to increase volunteer wheat. Jim Nelson and Dave Wichman (Mocassin) found that volunteer wheat was the 16th most common weed in conventionally-tilled fields, but the 7th most common in conservation tillage (13).

- Seeding winter wheat before or just after spring wheat is harvested

- Hail contributes to infection by causing head shattering which in turn creates volunteer that is difficult to control. Wet conditions in August are conducive to germination of volunteer. If a warm dry fall and early warm spring occurs, mite survival, reproduction, an movement can occur (5).

- Cool, moist spring weather reduces severity and hot, dry weather increases it.

- Warm, dry weather in the fall is conducive to mite reproduction. Normally by mid-September conditions no longer favorable for mite. In cool weather damage can be minimal.

- In Kansas, WSMV is a problem where wheat is grown in fallow systems, but it is not a problem in continuous wheat production where it is essential to control volunteer (9).

-Presence of green plant material in field required for both vector and virus to survive. Eliminate living plant material = eliminate disease.
 

Pattern in field: Symptoms often first observed on field edges or where grassy waterways pass through fields before progressing into center of fields.

Infestation levels: In general, symptom severity is related to the developmental state of the plant when infected, plant nutrition, air and soil temperature, and the particular virus strain involved.

EFFECT OF GROWTH STAGE:

1) Infection before tillering: Growth usually is halted and few or no heads are formed. When fields are infected with a virulent WSMV strain as wheat is emerging, losses can approach 100% (12).

2) Infection between tillering and jointing: Grain can be formed but florets often are sterile.

3) Infection after jointing: Lower test weights (6).

Losses may become progressively less the later the crop stage that is initially infected. Partial infections, mild strain of the virus, good moisture conditions which reduce plant stress, and cool weather generally will reduce WSM severity (12).

Life cycle: Vector: wheat curl mite (eriophyid mite, Aceria tulipae). The mite increases rapidly during warm fall weather. A Kansas study (reported in Plant Disease) found that perennials were not harboring WSMV as a rule but some annuals were. The disease is moved when the mites move from the harvested field or from volunteer into the young cereal.

History in MT and US: Sporadic outbreaks in scattered fields have been common in Montana, the Great Plains, and the Midwest for many decades. However, in 1993 and 1994 patterns of disease severity and spread were different than in the past, especially in the Golden Triangle region of Montana. The differences included severe disease in spring wheat, disease in fields where volunteer control had been good, very rapid spread of the disease over entire 200 acre fields in 3-4 days and extremely low numbers of mites detected on leaves (MB).

Crop losses: Crop losses depend on growth stage affected, mite populations, moisture, and temperature conditions. Later infections, moist weather, and moderate temperatures may minimize losses. However, if wheat is infected before tillering, yield losses can approach 90%. Heads may be fewer in number and shorter with fewer kernels (5).

Controls: Once plants are infected, controls are no longer available in the current crop. Cultural methods and resistant varieties are the best means of control.

Cultural controls:

1) ELIMINATE "GREEN BRIDGE"

Remove ALL living plant material to eliminate aphid vectors from spreading BYD via the "green bridge". Two weeks without green material in the field before the next crop emerges should be sufficient (5).

2) SEEDING DATES:

Late fall planting of winter wheat and early planting of spring wheat can reduce infections since the aphids usually are less active at these times. Fall planting should be flexible depending on local weather conditions. However, the planting dates listed below are good guidelines:

REGION OF MT FALL PLANTING DATE SPRING PLANTING DATE

North- September 5 by late April

Central- September 15 by late April

Southeast- September 20-22 by late April

Seed near CRP last in fall and first in spring. If winter wheat is heavily infected, reseed to barley (non-host).

3) ROTATION: In high inoculum level situations, rotate to barley or alfalfa.

Resistant varieties: Based on one-year trials: WINTER WHEAT: Tiber and Neely were the most resistant; Norwin, Roughrider, and Winalta, then Judith and Agassiz were the most susceptible. SPRING WHEAT: Amidon and Thatcher were the most resistant; Rambo, Fortuna, Olaf, and Len were the most susceptible.

Disease ratings winter wheat varieties to WSMV*

(*4 = Most Tolerant 1 = Least Tolerant)(4).
 

AC Readymade 2.3
Agassiz 1.3
Arapahoe 2.3
Blizzard 2.0
Centurk 2.0
Cree 2.7
Judith 1.3
Kestrel 2.7
Lamar 3.0
Manning 2.7
Meridian 3.7
Neeley 3.0
Norstar 2.3
Norwin 1.0
Promontory 3.0
Quantum 542 2.7
RAM 2.0
Redwin 2.3
Rocky 2.3
Roughrider 1.0
Seward 2.7
TAM 107 2.7
Tiber 3.7
Vona 3.0
Weston 3.0
Winalta 1.0
Winridge 2.0
Yuma 3.0
 

Joe Martin (U. of Kansas-Hays) has been breeding lines of wheat for resistance to WSMV for many years. Through transfer of resistance genes from spelt to wheat, he now has a variety (KS 91H184) that had 0.3% yield loss relative to 20% loss in the variety Larned which is grown on 20% of KS wheat acreage. He has found that the resistance breaks down at high temperatures and that mite populations build up just as much on resistant as on susceptible varieties (10).
 

Biological controls:

Chemical controls: Miticides have not been effective. There are pesticides that could lower mite populations, but achieving adequate spray timing is the real problem.

References: 1) 2) 3) Pacific Northwest Plant Disease Control Handbook, 1990 4) Winter wheat varieties. 1994. Montana State University Extension Service, Bulletin 1098. 5) Guide to Herbicide Injury Symptoms in Small Grains, Second edition. Agri-Growth, Inc., Hollandale, MN, pp. 87-88. 6) "Barley yellow dwarf and wheat streak mosaic in small grains" 1988. Montguide, Montana State University Extension Service publication MT8802A. 7) Wheat Diseases. 1984. Cooperative Extension Service, Kansas State University, Manhattan, pp. 4-5. 8) Christian, M.L. and Willis, W.G. 1993. Survival of wheat streak mosaic virus in grass hosts in Kansas from wheat harvest to fall wheat emergence. Plant Disease, v. 77 (3), pp. 239 - 242. 9) Martin, J. Report, Wheat Streak Forum, Bozeman, MT, October 7 - 8, 1994. 10) Seiters, D., Report, Wheat Streak Forum, Bozeman, MT, October 7 - 8, 1994. 11) Langham, M., Report, Wheat Streak Forum, Bozeman, MT, October 7 - 8, 1994. 12) Riesselman, J. 1993. Wheat streak identified. Montana Crop Health Report, May 7, 1993, Issue No. 3. 13) Riesselman, J. 1993. Why is wheat streak mosaic so severe this year? Montana Crop Health Report, July 2, 1993, Issue No. 7.