Corn planting is underway. Persistent rain has increased the concern about potential seedling diseases and the need to replant. Corn seedlings in DE and most of MD can be affected by two main pathogen groups 1) Pythium spp; and 2) Fusarium spp. Pythium is an oomycete, whereas Fusarium is an ascomycete. Even though these organisms are very different, they can damage corn seedling in similar ways. All of these organisms produce recalcitrant (hearty, tough, long lasting) overwintering structures in soil and litter. When conditions in the soil are wet and the seedling is under stress these structures are able to easily colonize seedling tissues. If colonization is severe, the seedling may never reach the soil surface. This is called PRE-EMERGENT damping off. Other times the seedling emerges, but the pathogen then colonizes root tissues and causes a rapid root decay. This is called POST-EMERGENT damping off.
Both pre and post emergent damping off is facilitated by damp soil conditions. Typically seedling disease issues are not problematic in hot, dry planting seasons. Why does this tend to occur?
Many times saturation of the soil by water is the main cause of damping off and stand issues, with a lack of oxygen in the soil choking off the germinating seedling or newly germinated roots. The stressed, suffocating plant then is easily colonized by damping off organisms that may, under optimal conditions, be considered weakly pathogenic or even saprophytic (living off of decaying or dead plant tissues). The direct effects of stress and a lack of oxygen to germinating or newly-germinated seedlings is one reason that seed treatments may have limited utility for controlling seedling diseases in heavily saturated soils. A second reason for limited efficacy of seed treatments during periods of prolonged soil saturation is the zone of activity of the fungicide as well as the effective duration of control. Seed treatment fungicides may be contact fungicides, which typically only have activity on the seed coat and the immediate seed area. This means that they will not protect seedling roots once the plant has germinated and started to grow. Prolonged, wet periods serve to dilute these compounds in the soil, which may be locked up in organic matter in the soil, limiting their bioavailiblity. Other fungicide active ingredients may be taken up by the plant roots and move within the plant. This allows better overall activity of the fungicide; however, activity typically lasts 10-14 days, depending on the compound and environmental conditions. Other seed treatments, such as those containing biological controls (mostly bacteria) may interfere with pathogen activity by competing with the damping off pathogens for space on the roots, hiding signals that may stimulate germination of spores or preventing growth of the pathogen to the root. As with any biological product, activity is going to be highly variable, even within the field. Remember that these are living organisms and that, just like damping off pathogens, the environment needs to be just right for them to establish properly.
To minimize damping off issues consider the following practices:
- Plant into well drained, warm soil
- Avoid planting excessively early
- Use high quality, fresh seed
- Do not plant too deep
- Consider a seed treatment, but realize it will only have a marginal effect in severely saturated soils