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Chapter 4 – Disease Control, No. 16, July 1992

"Green Bridge" Key to Root Disease Control

Roger Veseth *

* Roger Veseth, Extension Conservation Tillage Specialist, Washington State University and University of Idaho

Root diseases caused by soilborne pathogens can be important production limitations when spring barley or spring wheat are planted after cereals, particularly under conservation tillage systems. Recent research has shown that volunteer grain and weeds can serve as a "green bridge" for buildup of some root diseases between the harvest of one cereal crop to the planting of the next spring cereal. Interrupting the green bridge by early control of volunteer grain and weeds ahead of seeding spring cereals recropped after cereals has significantly reduced yield losses due to root diseases. There are no additional equipment or production costs associated with this green bridge disease management tool - it is only a matter of timing.

Understanding and controlling green bridge effects on root diseases has been a research focus of several scientists in the Pacific Northwest. Leaders in the effort include R. James Cook, Plant Pathologist with USDA-Agricultural Research Service in Pullman, WA; Richard Smiley, Plant Pathologist with Oregon State University in Pendleton, OR; and Alex Ogg, Jr., Plant Physiologist with the ARS in Pullman. Five years of research by these scientists has helped to provide growers with an effective management tool for root diseases. The research projects have been funded in part through grants from the Oregon Wheat Commission, Washington Wheat Commission, and Washington Barley Commission, the USDA tristate STEEP project and other USDA programs.

Green Bridge Background

A common practice used with no-till and minimum tillage seeding of spring grains in the Northwest is spraying volunteer cereals and weeds with a non-selective herbicide, such as glyphosate, just before seeding. This practice is also used when spring grain is direct seeded into winter wheat fields that have high winterkill losses or when crop acreage adjustments are required by federal subsidy programs. Field observations and research indicate that a short time interval between spraying and seeding can increase the potential for some root diseases, particularly Rhizoctonia root rot (primarily caused by R. solani AG-8 or R. oryzae) in spring wheat and barley. Incidence of Pythium root rot of wheat and barley, and take-all of wheat also can increase with a short interval between spraying and seeding spring cereals after cereals.

How it Works

The scientists explain that the activity and survival of some soilborne pathogens that cause root diseases, such as Rhizoctonia root rot, depend primarily on a "green bridge" of living roots of host plants - mainly wheat, barley, grass weeds and some broadleaf crops and weeds. As the plants begin to die from a glyphosate application, the natural disease resistance of healthy living roots slowly breaks down. Root pathogens, already on and in the roots of these dying plants, have a major initial advantage in taking possession of these roots as a food source first, compared to nonpathogenic soil fungi and bacteria involved in organic matter decomposition.

The researchers determined that populations of root pathogens increase sharply and commonly peak within a few days after application of glyphosate, and then begin to decline. They think that the pathogen is eventually displaced in the dead roots by successions of soil microbes involved in organic matter decomposition. This biological succession limits the duration of the pathogen activity and is essentially a form of' 'biological control" that growers can take advantage of by early killing of volunteer and weeds before seeding. The actual time and length of high pathogen activity is influenced by the types and populations of pathogens and other soil microbes present initially, and the soil temperature and moisture, and other conditions that may favor the pathogens or their microbial competitors.

If a cereal crop is germinating or in early growth stages when the root pathogen activity is high, chance of crop loss from root disease is also high. Fortunately, researchers have found that glyphosate application at least 2 to 3 weeks ahead of seeding can greatly reduce root disease potential associated with the green bridge, even with direct seeding of the crop without prior tillage, The longer the interval between spraying and seeding the better, but a 2- to 3-week time period seems to be sufficient in most cases for the root pathogen population to peak and for the natural succession of microorganisms to reduce the pathogen population to a "safe" level before planting the spring crop.

The biological control of root pathogens achieved through competition from beneficial soil microbes frequently appears to be accelerated by any tillage before seeding. The researchers hypothesize that tillage accelerates root death and also the activity of beneficial soil microorganisms compared with a glyphosate application alone without tillage. Thus, the roots are comparatively more accessible to colonization by residue decomposing microbes, thereby limiting root pathogen activity. However, the researchers have found that a glyphosate application at least 2 weeks before direct seeding of spring cereals results in yields and root disease levels that are not significantly different than where tillage was used immediately before seeding, either tillage alone or after a glyphosate application.

