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Chap. 4, No. 18, September 1993

"Green Bridge" Control Starts in the Fall!

-- New Video and Publications Show the Benefits --

Roger Veseth

Recent results of Northwest research show that volunteer grain and weeds growing between crop harvest and spring planting under conservation tillage can serve as a "green bridge" host for build up of some root diseases caused by soilborne pathogens. But early control of this green bridge -- beginning in the fall when possible -- has proven to be an effective management tool for minimizing losses from some root diseases, such as Rhizoctonia root rot, Pythium root rot and take-all. In addition, fall control can also reduce the incidence of Cephalosporium stripe, improve control of winter annual grassy weeds, such as downy brome and jointed goatgrass, and reduce populations of Hessian fly. A new video and several PNW Extension publication are now available to provide growers with more information on early green bridge control as an effective pest management tool to enhance crop yields under conservation tillage systems.

Root Diseases

No-till seeding of spring barley and spring wheat after cereals in the Northwest has commonly included spraying volunteer grain and weeds with a non-selective herbicide, such as glyphosate, shortly before seeding. Although volunteer grain and weeds were controlled with this spray timing, crop yields were often lower than expected, thus limiting the use of spring no-till seeding. Recently, research has shown that the build-up of root diseases associated with the roots of volunteer grain and weeds is a major cause of the yield reductions. Control of this green bridge, beginning in the fall or early spring well ahead of seeding time, largely eliminates the problem. This management breakthrough makes conservation tillage seeding of spring crops a promising production option.

Spraying at least 2-3 weeks before seeding commonly increased yields of direct-seeded spring cereals after cereals by 20 to 50 percent compared to spraying 1-3 days before seeding. This 1- to 3-day time interval between spraying and seeding greatly increases the level of root diseases. When extensive germination and growth of volunteer and weed occur in the fall, control before winter lengthens the "host-free" period between susceptible crops and further reduces root disease potential. If volunteer and weeds are not controlled in the fall, the pathogen inoculum level would continue to increase over winter and into the spring as plant populations and root mass increased.

With early green bridge control, the yields of spring crops can be further increased under conservation tillage because of improved soil water storage through reduced evaporation and runoff. A healthier root system allows the crop to take advantage of this higher yield potential. More intensive tillage, on the other hand, can also effectively minimize the build up of root diseases associated with the green bridge. However, an important tradeoff is reduced soil water storage and associated yield potential, because of greater evaporation and runoff losses. Soil erosion potential would also be increased.

A PNW Extension publication and WSU Cooperative Extension video provide more details on how early green bridge control in the fall or early spring can reduce root disease problems in spring crops under conservation tillage. "Green Bridge Key to Root Disease Control" (PNW Conservation Tillage Handbook Series No. 16 in Chapter 4) summarizes 5 years of field research in the Pacific Northwest.

A colorful and fast-moving video titled "Managing the Green Bridge: Root Disease Control in Conservation Tillage," video VT0040, was completed in January 1993 by Washington State University Cooperative Extension. The 17-minute video includes a series of animated graphics to clearly explain how the green bridge can increase root diseases and how it can be managed with early control. It features USDA-Agricultural Research Service scientists R. James Cook and Alex Ogg, Jr. explaining actual field trials comparing early versus late green bridge control for spring barley after winter wheat under no-till and conventional tillage. Two Washington farmers also share their experiences of how early green bridge control has increased the profitability of spring crops through the use of conservation tillage.

Weed Control

Controlling volunteer grain and winter annual grassy weeds, such as downy brome and jointed goatgrass, in the fall before the onset of winter can also improve weed control in the following spring crop, in summer fallow and throughout the rotation. In the green bridge video mentioned above, Alex Ogg points out that fall control can prevent the build-up of a heavy sod that can interfere with spring crop establishment and growth.

He explains that herbicides are usually very effective late in the fall because of the added benefit of winter stress to improve control of the volunteer and weeds. As a result, you may be able to use the lower labeled rated of non-selective herbicides, such as glyphosate, that would control the weeds. He also points out that, in dryer regions and dryer areas of fields, there is also the potential benefit of not allowing the volunteer and weeds to use stored soil water in the fall and winter, which could improve yield of the following crop.

