White mold of stem

The initial symptoms include the appearance of water-soaked, light-brown spots on the stems. These lesions expand rapidly, and within a short time, they become covered with a dense, white, cottony fungal growth, which is the hallmark of the infection.

Inside the infected stem tissues and sometimes on the exterior of the cottony growth, large, black, irregular sclerotia develop. These bodies are essential for identifying the presence of Sclerotinia in the field.

As the disease progresses, the plant's vascular system is compromised, leading to sudden wilting and eventual death of the affected plant. The foliage often turns chlorotic before collapsing.

Stem lodging is a common sign of late-stage infection. The stem becomes fragile and brittle at the site of the lesion, often breaking under its own weight or during windy conditions, causing significant structural failure.

In dry conditions, the fungal mat may desiccate, leaving the stem bleached and fibrous. This "shredded" appearance of the stem is a classic diagnostic feature observed during the harvest stage.

White stem rot, commonly known as sclerotiniosis, is a widespread fungal disease caused by members of the Sclerotinia genus, most notably Sclerotinia sclerotiorum. It is a polyphagous pathogen, meaning it can infect hundreds of different plant species, including sunflowers, oilseed rape, soybeans, and various legumes.

The fungus is characterized by its ability to produce sclerotia — hard, dark, seed-like structures of fungal mycelium. These sclerotia can survive in the soil for many years, serving as the primary source of inoculum for future infections in the crop rotation.

The infection process occurs via ascospores dispersed by wind or through direct infection from germinating sclerotia in the soil. The pathogen secretes enzymes that break down plant tissues, allowing the mycelium to invade and colonize the host systemically.

Because the pathogen is not host-specific, it thrives in agricultural systems that frequently use susceptible broad-leaved crops. Its biology allows it to strike at various stages of plant development, from seedling emergence to maturity.

The rapid spread of the disease is facilitated by the pathogen's ability to maintain high infectivity levels in organic debris, making it a persistent challenge for modern sustainable agriculture.

White stem rot thrives under conditions of high humidity, typically above 80%, and moderate temperatures ranging from +15°C to +25°C. Wet weather during the flowering period is particularly conducive to infection.

Poor aeration in dense crop canopies creates a microclimate that prevents moisture from evaporating, providing the perfect environment for spore germination and rapid mycelial colonization.

High levels of soil moisture, especially during the formation of apothecia (the fruiting bodies of the fungus), significantly increase the risk of disease spread within the field.

Mechanical damage from agricultural machinery or pest activity facilitates entry points for the pathogen, allowing it to penetrate healthy tissue more effectively than it could on intact surfaces.

A history of susceptible crops in the field creates a high soil inoculum density. Without sufficient intervals between host crops, the disease pressure remains high, leading to frequent outbreaks.

The primary damage caused by white stem rot is a significant reduction in overall yield. Infected plants often fail to reach full maturity, resulting in poor grain filling and diminished oil content in oilseed crops.

Lodging caused by the weakened stem structure makes mechanical harvesting extremely difficult and leads to increased yield losses on the field floor. Additionally, infected plants are more susceptible to secondary pathogens.

The quality of the harvest is degraded by the presence of sclerotia in the grain. This contamination is often rejected by processing plants and poses a severe risk for long-term storage due to the potential for mold development.

Because sclerotia can be transported with seeds, the disease can spread to previously uninfected areas, threatening the long-term viability of high-value agricultural zones.

The economic impact also includes the cost of intensive fungicide applications, which are often required when weather conditions become favorable for the disease during critical plant growth stages.

Effective management begins with strict crop rotation. It is recommended to avoid planting susceptible host crops in the same field for a period of at least 6 to 8 years, alternating with non-host crops like cereals.

Using certified, disease-free seed is crucial to prevent the introduction of sclerotia into the soil. Seed treatments with appropriate fungicides can help protect seedlings during the early establishment phase.

• Maintaining optimal plant density to improve air circulation and reduce canopy humidity.
• Rigorous control of weeds that can serve as alternative hosts for the pathogen.
• Deep plowing to bury sclerotia at depths where they are less likely to germinate or survive.
• Application of foliar fungicides during the critical flowering stage when infection risk is highest.

Developing and deploying resistant varieties is the most sustainable long-term solution. Breeding programs focused on Sclerotinia resistance provide farmers with the best chance to minimize losses without relying solely on chemical intervention.