Common leaf spot

Morphology and biology. P. medicagini is a specialized parasite infecting Lucerne only. Small, circular, dark brown to black spots, to 3 mm in diameter develop first on leaves, leafstalks, stems, or developing pods. The spots never merge. A tiny light-brown disk-shaped fungal fruiting body forms in thickened center of completely developed spots on the upper leaf surface. The fungus is known only in ascostages; reports on conidial stages (Phoma medicaginis) are doubtful. Central apothecium is about 350 microns in diameter and daughter apothecia located around it are 180 microns in diameter. Ascus is 70 x 10 microns, clavate, usually containing 8 ascospores. Paraphyses are threadlike, colorless, expanded at ends, to 75 microns long. Ascospores are oviform or elliptical, straight or weakly bent and colorless, unicellular, 10 x 5 microns. Large amounts of spores drift into the air during cool to warm wet weather, being carried by wind flow and rain splash to other plants. Thus, the disease can spread quickly throughout a field. The causal fungus over-winters as apothecia in fallen, un-decayed leaves and leaf fragments on soil surface. It is not known to be seed borne. In spring the apothecia ripen and form ascospores becoming the source of initial infection.