Surface Residue Effects

The effects of surface residue on root disease potential in spring cereals has often been a point of confusion over the years. Fortunately, research has now shown that an increased incidence of root disease associated with direct seedings or minimum tillage seedings of spring cereals after cereals is primarily due to the presence of the green bridge of volunteer grain and weeds until just before seeding, and not just the presence of surface residue. The most important effect of surface residue on root disease potential is that by protecting the soil from evaporation, it can create a cool, moist environment favorable to root diseases. Without the green bridge, spring cereal yield potential is commonly higher with surface residue than with a residue-free seedbed, because of reduced water loss by evaporation.

Winter vs. Spring Cereals

The researchers point out that the green bridge effect on root diseases appears to be more important for spring grains than winter grains. Where winter wheat is direct seeded into stubble of a wheat or barley crop harvested 2 or 3 months earlier, it appears that the inoculum potential of Rhizoctonia and other root pathogens in the roots of the crop just harvested is still sufficiently high at planting time and is not significantly enhanced by the presence of volunteer cereals and weeds.

In general, the dry soil conditions common in the fall limits the activity of beneficial decomposing microbes and preserves pathogen inoculum potential compared to cool wet conditions in the winter and spring. These same dry conditions often limit establishment and growth of volunteer and weeds before fall seeding time. With spring cereals, however, 6 months or more may pass between harvest and planting. During this time the beneficial decomposing microbes can displace much or most of the root pathogen inoculum contained in the old and ever disappearing root mass of the most recent cereal crop. Meanwhile, the presence of volunteer and weed roots growing over the wet winter and spring provide an alternative host for root pathogens to maintain and more commonly increase their inoculum potential, and do significant damage in spring crops.

Combine-row Effect

Where spring wheat or spring barley are direct drilled following cereals shortly after a glyphosate application, it is not uncommon to see the location of combine chaff rows, even late in the growing season. In these rows, stands are typically thin, and the plants are stunted and late maturing compared to areas between the combine chaff rows. Volunteer grain and weed seeds typically are concentrated by the combine in the chaff row and amplify their influence on the crop in these areas. The researchers believe that the so-called "combine-row effect" is largely a "volunteer-row effect" because of the green bridge enhancement of root diseases.

Research Highlights

Several research projects over the past 5 years have investigated the importance of the green bridge in root diseases and how it can be managed. The following are some brief highlights of the research results.

Pendleton, OR

Smiley conducted three experiments from 1987 through 1989 on green bridge management and root disease relationships. Experiments were located at the OSU Columbia Basin Agricultural Research Center near Pendleton in a 17-inch annual precipitation zone on Walla Walla silt loam soil. Barley was direct seeded with a deep furrow drill into winter wheat sprayed with glyphosate 38, 31, 24, 12, 10 or 3 days before seeding on March 30. Application of glyphosate at progressively shorter intervals before seeding led to an increasing proportion of disease stunted plants and a progressive decrease in grain yield (Fig. 1).

In a similar 1988 experiment, spring barley performance and disease incidence were again influenced by the time interval between glyphosate application and barley seeding (Fig. 2). Glyphosate was applied on growing winter wheat at 21, 14, 7 or 3 days before seeding with a double disk no-till drill on March 18. As the interval between glyphosate application and seeding decreased, yield and measured plant growth characteristics were reduced and Rhizoctonia root rot incidence increased. Grain yield in plots sprayed 21 days before seeding was 28 percent (646 pounds/acre) more than plots sprayed 3 days before seeding, 2,955 vs. 2,309 pounds/acre, respectively.

In a 1989 experiment, Smiley compared combinations of two glyphosate application dates with and without shallow tillage on two treatment dates before seeding. Stubble from the previous spring barley crop was chopped down by skew treading in the fall. Glyphosate was applied to a moderately dense stand of volunteer barley and weeds either 22 or 3 days ahead of seeding, or not at all. Shallow tillage (a disking and rod weeding at a depth of 2 to 3 inches) was performed either 21 or 2 days before seeding with each herbicide treatment, or not at all. Spring barley was seeded on April 6 with a double disk no-till drill.

Fig. 1. Grain yield and plant stunting, caused by Rhizoctonia root rot, as influenced by time interval between application of glyphosate on winter wheat and direct seeding spring barley, 1987, Pendleton, OR (Smiley, OSU, Pendleton).

