Did the pathogen host the party or just show up?

Corn seedlings planted April 18 with curled leaves and missing plants. Photo by Meaghan Anderson.

Several corn fields in southeast Iowa are having disease issues this spring. After visiting with several ag retailers and farmers, I had Alison Robertson and Tom Kaspar, a plant physiologist with the USDA-ARS, out yesterday to look at some fields with issues. When we put all the pieces together, we have to try to decide whether pathogens, such as Pythium, started the problem, whether they just took advantage of another problem, or whether it was a combination of issues.

With disease issues, “it’s all about the triangle.” The disease triangle describes the three necessary factors for disease development: pathogen presence, a susceptible host, and the right environment. Our spring seemed to be unusually dry in many areas of southeast Iowa, so what made the diseases show up? The environment would be the key to determine when and why the these problems occurred. A significant amount of corn was planted between April 14 and a rainfall that occurred on April 26. This rain came earlier than expected and was on the leading edge of a cold front that stuck around for days. Soil temperatures fell into the mid-50s after being above 60 degrees during the daytime, so it would seem that the cool, wet conditions were there to affect corn planted before that rainfall.

Multiple issues could have made these particular fields more susceptible to disease problems. It appeared that both fields we looked at had some planter setup issues that left open furrows, sidewall smearing, and could have prevented water in the furrow from penetrating deeper in the soil. All fields we looked at also had starter fertilizer in the furrow. We saw very few plants that looked to have obvious starter fertilizer injury, but this is a consideration when we put all the information together. All fields had a cover crop of winter rye. Recent research done by Tom and Alison showed that if rye was terminated less than 7 days before planting corn, there may be an increased risk of seedling disease depending on “the triangle.” The rye in the fields we visited had been terminated 3 to 14 days before planting.

Scouting and management

Notice the difference in aboveground symptoms of the two seedlings in the above picture.  One looks fairly healthy while the other is more yellowed and wilting. Photo by Alison Robertson.

It’s important to scout fields and take stand counts to look for early-season issues and perform a quality control check. Farmers will be looking for wilted or nutrient deficient plants, stand loss, and below ground symptoms like a rotting mesocotyl or browning root tips. Farmers can also collect diseased seedlings to send to the Plant and Insect Diagnostic Clinic in Ames for disease identification. Learn more about sending samples into PIDC here. This may help in choosing the appropriate management tactic for the future.

The seedlings above demonstrate differences in disease development. Notice the difference in browning on the mesocotyls. Photo by Alison Robertson.

With seed and seedling disease, nothing can be done at this point to save the seedlings, but plants with healthy mesocotyls and nodal roots already forming will likely survive.  Some areas may have enough stand loss or unevenness to warrant a replant, but this decision should be made considering the facts of the situation. Use a replant checklist to help make the decision.

Planning for next year

Management of seedling disease is a tricky business. Seed generally goes into the ground with almost 100% yield potential, and farmers can do their best to minimize problems that would reduce that yield potential. Plant high quality, vigorous seed at a depth of 1.5 to 2 inches; choose an appropriate hybrid for the field. Check planter adjustments often and plant into the best seedbed conditions when possible. We've learned that sidewall smearing and compaction can be a problem even if the field seems dry during planting. If using a cover crop prior to planting corn, the safest bet is to terminate the cover at least 7 days prior to planting corn. This may eliminate the risk of seedling disease development.

Fungicide seed treatments are available for management of seedling pathogens during a short window (~2 weeks) following planting. We don't have good information about seed treatment effectiveness for corn. We are quickly learning that fungicide seed treatment effectiveness may vary depending on the particular species of pathogen in a field. The good news is there are new modes of action in the pipeline that will hopefully provide protection against different species.