Conditions have been wet and cool across many midwest states this planting season. What does this mean for your planted soybean fields?
Soybean stand loss early in the season is often due to “damping off” which is a broad term which refers to seed and seedling diseases. The four big ones are pythium, phytophthora, fusarium and rhizoctonia. Pythophthora and pythium are often the two most common and troubling for us in our market area. These two pathogens are sometimes also referred to in slang as “water molds” as they thrive in saturated soils with free water. They have spores that can survive in soil and crop residue for long periods of time and when soils become saturated with free water spores can detect plant root exudates. They then literally swim to the root and infect. Quite simply, without wet soils they are not able to readily infect, so in drier years they are typically not an issue.
Stand loss early in the season with early-to-normal planting dates is more typically associated with pythium because it thrives in cold wet soils; while phytophthora infects more readily in somewhat warmer wet soils. With the recent cooler weather, pythium may be the leading candidate as the pathogen causing any stand loss/damping off within fields but an actual lab diagnosis can often be the only way to 100% confirm the pathogen in question as all four of the major soybean damping off diseases can be hard to distinguish from one another with the naked eye and their infection environments can overlap each other.
Many soybean varieties offer native genetic resistance to specific races of phytophthora which is very valuable. However, there are many different races of phytophthora present even within the same field, so one specific phytophthora gene may not always be effective. Partial resistance or “field tolerance” is also a rating which you will see in most product guides which is just as important. The fungicidal components of virtually all complete soybean seed treatment packages also offers a level of protection against the damping off pathogens. However, that protection can simply be overwhelmed under high pressure saturated soil conditions and begins to slowly fade following the first few weeks after planting.
Use of Pre-Emerge Herbicides
Soybean seedlings may also be further stressed when PPO soybean pre-emerge herbicides containing flumioxazin, sulfentrazone, or saflufenacil have been used in conjunction with cool and wet soil conditions. Soybean pre-emerge herbicides containing these actives are quite common in the industry and used on lots of soybean acres as they are generally good at controlling problematic small seeded broadleaves such as marestail and waterhemp. These herbicides can cause some stunting of seedlings most often due to some minor to moderate burning of the cotyledons and hypocotyl as the seedling emerges through the soil/herbicide layer. Cool wet conditions make it harder for the young seedling to metabolize the chemical.
It is quite possible that any fields currently showing damping off symptoms may have more than one thing going on. Variety, pathogen and herbicide may all play a part.
Contact your local Hoegemeyer DSM or Agronomist for more information.