White mold of sunflower head

The causal agent of white mold of the sunflower head is the fungus Sclerotinia sclerotiorum. This destructive disease is a significant threat to sunflower production, particularly in regions with high humidity, causing extensive tissue decay and yield loss during the late reproductive stages of the plant.

While sunflowers are the primary host, this pathogen is highly polyphagous, affecting various broadleaf crops including soybeans, canola, and beans. The disease cycle is driven by sclerotia, which are hard, black resting structures of the fungus that can survive in the soil for many years, acting as a persistent reservoir of inoculum.

The initial signs of infection typically appear on the back of the sunflower head as water-soaked, light-brown lesions. As the fungus advances, the tissue becomes soft and mushy, eventually covered by a characteristic white, cotton-like mycelial growth. Inside the rotting head and on the surface, distinct black sclerotia develop as the fungus matures.

The development and spread of the disease are strongly favored by cool to moderate temperatures and high moisture levels during the flowering stage. Air-borne ascospores are released and land on the heads, where they germinate and colonize the tissues, especially if the plants have sustained mechanical damage or have natural openings due to the stage of maturity.

The economic impact of white mold includes severe weight reduction of seeds, loss of seed viability, and potential total destruction of the harvestable head. Management relies on an integrated approach:

• Utilizing resistant or tolerant hybrids.
• Practicing crop rotation with non-host crops like cereals or corn.
• Deep plowing to bury sclerotia.
• Applying systemic fungicides during the R5.1–R5.5 flowering stages.
• Optimizing plant density to ensure better canopy ventilation.