How to Manage Pests
UC Pest Management Guidelines
Common Leaf spot
Pathogen: Pseudopeziza medicaginis
(Reviewed 3/17, updated 3/17)
In this Guideline:
Symptoms and signs
Symptoms of common leaf spot include small (0.12 inch), circular, brown-to-black spots on leaves. Margins of spots are characteristically toothed or uneven. As the disease progresses, infected leaves turn yellow and drop. In cool, moist weather circular, raised, brown fruiting bodies, called apothecia, are visible within the spots with the use of a hand lens.
Comments on the Disease
Common leaf spot is a cool-season foliar disease that requires moisture. The causal fungus overwinters in undecomposed leaves and leaf debris on the soil surface. In spring, spores are forcibly discharged into the air and some land on alfalfa leaves and initiate new infections.
Start looking for leaf spot in spring. Harvest infected alfalfa early because the severity of this disease increases over time. Although the disease does not kill plants, defoliation reduces vigor, hay quality, and yield. In irrigated fields in California, common leaf spot can cause more leaf loss during drying and harvesting than before harvesting. Fungicides are generally not necessary. Some cultivars may be less susceptible than others.
Crop rotation can reduce inoculum in the field. For more information, see CROP ROTATION.
UC IPM Pest Management Guidelines: Alfalfa
R. M. Davis, Plant Pathology, UC Davis (Emeritus)