Editor’s note: The following was written by Kyle Broderick, University of Nebraska Extension educator and coordinator of the UNL Plant and Pest Diagnostic Clinic, and Tamra Jackson-Ziems, Extension plant pathologist, for the university’s Crop Watch website.
The cool, wet, extended spring has resulted in many phone calls and questions about poor emergence and unknown symptoms in both corn and soybean.
While some emergence problems may be caused by soilborne pathogens such as Pythium, Phytophthora, Rhizoctonia and Fusarium, many are due to abiotic (non-living) factors such as flooding, soil type and agronomic practices.
One of the easiest ways to predict whether the emergence issues are due to a soilborne pathogen(s) or agronomic factors is to look at where symptoms appear in the field.
Many factors can cause emergence issues. Answering the following questions can provide important information to guide management.
Is it a large area, entire field or a few scattered plants?
Poor emergence due to abiotic factors can cover large parts of the field, with some growers reporting 50-80% of the field affected. When caused by pathogens, poor emergence is often spotty, with healthy stands gradually giving way to symptomatic plants.
Large areas may be diseased, but that often relates to soil type, moisture levels and temperature a few days or a few weeks earlier. For example, fields that had low areas with saturated soil may have experienced poor emergence due to root rotting pathogens.
Did damage develop gradually or suddenly?
Progression of symptoms generally is not observed when symptoms are due to disease and the symptomatic area does not spread. Injury due to biotic factors can be much more sudden and occur after a specific event such as hail, flooding or a chemical application.
Is there a pattern or is it random?
Emergence issues due to abiotic factors generally follow a pattern based on moisture, soil conditions or agronomic practices. Plant diseases tend to develop randomly in non-uniform pockets in the field and may be more severe in low spots of the field where water pooled. Injury due to pathogens often have a noticeable progression as well, with nearby plants becoming infected as the pathogen is able to spread through the area.
Are symptoms associated with field conditions — high or low areas, soil type, etc.?
Many plant pathogens require excess moisture to cause disease and may be associated with low spots that had standing water or heavier soils with poor drainage. If there is poor emergence in a well-drained, high area of the field, damage is less likely to be caused by a plant pathogen and may be related to moisture stress.
The level of organic matter and soil pH can influence the efficiency and breakdown of herbicides leading to injury. Damage caused by herbicide injury would usually be distributed over most or all of the field.