When spring tillage was not used before direct seeding of barley, grain yield in plots sprayed 22 days before seeding averaged 30 percent (847 pounds/acre) more than plots sprayed 3 days before seeding, 3,665 vs. 2,818 pounds/acre, respectively. When tillage was used at either time interval, with and without a herbicide application, grain yields were not significantly different than that of direct-seeded treatments sprayed 21 days before seeding.

LaCrosse and Dusty, WA

Ogg conducted a green bridge experiment in 1987 under a reduced tillage system in a winter wheat-spring barley-follow rotation near LaCrosse, WA. Average annual precipitation was 14 inches and the soil was a Walla Walla silt loam. Winter wheat stubble was left standing over winter. In the spring, there was a moderately dense and uniform stand of downy brome, and a sparse to light stand of volunteer winter wheat. Glyphosate was applied 12, 3 or 72 days before seeding or not at all. All plots were field cultivated (6-inch depth), disked (3-inch depth), fertilized with a shank applicator and rod weeded just before seeding with a conventional double disk drill on April 9.

Killing of volunteer and weeds with glyphosate 12 days ahead of seeding resulted in nearly a 52 percent (510 pounds/acre) higher yield than with the application 3 days before seeding (Fig. 3). The early herbicide treatments also resulted in a significantly higher yield than treatments tilled the day of planting but without a herbicide application. The results show that tillage may not reduce the high root disease potential associated with the green bridge, particularly with volunteer and weeds allowed to grow right up to the time of seeding. Glyphosate plus 2,4-D was also applied at the same three application intervals in the experiment and results were equivalent to treatments with glyphosate alone.

Fig. 2. Relative changes in Rhizoctonia root rot incidence, grain yield and plant characteristics as influenced by time interval between glyphosate application on winter wheat and direct seeding of spring barley, 1988, Pendleton, OR (Smiley, OSU, Pendleton).

Cook conducted an experiment in 1991 near Dusty, WA, under the same crop rotation and similar soil type and annual precipitation as in the LaCrosse experiment, although the tillage and planting systems were different. A one-pass double-disk drill with fertilizer shanks was used to deep band required fertilizer (nitrogen, phosphorus and sulfur) and direct seed spring barley into winter wheat stubble after a glyphosate application at either 23 or 2 days before seeding. In addition to time of glyphosate application before seeding, other experimental variables included chemical seed treatments and fertilizer banded 2 inches below seeding depth with fertilizer shanks positioned ahead of each seed opener, or 2 or 4 inches to the side.

Averaged across all fertilizer and seed treatments, the application of glyphosate 23 days before seeding produced an average grain yield 7 percent (217 pounds/acre) higher than when the application was 2 days before seeding, 3,345 vs. 3,128 pounds/acre, respectively. This response was both noticeable while the crop was growing and was statistically significantly at the 95 percent probability level. The best treatment combination was spraying 23 days before planting, deep shanking fertilizer directly below each seed row and treating the seed with Apron and PCNB (for Pythium and Rhizoctonia). This combination produced the highest average yield, 4,200 pounds per acre.

Fig. 4 shows the effect of early vs. late elimination of volunteer winter wheat and weeds before no-till seeding spring barley in a related study by Cook in 1992.

Fig. 3. Grain yield as influenced by the time interval between glyphosate application and direct seeding of spring barley after winter wheat, 1987, LaCrosse, WA (Cook and Ogg, USDA-ARS, Pullman, WA).

Lind, WA

Cook and Ogg evaluated the effect of glyphosate application timing during the fourth and fifth years of continuous no-till spring barley in a field experiment managed for high levels of root disease, particularly Rhizoctonia root rot. These studies were conducted in 1988 and 1989 at the WSU Dryland Agricultural Research Center near Lind, WA, on Ritzville silt loam soil in a 10 inch annual precipitation zone. The volunteer barley and weeds in the field were concentrated in combine chaff rows. To provide a more uniform "volunteer" stand for the experiment, they broadcasted spring barley (40 pounds per acre) in the fall and applied a sprinkler irrigation.

As an example of the study results, glyphosate plus 2,4-D was applied in the 1988 study at 115 (late November 1987), 10, 2 or % days before seeding. A one-pass double disk drill was used to direct seed spring barley on March 17. Fertilizer shanks deep banded required fertilizer (nitrogen, phosphorus and sulfur) between the rows. Application of glyphosate in late November resulted in significantly less Rhizoctonia root rot, greater plant height and higher yields than when it was applied at any of the spring application dates. Fall glyphosate application treatment yielded almost 53 percent (1,460 pounds/acre) more than the treatment with glyphosate applied 2 days before seeding, 4,235 vs. 2,776 pounds/acre, respectively.

