Ramularia leaf spot of sweetclover

Ramularia leaf spot of sweetclover is a fungal disease caused by the pathogen Ramularia meliloti. This infection is commonly observed in various forage legume species, where it targets the foliage, resulting in significant tissue damage and reduced biomass production for livestock feed.

The disease primarily affects plants belonging to the Melilotus genus. Ramularia meliloti invades the leaf tissues, but under favorable conditions, the infection can spread to stems and reproductive structures, causing severe physiological stress to the host plants during the growing season.

Symptoms appear as distinct spots on the leaf blades, which may change in color from pale brown to grayish-tan as they mature. A diagnostic sign of the disease is the presence of a white, mealy mold on the lesion surfaces, consisting of the pathogen’s conidia, which become visible during periods of high humidity.

The environmental requirements for the development of Ramularia meliloti include prolonged periods of leaf wetness and moderate temperatures. The fungus spreads through the air via windborne spores or rain splash, making it highly contagious within dense crop stands. The primary inoculum source is infected plant debris remaining in the field.

Economic damage is caused by the reduction of leaf area, leading to decreased yields and poorer quality of hay. Effective management includes:

• implementing crop rotation to break the disease cycle;
• removing and destroying crop residues;
• ensuring adequate plant spacing to promote airflow;
• utilizing certified, disease-free seed to prevent initial contamination.