Why do my alfalfa leaves look spotty??

Common leaf spot (Pseudopeziza medicaginis) has been very common in some alfalfa fields this spring due to our wet weather! Symptoms include small circular, brown-to-black spots on the leaves. As the disease progresses, infected leaves turn yellow and drop. Although the disease does not kill plants, defoliation reduces plant vigor, hay quality, and yield, especially with leaf loss during drying and harvesting.

Alfalfa stems damaged by the common leaf spot fungus.

Common Leaf Spot (Pseudopeziza medicaginis) in alfalfa - symptoms are small (.12 inch) circular brown/black spots on leaves, leaves turn yellow and drop.  Brown fruiting bodies (apothecia) can be seen within the spots with a hand lens.

Common leaf spot is a cool-season foliar disease that requires moisture and humidity, which we've had plenty of this year. The causal fungus over-winters in leaf litter on the soil surface. In the spring, under favorable wet conditions, spores become airborne and land on alfalfa leaves where they start new infections. Infections can come on fast, as in this photo of a field in Yolo County that looked fine just over a week ago, then showed damage, likely from rains favoring the disease a few weeks back.

At this time of year there isn't anything one can do for controlling leaf spot except harvest fields. With the onset of hot, dry, windy conditions, the disease will stop progressing, so the second cutting should be fine.

Fungicides are generally not needed for controlling this disease. However, monitor fields early in the spring for leaf spot and treat with Headline or Quadris (following the pesticide label) when the plants are 4-6 inches tall if significant leaf loss is occurring or anticipated.  This could reduce the incidence and impact of the disease. Some alfalfa cultivars may be more susceptible than others, but this is a disease for which ratings are not available as common leaf spot is not part of standard testing protocols for release of alfalfa varieties. 

See http://ipm.ucanr.edu/PMG/r1100611.html  for a complete description of the disease and for management and fungicide ideas.

Field in Yolo County damaged by common leaf spot, May 2017.