June 21, 2013

This is one of two stories in this week's CropWatch looking at potential causes of reduced soybean stands. For information on potential injury from preemergence PPO herbicides, see Complexities of Diagnosing Early Soybean Stand Issues — Herbicides.

Any time a herbicide is used, some level of stress is imposed on the crop. While this may not always show up like a yellow flash from a high rate of Roundup®, adverse environmental conditions can enhance crop stress from herbicide applications. Under optimum growing conditions herbicide stresses can be minimal. In recent days, there have been a number of inquiries about the potential cause(s) of symptoms in seedling corn and soybean. Many of the questions focus on differentiating between plant injury caused by preemergence herbicide applications and seedling diseases.

Seedling Diseases

Figure 1. Poor soybean stand in eastern Nebraska.  (Photo by Haley Oser)

This year, seedling diseases have been common in both soybean and corn. The primary reason for the increase in seedling diseases is early season weather conditions that were favorable for the common pathogens that cause seedling diseases. The most common seedling diseases have been those caused by the Pythium species. These organisms require wet conditions so that their swimming spores can move toward and infect plant roots. Frequent and/or heavy rainfalls and cool temperatures, like we’ve experienced this spring in Nebraska, are very conducive for infection by Pythium species in corn and soybean.

Seedling diseases are often difficult to diagnose because their symptoms are similar to those of several other seedling diseases as well as other problems. Seedling diseases can be confused with insect injury, herbicide damage, planting problems, or environmental stresses that have similar symptoms. Symptoms of seedling diseases include:

• rotted seed prior to germination,
• rotted or discolored seedlings after germination prior to emergence,
• postemergence seedling damping off, and
• root decay.

Scouting — Determine Symptom Distribution

Symptom distribution in the field and on the plants can be the most valuable clues to the cause of the problem. Diseased plants can be identified by their overall wilting and discoloration of leaves. They will have rotted, decaying roots and/or lower stems. Because the organisms that cause seedling diseases survive in the soil, their diseases usually occur in patches in the field — randomly scattered or often associated with low, wet areas. Typically, when chemical injury is the cause, a high percentage of the plants will exhibit symptoms and the distribution will be more uniform across a field or in patterns that may be associated with applications. Injury is not likely to be concentrated in pockets. Diseased seedlings also may be randomly scattered among otherwise healthy plants in the field. Field topography will be key in distribution factors. In very flat fields it is possible to have plants with seedling disease scattered across the field but this is rare.

Carefully dig up symptomatic plants/roots, as any rotted roots may be broken off more easily and lost in the soil. Distribution patterns on the individual plant root systems can offer clues as to the pathogen involved. For example, in soybean damage at the lower portion of the root system can often be associated with Fusarium, while lesions near the soil line are often due to Rhizoctoinia.

We encourage you to correctly identify what’s causing any stand problems to aid with future management actions. While nothing can be done now, variety resistance and seed treatment next year will help if your stands are being reduced from a seedling disease.

For more information on submitting samples, see the UNL Plant and Pest Diagnostic Clinic website and submission form.

Tamra Jackson-Ziems
Extension Plant Pathologist
Loren Giesler
Extension Plant Pathologist