Purple Spot Disease of Asparagus
Purple spot disease on asparagus spears and fern is caused by the fungus Stemphylium vesicarium. The fungus survives the winter as sexual spores (ascospores) in a sac (ascus) produced in overwintering structures (pseudothecia) that appear as small black dots on asparagus debris from the previous season. The ascospores are released from the ascus by rain, and can be carried by the wind to newly emerged asparagus plants, where they cause the primary infection of the growing season. These new infections result in spores (conidia) produced by an asexual process, which in turn can cause secondary infections, a process that is repeated as long as temperatures and rainfall are favorable.