A high stubble and weeds' matting around the alfalfa plants also contribute to disease severity. Common, or Pseudopeziza, leaf spot. Caused by the fungus Pseudopeziza medicaginis, this leaf spot occurs wherever alfalfa is grown. The disease appears to be most serious on soils that are acidic or low in fertility. Common leaf spot starts on the lowermost foliage and progresses up the plant.
Later cuttings are usually the one attacked most severely.
In the thickened center of fully developed spots, a tiny raised light brown disk-shaped fungus fruiting body forms on the upper leaf surface. This fruiting body is easily visible with a hand lens or reading glass. Large numbers of microscopic spores are shot into the air during cool to warm wet weather and are carried by wind currents and rainsplash to other plants. Thus, the disease may spread quickly throughout a field. When the spots are numerous, the leaflets soon turn yellow and fall off.