Dr. Alison E. Robertson, Professor and Extension Field Pathologist, Iowa State University, was invited to speak during a Seed World Strategy Webinar called Exploring no-till, cover crops and the relationship with seed treatments. She began by giving a brief overview of past research done that has led up to recent information pertaining to seedling diseases in cover crops.
A study done in 2011-2013, throughout the Midwest, showed Phytophthora spp. was not considered the main source of seedling disease as it had once been 20 years ago. Instead, the main fungal-like organism found in soil was pythium spp. Researchers discovered a total of 82 oomycete species in the soil during two years of totally opposite environments. Of this total only 2 to 4 Phytophthora species were in the mix, with 52 to 57 pythium species found. Not all of these pythium species were considered pathogenic. They also found various communities of pythium species found within different fields and this could be influenced by several factors such as soil temperature, drainage, management, planting date, etc. She also reported that these pythium species varied by latitude.
This research opened Pandora’s box and the next step was to figure out which pythium species were pathogenic. The next research, done in 2016, showed some specific pythium species favored cooler temperatures, while others favored warmer temperatures. Researchers then discovered that seed treatments have different sensitivities to different pythium species. They tested both metalaxyl and ethaboxam, which are common seed treatment components that protect against oomycete fungi. They found, for example, a particular pythium species that prefers warmer temperatures was less sensitive to metalaxyl and ethaboxam during warm conditions. They continue to work on this research.
The next study evaluated temperature swings after planting, which are very common in Iowa, to see if they affected the amount of seedling disease in a field. Researchers found, via greenhouse work, that there was a greater amount of seed disease in soybeans when there is cold stress 1 to 4 days after planting. For example, cold stress reduced emergence by as much as 50%, but the good news is that they saw seed treatments mitigated this stress.
We have always known that corn planted right after winter rye had reduced yields, but no one really knew why this was happening. Researchers discovered that winter rye was a host of the same seedling pathogens, such as pythium and fusarium, as corn. As the rye roots die, pythium spp. can increase in the soil. The objective of recent research was to find out if rye was serving as a green bridge for pathogens that could later infect corn seedlings. Cover crop termination trials revealed that corn planted into rye right after it was terminated had a higher amount of disease compared to corn planted after rye that had been terminated 14 to 21 days earlier. The primary disease isolated from these corn roots was pythium spp., not fusarium spp. I look forward to learning more.