Another advantage of fall control is that it usually results in lower weed populations and smaller plants in the spring. This would improve spring weed control with herbicides or tillage. It could also reduce the possibility of seed production by weeds in the spring when wet weather delays spring field operations. This was a major problem in the spring of 1993, resulting in extensive seed set by downy brome across the region.

Cephalosporium Stripe

The potential for Cephalosporium stripe in future winter wheat crops can be reduced through control of volunteer wheat and grassy weed hosts before winter. Winter wheat yield losses from Cephalosporium stripe were particularly severe and widespread in the Inland Northwest in 1984 and 1993. To develop effective management strategies for the disease, growers need a basic understanding of the disease cycle and how environmental and management factors affect disease potential.

Volunteer winter wheat and winter annual grassy weeds, such as downy brome, jointed goatgrass, growing over winter can be important "green bridge" hosts of the Cephalosporium stripe pathogen. They should be controlled as much as possible between crops and throughout the rotation to minimize inoculum production and carryover between winter wheat crops. Allowing them to grow overwinter can reduce or eliminate the effectiveness of crop rotation to reduce pathogen inoculum levels.

Use of fall green bridge control for reducing the production and carryover of the disease pathogen is included in the September 1993 PNW Extension publication "Managing Cephalosporium Stripe in Conservation Tillage Systems," PNW Conservation Tillage Handbook Series No. 17 in Chapter 4.

Hessian Fly

Hessian fly populations can be reduced through early control of volunteer wheat and other susceptible green bridge hosts. Hessian fly caused significant yield losses in spring wheat in the eastern Washington and northern Idaho in 1993. Spring wheat is the primary crop damaged by Hessian fly in this region, but spring barley and winter wheat can sustain some yield losses in areas where weather and management conditions are highly favorable to the fly.

The high Hessian fly population and crop infection levels in 1993 were attributed in part to periods of wet weather in late summer and fall in 1992, and the extended wet spring weather in 1993, which also delayed spring seeding and increased infection potential. In addition to facilitating emergence and reproduction of the fly, periods of wet weather in late summer, fall and spring increase populations of volunteer wheat and other host weeds, which help increase fly production and the number generations.

Control of volunteer wheat and other host grassy weeds in fallow, after harvest and through planting of the next crop can reduce Hessian fly populations. Volunteer wheat and other grassy weed hosts which are allowed to grow for 2 to 3 weeks during fly emergence periods can become infested with Hessian fly and increase populations.

Non-selective herbicides often replace some or all tillage operations for controlling volunteer plants and weeds under conservation tillage. Spray early before Hessian fly emerges and "flaxseed" larvae develop.

A new PNW Extension publication titled "Hessian Fly Management in Conservation Tillage Systems for the Inland Pacific Northwest" (PNW Conservation Tillage Handbook Series No. 15, Chapter 8) was published in September 1993. It includes early control of the green bridge as part of the pest control strategy.

Other Crop Pests

Early control of volunteer grain and other weeds growing between crops can also increase populations of Russian wheat aphids, aphids carrying barley yellow dwarf virus, and other insect and disease pests on these host plants which can attack fall or spring crops.

How to Get a Copy of the Publications and Video

For copies of the PNW Conservation Tillage Handbook Series publications mentioned here, contact your county Cooperative Extension or Conservation District office in the Inland Pacific Northwest, or contact Roger Veseth, WSU/UI Extension Conservation Tillage Specialist, at the Plant, Soil and Entomological Sciences Dept., University of Idaho, Moscow 83844-2339, phone (208) 885-6386, FAX (208) 885-7760.

For a copy of WSU video # VT0040 "Managing the Green Bridge," send a $15 check (payable to WSU Cooperative Extension Publications) to the Bulletin Office, WSU Cooperative Extension, Cooper Publications Bldg., Pullman, WA 99164-5912 or call (509) 335-2857. Copies of the video are also available through county offices of WSU Cooperative Extension in eastern Washington.


Contact us: Hans Kok, (208)885-5971 | Accessibility | Copyright | Policies | WebStats | STEEP Acknowledgement
Hans Kok, WSU/UI Extension Conservation Tillage Specialist, UI Ag Science 231, PO Box 442339, Moscow, ID 83844 USA
Redesigned by Leila Styer, CAHE Computer Resource Unit; Maintained by Debbie Marsh, Dept. of Crop & Soil Sciences, WSU