An article from the desk of 20/20 Seed Labs Inc.
As soybean production continues to expand across Canada, growers are placing greater emphasis on seed quality and early-season crop establishment. While foliar diseases and root rots often receive the most attention during the growing season, some of the most significant threats to crop performance begin long before plants emerge. Seed-borne diseases can be present on or within the seed itself, reducing germination, weakening seedlings, and creating uneven stands that limit yield potential from the start. Healthy seed is the foundation of a successful crop, making seed health an essential component of soybean production.
In Canada, seed-borne and seed-associated pathogens can affect soybean quality both in the field and during storage. Environmental conditions during flowering, pod fill, maturity, and harvest often play a major role in determining the level of disease present in a seed lot. Wet weather at the end of the growing season can significantly increase infection levels, affecting not only emergence the following spring but also seed marketability and overall crop performance.
Phomopsis Seed Decay: A Major Seed Quality Concern
Among the most important seed-borne diseases of soybeans is Phomopsis seed decay, caused by fungi in the Diaporthe complex. The disease is particularly prevalent when warm, wet conditions occur during pod fill and crop maturity. Infected seed often appears cracked, shriveled, lightweight, or covered with a distinctive white, chalky mould. Even when symptoms are not obvious, infection can significantly reduce germination and seed vigour.
The pathogen can survive on infected seed and crop residue, providing a source of inoculum for future crops. Delayed harvest is one of the primary factors contributing to increased infection because mature seed remains exposed to rainfall and prolonged moisture. As a result, timely harvest and proper seed testing are important management tools for preserving seed quality.
Fusarium: More Than Just a Root Rot
Several species of Fusarium can infect soybean seed and seedlings. These fungi can cause seed decay, seedling blight, and root rot, resulting in poor emergence and reduced plant populations. Infected seedlings often appear weak and stunted, with brown lesions on roots and lower stems. Under cool, wet planting conditions, losses can be significant.
One of the challenges with Fusarium is its ability to survive in both seed and soil. Because many Fusarium species have broad host ranges, crop rotation alone may not completely eliminate risk. High-quality seed, appropriate seed treatments, and favourable planting conditions remain essential components of disease management.
Anthracnose: A Hidden Threat
Anthracnose, caused by Colletotrichum truncatum, is frequently associated with soybean residue but can also be carried on seed. The disease is often most noticeable later in the season, causing dark lesions on stems, pods, and petioles. However, infected seed may lead to reduced germination, poor vigour, and the introduction of inoculum into new production areas.
Warm, humid weather favours disease development, particularly during flowering and pod development. Because infected seed may not always show obvious symptoms, laboratory testing can play an important role in identifying affected seed lots before planting.
Purple Seed Stain: When Quality Suffers
Purple seed stain, caused by Cercospora species, is one of the most recognizable seed diseases affecting soybeans. Infected seeds develop a distinctive purple discoloration that can range from small specks to nearly complete seed coverage. While yield impacts may be limited in some cases, seed staining can reduce seed quality, affect marketability, and contribute to lower germination under severe infection levels.
The same pathogen complex is also responsible for Cercospora leaf blight, making it a concern throughout the growing season. Warm temperatures and high humidity during seed development favour disease infection.
Pythium and Phytophthora: Early-Season Challenges
Both pathogens thrive in cool, wet soils and can cause seed rot, damping-off, root rot, and seedling death before plants become established.
Poor emergence is often the first sign of a problem. In fields with heavy soils or poor drainage, prolonged wet conditions can create ideal environments for disease development. Fungicide seed treatments, resistant varieties, and improved field drainage are among the most effective management strategies available to growers.
Not All Important Diseases Are Seed-Borne
Some of the most economically significant soybean diseases are not primarily spread through seed. White mould, also known as Sclerotinia stem rot, remains a major concern in many Canadian soybean-growing regions, particularly in years with dense canopies and prolonged moisture. However, the disease survives primarily in soil and crop residue rather than being transmitted through seed. Understanding the distinction between seed-borne and soil-borne pathogens helps growers develop more targeted disease management strategies.
Building a Strong Foundation
Managing seed-borne diseases begins long before the planter enters the field. Seed testing provides valuable information about germination, vigour, and the presence of pathogens. High-quality seed, fungicide seed treatments, resistant varieties, crop rotation, and proper harvest timing all contribute to reducing disease risk and protecting stand establishment.
As soybean acres continue to grow across Canada, the importance of seed health will only increase. Whether the challenge is Phomopsis seed decay, Fusarium, Anthracnose, Purple Seed Stain, Pythium, or Phytophthora, investing in seed quality remains one of the most effective ways to protect yield potential and ensure strong crop performance from the very beginning.
When it comes to disease diagnostics for soybeans, 20/20 Seed Labs offers screening for single pathogens and for a more complete picture we recommend adding Complete Disease Diagnostics to your sample.
Visit www.2020seedlabs.ca/services/seed-testing to get started today.
