Purple Spot Disease of Asparagus
(text from Mary K. Hausbeck, Professor and Extension Specialist, Michigan State University, Department of Plant Pathology, E. Lansing)
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 favourable.
The emergence of purple spot as a significant problem in the production of asparagus in Michigan may be due to the adoption of a no- till cultural system, whereby the dried fern from the previous season is chopped in April and left on the soil surface. This residue persists through the harvest season (mid-June) and is visible through the fern growth period (late June to September) and is the source of ascospores which start infections early in the growing season. Symptoms of the disease are: The disease appears as numerous, slightly sunken, purplish spots with brown centers occurring on harvested spears and fern. Lesions on spears are often found on the windward side, because blowing sand causes wounding which favours infection. During epidemic years spotting can occur on 60-90% of the spears and may result in rejection of the crop, especially for fresh market sales. Spots also occur on the asparagus ferns, affecting the main stem, secondary branches and needles (cladophylls). Severe infection of the fern can result in premature defoliation of the plant. Increase in the severity of purple spot disease is associated with extended periods of rainfall, fog or dew.