UPDATE - December 1995
"Green Bridge" Alert!! -- Plan Fall or Early Spring Control!
Roger Veseth, WSU/UI Conservation Tillage Specialist
Don't let the "green bridge" cut your spring crop yield potential. Abundant fall rains and mild temperatures across the Inland Northwest have resulted in extensive populations and growth of volunteer grain and weeds this fall. Under minimum tillage or direct seeding of spring crops, the volunteer grain and weeds growing between crop harvest and spring seeding can serve as a "green bridge" host for root diseases and other pests. Growers should control this green bridge in late fall or early spring while maintaining surface crop residue to optimize water conservation -- in order to achieve the high spring crop yields now possible in much of the region.
Northwest research has shown that spraying volunteer and weeds with a nonselective herbicide in late fall, or at least about 3 weeks before spring seeding, can increase yields of direct-seeded spring cereals after cereals by 20 to 50 percent compared to spraying 1 to 3 days before seeding. The 1- to 3-day time interval that has commonly been used between spraying and direct seeding can greatly increase the level of root diseases such as Rhizoctonia root rot, Pythium root rot and take-all. Spring pulse crops can also be impacted by the green bridge.
When weather conditions permit, fall spraying when plants are still actively growing can provide the longest possible "host-free" period for reducing spring crop pests associated with the green bridge. Nonselective herbicides can be very effective in late fall because added winter stress helps kill the plants. In addition to minimizing root diseases, fall green bridge control can improve control of winter annual weeds, reduce populations of Hessian fly, Russian wheat aphids, and aphids carrying barley yellow dwarf virus, and reduce the carryover of disease inoculum for Cephalosporium stripe of winter wheat.
Why not till? Intensive tillage generally provides effective control of root diseases and other crop pests associated with the green bridge, although some yield losses have been documented when the green bridge was tilled out shortly before spring seeding. The major disadvantage with more intensive tillage is greater evaporation and runoff losses which reduce soil water storage and associated crop yield potential.
On-farm testing is an easy and effective way for growers to determine the yield benefits of early green bridge control with their tillage system, crop rotation and volunteer/weed situation. If growers plan to spray the whole field, they can turn half the sprayer boom off and on in the field to make four side-by-side comparisons. An alternative approach could be to only spray four strips in the field, each with an adjacent no-spray strip for yield comparison. Each strip should be wider than the combine header and as long as possible, preferably 500-700 feet or more, for accurate yield comparisons.
Growers interested in setting up a green bridge on-farm test are encouraged to contact their county extension agent or Roger Veseth, WSU/UI Conservation Tillage Specialist (208-885-6386) for assistance in establishing and evaluating the trials. Ask for a copy of "Green Bridge Key to Root Disease Control," PNW Conservation Tillage Handbook Series No. 16 in Chapter 4, that summarizes 5 years of field research on green bridge management in the Northwest. A 15-minute video (#VT0040) on the topic is also available through WSU Cooperative Extension.
us: Hans Kok, (208)885-5971
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