Plenodomus meliloti

Plenodomus meliloti is a specialized fungal pathogen belonging to the kingdom Fungi, class Dothideomycetes. It is widely recognized as the causative agent of black stem and leaf spot diseases affecting various forage legumes, most notably sweet clover (Melilotus) and alfalfa (Medicago sativa).

The primary symptom of infection is the appearance of small, dark brown or black spots on the stems, petioles, and leaves of the host plant. As the disease progresses, these lesions enlarge and coalesce, often girdling the stems, which leads to necrotic tissue formation and severe chlorosis of the foliage.

The life cycle of the pathogen is centered on the persistence of pycnidia within crop debris. During favorable environmental conditions, spores are released and dispersed by rain splash and wind currents. The fungus primarily overwinters on infected plant material left on the soil surface, providing a source of inoculum for the next growing season.

Environmental conditions characterized by cool, wet weather in the spring and early summer are highly conducive to disease development. Dense canopy structures enhance the spread of Plenodomus meliloti by retaining moisture at the base of the stems, creating an optimal microclimate for fungal proliferation.

The economic impact of this pathogen is significant, as it reduces both the yield and the quality of forage crops. Severe infestations result in stem breakage, stunted plant growth, and a reduction in protein content. Furthermore, infected fields may experience reduced winter hardiness and premature plant death.

• Implementing crop rotation with non-host species for 3 or more years.
• Using fungicide seed treatments to minimize early-season infection.
• Removing and destroying infected crop residues after harvest.
• Managing plant density to improve air circulation and reduce humidity within the canopy.