Cercospora leaf spot of alfalfa

Cercospora leaf spot is a destructive fungal disease affecting alfalfa (lucerne) crops, caused by the pathogen Cercospora medicaginis. This fungus thrives in humid conditions and can significantly impact the quality and quantity of forage yield if not managed properly during the growing season.

The disease primarily infects alfalfa, attacking the foliage of the plant. Under favorable conditions, the infection spreads rapidly throughout the field, leading to premature defoliation. This loss of leaves significantly reduces the nutritional value of the hay, particularly in terms of protein content and digestibility for livestock.

Symptoms typically begin as small, roughly circular brown to black spots on the leaflets, often surrounded by a yellowish halo. As the pathogen matures, these spots expand and coalesce. A key diagnostic feature is the presence of a grayish mold-like appearance on the surface of the lesions, which consists of the fungus's conidia.

The development of Cercospora medicaginis is highly dependent on environmental factors, specifically high relative humidity and temperatures ranging between 20°C and 25°C. The pathogen overwinters on crop residues and infected plant debris remaining in the field, which serve as the primary source of inoculum for the next crop cycle.

Effective control measures focus on an integrated approach:

• Selecting resistant alfalfa cultivars
• Ensuring proper field drainage and airflow
• Removing or burying infected crop residues after harvest
• Using authorized fungicides if the disease threshold is exceeded