Soybeans: Seed Rots and Seedling Blights

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Introduction
2. Appearance
3. Disease Cycle
4. Management Strategies
5. Updates on Soybeans: Seed Rots and Seedling Blights
6. Related links...

Introduction

Appearance

It is difficult to distinguish between these pathogens since they seldom produce distinctive symptoms. Seed infected by these organisms can rot in the ground or die shortly after germination. This phase is referred to as "preemergent damping-off," since the seed fails to emerge from the ground, resulting in "gaps" or stand problems. Seedlings that do emerge can be diseased as well. "Postemergent damping-off" symptoms include delayed growth, wilting, dieback and/or death. In most cases, infected seedlings will have a "pinching" or "girdling" of the stem near the soil line. Lesions that appear water-soaked with brown or purple roots or lower stems are often the result of infection by Pythium, Phomopsis or Phytophthora. A reddish or brown lesion near the soil line is characteristic of Rhizoctonia or Fusarium, respectively. Growth and vigour are often reduced in those plants that do survive.

Plate 63. Fusarium root rot causes a brown discolouration of the internal root tissue.

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Disease Cycle

Although these organisms can be seed-borne, they are present in most fields to some degree. Seedling diseases are prevalent under cool wet conditions that keep the soil temperatures below 13°C (55°F). These organisms often survive as saprophytes, living on dead plant material, or as dormant mycelium or spores. Root exudates from germinating seedlings or growing roots stimulate the inactive fungi.

Management Strategies

Low-lying or poorly drained areas of the field are often the first to show disease problems. Increased drainage through tiling will remove excess moisture, increase soil warming in the spring and help maintain a well-drained seedbed. These diseases can be more severe in no-till or reduced tillage fields. Heavy residues keep soil temperatures cooler and wetter longer than conventional fields. Timely planting into good soil conditions will help minimize losses to early season dieback, regardless of the tillage system. When dealing with these diseases, rapid emergence and good early-season vigour are important. Other factors that delay germination and emergence include environmental conditions, compaction, crusting and deep planting. Fungicidal seed treatments increase germination and emergence and therefore are recommended. Delayed germination will also affect the performance of a fungicidal seed treatment. The average fungicide seed treatment provides up to 2 weeks of protection. There is no replacement for timely planting into a good seedbed. For seed treatment recommendations, refer to OMAFRA Publication 812, Field Crop Protection Guide. (Order OMAFRA Publication 812)

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Updates on Soybeans: Seed Rots and Seedling Blights

• Field Crop Protection Guide - Publication 812
  • Seed and Soil-Applied Fungicide
  • Phytophthora Root Rot
  • Rhizoctonia Damping Off
  • Pythium Damping Off
  • Fusarium Seedling Blight

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Related links...

• Diseases
• Soybeans

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