Green
Bridge Management Tips for Fall Seeding
By Diana Roberts
September 2004
It’s been a long time since we’ve had enough moisture in the fall to germinate weed seeds, let alone enough rain to get the new winter wheat crop off to a good start. Yet this year (2004) most farmers have fields already green with volunteer after the soggy harvest, so “green bridge” issues are important to consider.
A green bridge may occur any time one seeds into a field that has, or recently had, green growth from weeds or crop volunteer that forms a “bridge” over which pathogens move from the old crop to the new. Effective green bridge management is important to prevent crop damage from soil borne pathogens or pathogens above the soil surface. The best way to avoid both these situations is to ensure you have at least 10 to 14 “brown days” where there is no green material in the field prior to seeding.
Green bridge below the soil
Soilborne
pathogens such as Rhizoctonia and Pythium live on the roots of many crop
and weed species, and if they transfer on to the roots of germinating
crop seedlings, which are young and vulnerable, they will
establish themselves in the new host and cause a lot of damage and subsequent
yield loss. If weeds and crop volunteer carrying these pathogens are
sprayed with
herbicide, they will die over a period
of days. During this process, the pathogen populations
will increase immensely as they feed and multiply
on the weak, dying plant tissue. The population
will then drop again as the food source diminishes. A classic
green bridge situation occurs if the new crop is seeded and
begins to germinate right as the pathogen population peaks,
when the young seedlings do not have the ability to withstand
infection.
Right now many farmers have fields heavy with lentil volunteer, which is a crop that carries Pythium. If you’re wondering what is the most economical way of seeding winter wheat into these fields, be aware that the highest risk of having a green bridge comes with spraying the field with a burn down herbicide such as glyphosate and seeding directly afterwards. The pathogen populations are likely to peak right as the new crop is germinating. The lowest risk situation is to spray out the weeds and volunteer and leave it for two weeks prior to seeding. Not many farmers want to wait that long to start seeding. Tilling the field is an option to consider as this operation kills the volunteer quickly and aerates the soil, reducing the risk of infection. However, it also buries what little residue is left from the lentil crop and greatly increases the potential for soil erosion.
Another scenario that some farmers are considering is to direct seed into the lentil volunteer without a spray or tillage operation. If the weather stays cool, competition for soil moisture between the new crop and the volunteer should not be an issue. These growers are hoping to save on herbicide costs in the hope that the lentils will die out over the winter, which is likely unless we have a really mild season. Tim Paulitz, USDA-ARS plant pathologist at Pullman, said that if the lentils do survive the winter it will be important to spray them out early in the spring as it then becomes a situation in which the wheat and the volunteer are competing for moisture and nutrients. There is some risk of Pythium infection in this scenario, though using a higher disturbance drill would warm the soil and create some tillage action in the seed row. As the lentils die out, Pythium populations will increase on their roots. The transfer of pathogen will depend on when this occurs and how vigorous the wheat crop is at this point. Paulitz says there are indications that the wheat roots are more resilient to high pathogen populations when soil temperatures are relatively high (around 70 0C) in the fall rather than when the soil is cooler in the spring. Consequently, seeding immediately after spraying is more risky in the spring than in the fall. In any event, including phosphorus as a starter fertilizer will help the wheat plants develop strong, healthy roots early in their development.
Farmers who have a lot of wheat or barley volunteer in their fields should be aware that Rhizoctonia may be an issue. Once again, seeding right after spraying carries the highest risk of incurring a green bridge situation. If some volunteer survives the winter and you decide to spray it out, remember that seeding into cold, wet soil that is heavy with pathogens puts your new crop at a high risk of infection.
A really low input option that some are considering is to leave the volunteer in the field and hope it makes a crop next year. I have seen some pretty good-looking fields managed in this way, but I don’t know how the economics penciled out at the end of the season.
Above ground green bridge
Some farms had hail damage earlier this season, which has the potential to create a green bridge for foliar pathogens. In 1993 a similar situation lead to the first major outbreak of wheat streak mosaic virus (WSMV) in Washington state. That year there was widespread hail in northern Lincoln County when the winter wheat was in the soft dough stage and the grain was physiologically mature. Grain germinated in fallen heads, so there was green volunteer in the fields during harvest.
The wheat curl mite, which is the vector for WSMV, was able to move from green spring wheat on to the green volunteer and then on to the new winter wheat crop that was emerging in adjacent fields. Consequently, the population of the wheat curl mite and the infection rate of WSMV were far higher than normal. According to Clarence Peterson, a former wheat breeder at WSU, WSMV can be found at very low levels across the state each year. It was the unusual sequence of events that year made it far more visible in the 1994 crop.
In order to prevent a recurring WSMV outbreak in areas where there was hail, it is important that farmers break the green bridge by ensuring that they have no green wheat volunteer and weeds in their fields for at least 10 days prior to the emergence of the next winter wheat crop. The wheat curl mite is not closely related to other mites, and traditional miticide sprays are not effective against it, so chemicals are not a management option in this situation.
If you have further questions, contact Diana Roberts at 509-477-2167 or e-mail at robertsd@wsu.edu
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