Fall weed control and smut on downy brome
by Joe Yenish,
John Burns, Lori Carris, and Tim Murray
9/24/04
Rainfall in August and September has resulted in much
emergence of volunteer grains and winter annual grass weeds. The past few years, many growers have controlled
grass weeds with a single spring application of Roundup or other nonselective
herbicide prior to planting a spring crop or establishing fallow. This year, growers should consider splitting
applications between the fall and spring to effectively control grasses for
timely spring planting and eliminating seed production prior to fallow.
The thick "sod" produced this fall will require higher rates
or multiple herbicide applications for effective control. Dense stands of weeds tend to require higher
herbicide rates and burndown may be slowed.
Slow burndown or, worse yet, a second spring herbicide application to
avoid the green bridge will delay spring planting and reduce crop yield. Fall applications allow killed sod plenty of
time to decompose and greater likelihood that a single spring application will
effectively burndown vegetation.
Growers should consider a fall application of a burndown herbicide
across all intended spring crops or at least those fields that are first up to
plant in the spring. Fall burndown
applications may also pay off on the subirrigated flats in which spring entry
is severely delayed due to wet conditions.
Fall application of burndown herbicides may also benefit
growers in wheat/fallow regions this coming year. Late summer/early fall emergence means volunteer grains and
annual grass weeds will have more advanced growth going into winter. Without a fall application, spring
applications will need to be made earlier to prevent seed production of annual
grasses such as downy brome (cheatgrass).
It is possible that early plus late spring herbicide applications will
be needed to prevent weed seed production by fall and spring emergence
weeds. If two spring burndown
applications are needed prior to fallow establishment, growers are better off
splitting applications between the fall and spring than early and late
spring. Multiple applications of
herbicides cost money and time, but an upside to current fall conditions is
that flushes of annual weeds will greatly deplete the soil weed seed
reservoir. If plants are not controlled
and allowed to go to seed, the advantage is lost.
A wetter fall also means more winter annual grasses such as
downy brome and jointed goatgrass in fall seeded wheat this year. It is important to control winter annual
grass weeds prior to planting. Should
rains immediately follow wheat planting and downy brome or jointed goatgrass
emerges ahead the crop, a nonselective herbicide application prior to wheat
emergence may benefit. Otherwise,
growers should consider fall applications of Maverick, Olympus, Beyond, or
other selective herbicides if stands of downy brome thicken this fall.
A cheatgrass
(downy brome) bunt fungus that looks and smells like dwarf bunt was
sufficiently abundant in some wheat fields in Washington this year [or [in
2004] to result in smutty wheat at harvest. The cheatgrass bunt, (Tilletia
bromi), is closely related to dwarf bunt (Tilletia controversa)
and common bunt (Tilletia tritici) but it does not infect wheat (see
attached photos). There were several problems resulting from the high
level of cheatgrass bunt spores in 2004. There were enough spores present
in some fields to affect the quality of the wheat, and in some cases the spores
were misidentified as dwarf bunt. Additionally, wheat growers with this
problem in their fields need to know that the source of the spores is
cheatgrass and not wheat so they can take appropriate control measures. The
heavy infestation of infected cheatgrass was apparently the result of a combination
of favorable environmental factors. A heavy freeze last Halloween thinned
out the emerging winter wheat crop in some areas, and the timing of the spring
rains was favorable for cheatgrass. The result was a heavier
infestation of both cheatgrass and the bunt in wheat fields than is usually
seen.
Thus, controlling downy brome during the fallow period to
reduce soil seed reserves and effective control during the cropping year will
prevent real or perceived losses in quality and value of wheat from the Pacific
Northwest. Although the moist fall has
brought on heavier weed populations, it could be used to our advantage for
long-term control
.