There were no significant differences in root disease, plant growth or yield among the three spring glyphosate treatments applied at 10,2 or Y2 days before planting. The researchers believe that the glyphosate application 10 days ahead of seeding was not early enough to significantly reduce the inoculum level available for attack of the new spring barley crop, due to the unusually high pathogen inoculum level associated with the large root mass from the "volunteer" and weeds, and cropping history which experimentally increased root disease potential.

Fig. 4. This May 1992 picture shows the effect of early elimination of volunteer winter wheat and weeds (right) with glyphosate application 2 weeks before no-till seeding spring barley compared to a glyphosate application 3 days before seeding (left). Severe stand loss and plant stunting from Rhizoctonia root rot occurred with the application just before seeding. The research project is being conducted in a 14-inch precipitation zone near Dusty, WA, by R. James Cook, USDA-ARS, Pullman, WA.

Fig. 5. Schematic representation of the effects of the "green bridge" and timing of glyphosate application before seeding of spring barley on pathogen inoculum potential for Rhizoctonia root rot (Smiley, OSU, Pendleton, OR; Cook and Ogg, USDA-ARS, Pullman, WA).


The researchers offer a schematic representation (Fig.5) to help illustrate how the green bridge and timing of killing volunteer and weeds before seeding spring grains can affect root disease potential, particularly Rhizoctonia root rot. After harvest, pathogen inoculum levels in the soil tends to decrease slowly because of the combination of dry soil conditions, that limit activity of beneficial decomposing microbes, and lack of growing roots of host plants. Pathogen inoculum level begins to increase when volunteer grain and weeds emerge after fall rains. If glyphosate is applied in the fall after most of the volunteer and weed seeds have emerged, pathogen inoculum levels would rise sharply and then decline to a relatively low level by spring seeding time, assuming few additional volunteer and weeds emerged.

If volunteer and weeds are allowed to grow in the fall, the pathogen inoculum level would continue to increase over the winter and into the spring as plant populations and root mass increased. If glyphosate is applied in early spring (at least 2-3 weeks before seeding), the pathogen inoculum level would increase sharply (but temporarily) and then decline again by seeding time. However, if the application was delayed until just 1 to 3 days before seeding, inoculum level would still be high during crop germination and early growth stages, and could result in significant crop damage.

There are a number of factors that influence the importance of early application of glyphosate or tillage to eliminate the effect of the green bridge on root diseases. An important factor is the initial pathogen inoculum level in the soil, determined largely by cropping history. Recropping cereals after cereals favors many cereal root diseases. Another factor is the population of volunteer grain and weeds, and when they are eliminated before the next planting. The higher the population and the longer they are allowed to grow, the greater the potential for disease.

Tillage and planting system choice can influence green bridge impact on root diseases in recropped cereals and the need for early application of glyphosate. Use of some tillage before (or possibly with) seeding generally reduces root disease potential associated with the green bridge. If glyphosate is applied 2-3 weeks ahead of seeding, however, tillage does not appear to provide additional disease control benefits. Furthermore, lower yields have been reported for intensive tillage compared with direct seeding systems in root disease experiments under dry growing conditions.

Research indicates that the green bridge is the primary factor which influences the potential for Rhizoctonia root rot, through increased pathogen inoculum level. In contrast, presence of surface residue in conservation tillage systems has a secondary role in disease development by keeping the surface soil cooler, and wetter and therefore more favorable to the pathogen. With early elimination of the green bridge, and other appropriate pest management strategies, surface residues can increase yield potential by increasing soil water storage. Precipitation amount and seasonal distribution can also influence the importance of green bridge management. Wet winter and spring conditions can increase the potential for root disease when volunteer grain and weeds are present. Mild winters, such as in 1991-92, are highly favorable to growth of volunteer grain and weeds.

In summary, research illustrates that glyphosate application even 2 or 3 weeks ahead of seeding, instead of just before seeding, can make a major difference in the health and yield of spring cereals recropped after cereals under

direct seeding systems. With this earlier application date, tillage before seeding has not significantly increased yields or reduced the level of root disease compared to direct seeding.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.


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