Pythium Seedling Blight in Soybeans: Causes, Prevention, and Sustainable Disease Management

PYTHIUM SEEDLING BLIGHT or soybean seedling blight – It is one of the initial and most aggressive soybean seedling diseases which occurs during soybean crop establishment worldwide. The disease is caused by a number of Pythium species (soilborne oomycetes, or water molds), and attack is initially on seed before or immediately after the seeds germinate. After infection, seed can rot in the seed banks or germinate into weak seedlings which die shortly after emergence. This causes uneven stands, decrease in plant population, delays planting canopy, and ultimately a decrease in yield potential for soybeans. The injured plants may not be noticed by the grower as the plants may have poor emergence of seedlings that is noticed when the plant growth is visible throughout the field.
 

Why does Pythium occur in some fields, and not others?

Soybean yields will be impacted due to pythium seedling blight drop-outs in Southeast fields due to environmental factors though not all fields in the region will have a large impact. Pathogen is highly active in cool, wet soils, particularly when wet and cool during planting and germination. Inadequate water drainage, compaction, prolonged periods of wet conditions and irrigation that over-water the soils all provide ideal conditions and lack of oxygen around developing roots, favoring infection.

Low temperatures cause soybean to be delayed in emergence which can increase emergence time for germinated seeds, allowing more time for the pathogen to be able to attack tender tissues. Occasionally these conditions result in soybean damping off of the seedling stage of the soybean crop, one of the earliest symptoms of seedling blight of soybean. The disease pressure is significantly reduced in fields with good drainage, a good soil structure and early field emergence.

How Does Pythium Infect Soybean Seed and Complete its Disease Cycle?

A knowledge of the disease cycle is useful to the grower in determining the best times for the disease to be prevented. The pathogen Pythium is highly winter hardy, found in the form of hardy survival structures in crop residues and soil between growing seasons. These structures germinate in areas of increased moisture and initiate soybeans during planting and produce spores that infect emerged soybeans and early roots.

The pathogen infects the seed on germination, invades the Hypocotyl and young roots and causes retardation of seedling development. The infected seedlings will rot out before they emerge or soon after emergence causing a loss of plants. 

Infestation will happen if, throughout the season, environmental factors are favourable for the survival of the pathogen in soils and crop residues. Therefore, in fields which are subject to frequent wetness and lack good drainage, seedling blight of soybean, soybean damping off and soybean seedling blight are troublesome.

If Pythium were present in the soybeans how would you be able to detect it?

There are various soilborne diseases that can be a challenge to emerge in soybeans and diagnosis is critical. Symptoms of Pythium usually cause irregular areas of missing plants in the field, poor emergence, water-soaked hypocotyls, soft seed decay, brown roots, seedling collapse and seedling mortality.

Pythium fungi affect very young tissues under extreme wetness, as opposed to the sometimes-greening of roots found with Fusarium and sometimes reddish-brown lesions in the stem above soil level noted with Rhizoctonia. Roots that are affected look soft and water-soaked, instead of dry and tight. With the use of a trustworthy soybean seedling disease guide growers will be able to make accurate identification to make better treatment decisions and avoid unnecessary pesticide applications. Early diagnosis also aids in differentiating the soybean damping off due to Pythium from other soybean seedling diseases which have similar above ground symptoms.

Why Is Preventing Stand Loss More Effective Than Treating Established Disease?

Sometimes, soybean seedlings may be killed or suffer severe damage and are unlikely to recover. Once the seedling is dead it can not be replaced, thus stand density is permanently reduced for each loss. There will be lower numbers of plants, higher weed competition, less light interception, which will lead to lower yield and lower farm profitability due to delay in plant population.

Soybean seedling blight is most economically controlled with prevention, because it occurs at the earliest stages of crop establishment. Disease management involves keeping the plants healthy and avoiding disease rather than treating diseased plants. Early losses will be avoided, reducing the long term losses typical with extreme soybean seedling diseases.

How to prevent Pythium seedling blight in the most sustainable way?

Species suppression will require multiple agronomic practices, not just one, over the long-term. Good field drainage to keep moisture out for longer periods of time helps to inhibit pathogen growth and less compaction allows increased root system growth and oxygen in the soil.

Crop rotation helps minimize pathogen pressure over time and planting in favourable soil temperatures gets seeds germinating quickly, decreasing the amount of time seedlings are exposed to disease pressure. Healthy soil biology and use of high quality seed will further contribute to early crop establishment.

Chemical protection of several kinds are still valuable, and many growers have soybean seed treatments as part of a comprehensive disease management program which might include fungicide for soybean seed treatment as well as other soybean seed treatment fungicides. But, in general, if seed treatment is used to achieve chemical protection, plus good agronomic practices, the effectiveness is more predictable and consistent over time.

What are reasons for the wide use of Trichoderma harzianum in biological control of pythium seedling blight ?

Use of biological disease management for soybean seedling blight has become more and more critical in sustainable soybean production. Trichoderma harzianum is one of the most widely researched plant growth promoting fungi which is naturally occurring in the root zone and offers protection against various soil borne pathogens.

The competition for nutrients and space, direct parasitism of the pathogenic fungi by mycoparasitism, antifungal enzymes to inhibit the development of pathogenic fungi and enhanced induced systemic resistance in plants are some of the research conducted by Trichoderma harzianum for soybean seed treatment. It prevents disease, promotes root growth, nutrient absorption and seedling vigor, creating a more healthy and robust plant during the very critical establishment phase.

These benefits have been substantiated by scientific research. Harman et al. (2004) showed the beneficial effect of Trichoderma species on plant growth and biological disease control, and Woo et al. (2014) pointed out their positive involvement in sustainable agriculture, biological control and integrated disease management.

What Value Does Novobac HarzShield Contribute to Sustain a Soybean Stand?

Novobac is engaged in developing microbial and biological crop protection products for the grower based on the sustainable agriculture concept to fight soilborne diseases. Novobac HarzShield for soybean seed treatment, containing Trichoderma harzianum, is a biological agent to colonize in the crop root zone during early crop establishment to help control soilborne plant diseases. The product can be used to protect germinating soybean seed, promote better root development, reduce disease pressure and provide a more effective enhancement of seedling stand establishment in disease prone situations. Soybean producers in both the USA and Canada and other countries including Brazil and Argentina, as well as Europe and India have used HarzShield as a component of their integrated disease management programs to boost crop resilience and lower the need to apply chemicals repeatedly.

An Integrated Soybean Seedling Protection Program - What Does It Look Like?

A  soybean seedling disease guide is considered a resilient soybean establishment program by the use of a series of soybean establishment practices. Healthy soil management enhances soil microbes and root well-being and good drainage will reduce extended wet soil conditions favoring disease development. Some soybean seed treatments include crop rotation that helps to keep pathogens from being established from season to season as well as regular scouting in the fields helps identify new disease issues early on.

Combination of biological seed protection and regular monitoring of diseases, plant species selection and soil care results in more uniform stands of stronger crops. This integrated approach, rather than depending on a single control method, reduces stand loss, promotes sustainable (long-term) soybean production without compromising long-term field productivity by minimizing the effects of soybean damping off and other early season soybean seedling diseases.

Reference:

•  Pimentel, Mirian F., et al. "Reduction of Pythium damping-off in soybean by biocontrol seed treatment." Plant disease 106.9 (2022): 2403-2414.
• John, Rojan P., et al. "Mycoparasitic Trichoderma viride as a biocontrol agent against Fusarium oxysporum f. sp. adzuki and Pythium arrhenomanes and as a growth promoter of soybean." Crop protection 29.12 (2010): 1452-1459.