Purple spot is a fungal infection caused by Stemphylium vesicarium. The pathogen can damage most above-ground parts of asparagus plants, reducing the intake and movement of nutrients. The lesions can also impact the marketability of the crop.
BIOLOGY
The sexual stage of S. vesicariumoverwinters on crop debris from the previous year. During the spring and summer, the fruiting bodies on the crop debris release spores, which are spread through water and wind to new hosts. Early in the year when the weather is wet and cool, purple spot develops on emerging asparagus spears. Asparagus with pre-existing wounds, often caused by wind-blown soil, are more susceptible to infection. However, if the spores stay on wet plant tissue for four or more hours, the disease can infect uninjured asparagus through the stomata. As the fungi mature on the infected asparagus, more spores will be released to infect more crops. