HomeMy WebLinkAbout1993-112-632-3 WSM VirusAgdex 112/632-3
Wheat Streak Mosaic in Alberta
and Its Control
T he information in this factsheet was prepared five
decades ago by Dr. J.T. Slykhuis and Dr. J.E.
Andrews at the Agriculture Canada, Lethbridge Research
Station. With a few minor revisions, all their virus control
information is as valid now as it was then.
Wheat streak mosaic, which is probably native to southern
Alberta, was first recognized here in 1952. In both 1952
and 1953, wheat streak mosaic caused severe damage to a
number of winter and spring wheat crops. It did so again
on winter and spring wheats in 1992 in the Claresholm to
Mossleigh area.
This disease can be controlled or reduced to a minimum
by following certain cultural and agronomic practices. The
following information is provided to answer some of the
questions being asked by farmers about wheat streak
mosaic and to suggest methods for its control.
What is wheat streak mosaic?
Wheat streak mosaic is a disease of wheat caused by a
virus. The virus enters the leaves and spreads to all parts of
the plant. The first signs of the disease are light green
dashes or streaks in the leaves caused by the destruction of
chlorophyll. As the disease progresses the light green
dashes and streaks turn to yellow streaks and blotches
giving the leaf a green and yellow pattern of colour called
a “mosaic”, hence the disease is called “streak mosaic”.
How does streak mosaic damage
the wheat plant?
The virus causes destruction of the green chlorophyll that
harnesses the sun's energy for the manufacture of sugar in
the plant. Infected plants are usually stunted and produce
fewer seeds, which are often badly shrivelled. The earlier
the infection, the greater is the effect on the plant. Thus
plants may die, fail to set seed, be stunted or show little
effect depending on when they were infected.
How can the disease be
recognized?
In young plants, the disease can be recognized by the
presence of light green or yellow streaks in the leaves.
These streaks distinguish wheat streak mosaic from
yellowing due to other causes.
Later as the crop matures, stunting of diseased plants is
the most obvious indication of wheat streak mosaic. These
stunted plants are usually intermingled with plants of
normal height. Thus heads occur at two levels throughout
large areas of the field. This contrasts with the effect of
“take-all” root rot that causes stunting in scattered,
irregular patches. Blackened bases and roots are typical
signs of “take-all”.
How serious is streak mosaic?
Damage ranging from complete crop failure to severe
reduction in yield can result from mosaic infection. Such
severe damage usually occurs in localities where a
combination of conditions happens to favour the disease.
Sometimes, as in western Kansas in 1949, streak mosaic
causes severe losses in many fields over extensive areas.
However, usually only an occasional field is severely
damaged. It is customary to find crops with little or no
damage from streak mosaic within a few miles and
sometimes even adjacent to fields that were ruined by the
disease.
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How does the disease spread?
The virus causing wheat streak mosaic, like most viruses,
can not spread without the aid of some carrier. Streak
mosaic has unusually interesting carriers, which are very
tiny, four-legged, worm-like mites. These mites are only
one-one hundredth of an inch long, and they cannot be
recognized without the aid of a strong magnifying glass.
In fact no one knew these mites occurred on wheat, nor
just how wheat streak mosaic spread in the field until
October 1952, when the mites were found and then
proven to be the carriers of the streak mosaic virus.
The mites, known by the scientific name of Aceria tulipae,
are not the actual cause of streak mosaic. They merely
carry the virus from diseased to healthy plants. It is an odd
coincidence, however, that conditions favorable for the
virus also favor the mites.
The mites, like the virus, must have living plants to survive
throughout the year, and they prefer wheat. They survive
the winter on winter wheat. The delicate looking mites can
actually stand more freezing than the hardiest varieties of
winter wheat, but if the wheat plants die from freezing the
mites also die.
Mites that survive the winter on winter wheat increase
rapidly during the warm spring and summer weather and
find their way to susceptible plants nearby. The mites have
no wings and cannot fly, and they are too small to crawl
far. They are so light that they are blown about by the
wind like specks of dust or tiny flakes of snow.
What conditions favor the disease?
Important conditions that favor the spread of the disease
are the presence of a source of infection coupled with early
planting of fall wheat. The most severe losses have
occurred in early planted fields of winter wheat although
the disease has been observed in other winter wheat fields
and in spring wheat. The streak mosaic virus and its
carrier, the mite, need living plants to survive not only the
winter but throughout the year.
During the warm weather of spring and early summer
they can spread from winter wheat on which they
overwintered to other winter or spring wheat nearby. They
can spread continuously throughout the summer as long
as the infected wheat plants remain green.
