By Tim Murray, Plant Pathologist
The long, late winter is finally giving way to spring and with that comes field work and the reality of dealing with a compressed field season. Because of that, some are thinking how to get the most done in the shortest amount of time, so they can still make a timely seeding of their spring crop. I recently had a question about herbicide choice for the green bridge and how long to wait before planting; specifically, whether spraying paraquat to get a quick burn-down would shorten the green bridge compared to spraying glyphosate. Our recommendation for green bridge management is to wait a minimum of 2 weeks after spraying before planting.
As a reminder, the green bridge is that time after spraying weeds and volunteer cereals when plants are still alive and able to serve as a bridge to carry soilborne plant pathogens to the next host plant. The soilborne fungi that cause Rhizoctonia root rot and Pythium root rot can build-up on roots of volunteers as they are dying (technically, increased inoculum potential), so it’s critical to allow enough time after spraying for the roots to die, otherwise you will end up with severe disease when Rhizoctonia and Pythium hop onto the new seedling roots.
Back to the original question: Does this recommendation change if paraquat is sprayed instead of glyphosate? The short answer is NO, you still need to wait at least 2 weeks before planting. To get the details on why, I consulted Dr. Tim Paulitz, our USDA-ARS expert on these root diseases, and Dr. Drew Lyon, our resident herbicide expert. Paulitz told me there are no data to support a recommendation on paraquat because the research has been done only with glyphosate. Even though there are no data on paraquat, he wouldn’t change that recommendation because of the way these two herbicides work to kill plants. Lyon confirmed that when sprayed on a plant exposed to light, paraquat acts as a powerful oxidizer that generates reactive oxygen species and quickly destroys living cells of the foliage first, with roots dying later. In contrast, glyphosate is translocated to roots where it inhibits the shikimic acid pathway, which is involved in plant defense. Pathogens like Rhizoctonia and Pythiumare are then able to more easily colonize and kill roots before the foliage dies. The result with both herbicides is that roots survive long enough to serve as a green bridge for a couple of weeks. So, although it’s tempting to spray and plant as soon as possible, it’s better to wait a couple of weeks before planting that spring crop after spraying weeds and volunteers, regardless of which herbicide is used.