Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu; @Delmarplantdoc
Pythium is a common pathogen we encounter in corn fields throughout Delaware. The pathogen colonizes roots, causing decay of feeder roots. This decay reduces water and nutrient uptake by the seedling and can cause seedling death. Many times seedlings will grow out of initial infections, and no subsequent impacts are noted. However, on occasion the root decay is so severe that seedlings die before emergence or soon after emergence. One way to determine if Pythium may be involved in an emergence or stand issue is to dig out a few affected plants. If the roots are brown and slough off easily in-between your fingers, Pythium may be the culprit. Confirmation can be carried out for you by submitting a sample of the affected root system to the UD Plant Diagnostic Clinic.
As with most seedling diseases, any environmental factor that reduces emergence will favor Pythium development. Most important is the amount of free water in the soil. Pythium thrives in water soaked conditions, as this allows the fungus to grow and produce specialized motile spores, as well as causes stress to the plant, weakening it and allowing for colonization of the pathogen with limited interference. There are numerous species of Pythium that can attack seedling roots, and all of them have different temperature optima. For this reason, water is the key factor impacting Pythium development on corn. Seed treatments containing metalaxyl, mefanoxam, or ethaboxam may provide some benefit in fields that tend to hold water. However, seed treatments will not protect flooded plants nor will they provide protection for more than 2 weeks. Planting into warm, well drained soils that favor rapid emergence is the best way to minimize Pythium issues in your corn.
Figure 1. Corn seedlings showing characteristic symptoms of Pythium infection.