Fall
weed control and bunt fungus on downy brome
by Joe Yenish, John Burns, Lori Carros, and Tim Murray
September 2004
Fall weed control
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 single spring applications 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 sub irrigated 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.
Cheatgrass bunt fungus
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 cheats grass 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.
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