Sclerotinia spermophila

The pathogen completes its life cycle by forming sclerotia—dense, melanized structures of fungal mycelium that remain dormant in the soil or within the seed lot until favorable environmental conditions return.

In the spring, these sclerotia can germinate to produce apothecia, which release ascospores into the air. These spores are then transported by wind to susceptible host plants, initiating a new infection cycle.

Morphologically, the fungus is characterized by its white mycelial growth under high humidity and the subsequent development of hard, dark sclerotia that often mimic the size and shape of host seeds.

The fungus is highly dependent on environmental factors, particularly moisture and moderate temperatures, which are essential for both the germination of sclerotia and the infection process of the plant tissue.

The primary economic impact of Sclerotinia spermophila is observed in seed production, particularly in clover crops. The pathogen directly infects the flower heads and developing seeds, leading to severe yield losses.

The infection results in the formation of diseased seeds that lose their viability or become shriveled, making them unsuitable for sowing and significantly reducing the marketable value of the seed lot.

Because the sclerotia can survive in the soil for several years, the pathogen creates long-term phytosanitary risks, limiting the farmer's ability to rotate crops effectively on infested land.

The fungus weakens the overall vitality of the infected plants, leading to uneven crop stands, increased susceptibility to other biotic stresses, and potential death of the inflorescences.

Furthermore, contaminated seed lots act as the main vector for the long-distance transmission of the disease, potentially spreading the pathogen to previously healthy agricultural regions.

Visible symptoms often include the premature browning and withering of flower heads during the flowering stage, which is often misinterpreted as drought or physiological stress.

A definitive diagnostic sign is the presence of sclerotia—small, black, hard, irregular bodies—found either within the flower head or replacing the actual seeds inside the fruit.

In highly humid conditions, a white, fluffy mycelial mat may develop on the surface of infected floral parts, providing a clear indication of active fungal growth.

Affected plants may show general chlorosis and stunted growth, but the most significant evidence is found upon detailed examination of the generative organs during the maturation phase.

Seed testing in laboratory settings reveals the internal presence of fungal mycelium and the characteristic structures of the pathogen, confirming the infection status of the lot.

The most critical management strategy is the use of high-quality, certified, and pathogen-free seed material. Mechanical seed cleaning to remove sclerotia is essential for preventing the introduction of the fungus.

Implementing a long-term crop rotation strategy with non-host species is vital to exhaust the population of sclerotia present in the soil, typically requiring a 3 to 5-year break.

Deep tillage (plowing) is a standard agrotechnical practice that buries sclerotia deep enough in the soil profile to prevent the development of apothecia and the subsequent release of ascospores.

Fungicidal seed treatments are recommended to reduce the initial inoculum and provide early protection to emerging seedlings against soil-borne infections.

• Effective weed management to remove alternative hosts.
• Maintaining optimal moisture levels to discourage fungal bloom.
• Using resistant or tolerant crop cultivars where available.
• Strict field sanitation and removal of crop residues.