Managing the Effects of the ‘Green Bridge’

Volunteer spring wheat next to emerging winter wheat

Google ‘Green Bridge’ and you may find the descriptions of a number of bridges across the U.S. but not the definition I’m about to give in this context of wheat production. A pretty succinct definition might be ‘the concurrent presence of host plants of different cropping cycle/generations in close proximity’. Simply translated, volunteer spring wheat in late summer/early fall in the same field as emerging winter wheat or corn in close proximity to a field of winter wheat in the fall.

Why do these situations pose a problem? Well, simply put, allowing for a green bridge negates the effect of crop rotation (or in our case the break winter provides) to break disease or pest cycles. And as a result, you create a situation in which disease and pest problems can develop much more quickly.

As it stands, crop rotation (and our winter) is one of the most effective biological control methods we have.  A number of our economically important disease and pest problems are so called obligate parasites, meaning that they can not survive/propagate outside the host plant and they either have no survival structures or survival/dispersal structures that are relatively short lived.  Consequently, the absence of a suitable host is a pretty effective means to either control or at a minimum keep those disease and pest problems at bay.  Examples of such disease and pest problems in wheat are leaf rust, barley yellow dwarf virus, and Hessian fly.

Volunteer wheat provides a bridge of green plant material for these pest and insect problems to get established already in the fall of the same year in the winter wheat. This may affect the survival of the winter wheat stand and reduce yield and grain quality of the winter wheat if left untreated.  More importantly, however, it may provide a local source of inoculum for the spring wheat next season at a time when normally no local population/source of inoculum is present.  This compounds the problem. Not only is the winter wheat crop affected, but now it also impacts the spring wheat crop at a time when it is much more vulnerable to damages.

Currently, the number of acres of winter wheat in Minnesota is dwarfed by the acres of spring wheat.   Nonetheless, vigilance to control volunteer wheat a minimum of 2 weeks prior to planting winter wheat and pushing the planting date of winter wheat to the later half of the planting window is already important enough to make you aware of the risks. After all, the potential rewards of winter wheat are large enough that it would be ill-advised to ‘ban’ the crop to the Nebraska or western South Dakota.