Stagonospora leaf spot of sweet clover

The pathogen responsible for this disease is the fungus Stagonospora meliloti, which belongs to the Dothideomycetes class. It is a specialized phytopathogen that affects various species of the Melilotus genus.

The fungus propagates through the formation of pycnidia, which are small fruiting bodies embedded within the host plant tissues. Inside these structures, conidia are produced and released for dissemination.

The pathogen overwinters primarily as mycelium within infected crop residues, as well as on the stems and crowns of living sweet clover plants.

Identification typically involves microscopic examination of the spots on leaves and stems to detect the presence of characteristic black pycnidia, which indicate the pathogen's activity.

Warm temperatures combined with high humidity are the primary drivers of the disease, facilitating spore germination and the infection of healthy plant tissues.

The primary host for this fungus is sweet clover (Melilotus), a crop widely used for forage and as a green manure crop in agricultural systems.

The disease causes premature leaf senescence, which significantly reduces total forage biomass and diminishes the quality of hay or silage harvested from the field.

Heavy infections weaken the plant, resulting in stunted growth and reduced vigor, which negatively impacts the plant's ability to survive the winter season.

The loss of foliage directly affects the plant's capacity to produce seeds, leading to lower seed yields and poor seed quality for future planting.

Damage to the crown and stem base can result in thinning stands, creating opportunities for weeds to invade the field and for secondary pathogens to take hold.

Symptoms of the disease usually emerge in the spring as plants begin their active growth phase following winter dormancy.

The spread of the infection is most rapid during periods of frequent rainfall and moderate summer temperatures, which support the transmission of spores via water splashes.

Conidia are spread throughout the field by rain and wind, leading to repeated cycles of infection on healthy leaves and stems during the growing season.

As the season progresses, the rate of new infection may fluctuate with weather conditions, though the fungus continues to colonize stems for survival.

A warm and moist autumn can extend the infection period, allowing the pathogen to build up a significant inoculum load for the upcoming growing cycle.

The disease manifests as small, circular or irregular spots on the leaves and stems, usually appearing brown or grey with dark margins.

A diagnostic feature is the appearance of small black dots, known as pycnidia, located in the center of the necrotic spots, which can often be seen with a magnifying glass.

As the infection advances, these spots may coalesce, covering larger areas of the leaves and leading to chlorosis, leaf yellowing, and eventually leaf drop.

Stem lesions can also form, which may interfere with the plant's vascular system, potentially causing wilting and dieback of the upper parts of the plant.

• Brown or greyish leaf spots with dark borders.
• Visible black pycnidia within the necrotic leaf tissue.
• Chlorosis and premature yellowing of lower leaves.
• Necrotic stem lesions and reduced plant vigor.

Crop rotation is the most effective cultural practice, ensuring that sweet clover is not replanted in the same field for at least 3–4 years.

Deep plowing and thorough incorporation of crop residues into the soil help to accelerate the decomposition of infected material and reduce the primary inoculum.

Using resistant or adapted varieties of sweet clover is a critical strategy to maintain productivity and minimize the impact of the disease without chemical intervention.

For seed-producing fields, the application of fungicides during periods of high disease pressure may be necessary based on field scouting and local disease risk assessments.

Timely harvesting or mowing of the forage crop before the disease reaches a critical level can help manage the infection and preserve the quality of the harvested biomass.