When the wheat crops are ripe, both survive in living
shoots of volunteer wheat and to some extent in
susceptible grass weeds in the wheat stubble fields. Their
abundance in the fall therefore depends to a large extent
on the amount of living volunteer wheat, and perhaps
susceptible weeds, that are allowed to grow in fields where
the virus and mite have increased during the summer. If
such infested plants are allowed to survive in stubble or
even in summerfallow fields and winter wheat is seeded in
or adjacent to such fields early in the fall, the disease has its
best chance to spread to the new winter wheat crop.
On the other hand, if the new winter wheat crop is sown
late the disease has less warm fall weather in which to
spread. Sources of infestation can be eliminated by
destroying volunteer wheat and susceptible weeds before
the new crop is sown. Prairie grasses are not an important
source of infection.
Is streak mosaic serious in crops
other than wheat?
Although oats, barley and some other grass-type plants
can be infected with the wheat streak mosaic virus, wheat
is the only cereal that is seriously affected. All types of
wheat including hard red winter, spring, soft and durum,
are susceptible to this disease.
Is streak mosaic a menace only
where winter wheat is grown?
Although spring wheat is as susceptible as winter wheat,
streak mosaic is not a menace in areas where winter wheat
is not grown. This is because the virus and mites
overwinter in living plants, and winter wheat is the most
favourable plant known for carrying the virus and mites
through the winter.
Is the virus carried in the soil,
straw or seed?
All experimental evidence to date indicates that there is no
danger of the virus being carried over in the soil, straw, or
seed. It is perfectly safe to sow seed from a streak mosaic
infested crop. When a plant dies or matures in the field
both the virus and the mites die.
Can the virus and the mites be
controlled with sprays
There is no chemical known to control the virus, nor to
cure infected plants. There are sprays that will kill the
mites, but as yet there has not been enough experimental
work to find a spray that will be of value in the field.
Possibly a good spray may be found to prevent the spread
of the mites and hence the virus. Then as now there is no
economic chemical control.
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Are any varieties of wheat resistant
to wheat streak mosaic?
No commercial varieties of wheat possess sufficient
resistance to streak mosaic to be of any value in
controlling the disease. Nevertheless, there is some hope
that a suitable source of resistance may be found and that
good resistant varieties may be developed. None have
been developed to date (1993).
Can wheat streak mosaic be
controlled on Alberta farms?
Any farmer in Alberta who is aware of wheat streak mosaic
on his farm can control the disease by cultural means if he
has a thorough understanding of these facts. It is still
better when his neighbors also understand the danger. By
a community effort, with little or no extra expense to any
farmer, the disease can be essentially eliminated.
Shortly before seeding time destroy any volunteer wheat
in or near fields in which winter wheat is to be seeded. It is
not necessary to bury the stubble but only to destroy the
growth. Perform all cultivating operations a week or two
before seeding. Do not rush it and thus upset your
cutworm control program.
Seed winter wheat between September 1 and 15. Later
seedings are safest from mosaic infection, but a
compromise must be made with good agronomic
practices.
There is no danger of streak mosaic occurring in spring
wheat if the virus does not overwinter on winter wheat
crops or volunteer plants. Therefore, be certain that there
is little or no mosaic in any winter wheat crop before
seeding spring wheat next to it. If much streak mosaic is
present in a field of winter wheat, seed a resistant grain like
barley or oats adjacent to it rather than spring wheat.
When a winter wheat crop is so severely infected with
mosaic that it must be destroyed in early spring, spring
wheat can be safely seeded in the same field if all infected
plants are killed. Allow several days for the diseased plants
to dry out or begin to decay before seeding spring wheat.
If the infected plants have not been completely killed then
seed barley, oats or canola.
Are there other advantages or
disadvantages of late versus early
seeding?
Seeding fall wheat early in August may be more
convenient to some farmers since this is a period when
other farm operations are at a minimum. In years of
unfavourable fall conditions and early freeze-up, the earlier
seeded fall wheat may have some advantage in attaining
adequate fall growth. However, in most years, the
disadvantages of early seeding far outweighs the
advantages.
As previously mentioned, streak mosaic has a much better
chance to spread in early-seeded than in late-seeded fall
wheat. In early-seeded fall wheat, there is also a greater
opportunity for development of root rots. Furthermore,
the early-planted fall crop uses up a lot of soil moisture
before freeze-up. Moisture used during this period does
not add anything to the yield of the crop. If there is a dry
fall, the early-planted crop may use up most of the
available moisture before winter and consequently may die
from drought before spring. Drought is one of the causes
of winter killing.
Seeding fall wheat during the first two weeks of
September appears to be the best date to satisfy all
conditions for the successful production of winter wheat
in southern Alberta.
Are there any recent
developments?
Since this fact sheet was written 50 years ago, there are no
recommended chemical controls or resistant wheat
varieties. The cultural control methods described are as
effective now as they were then.
Prepared by:
Dr. Ieuan R. Evans
1993
12/01/500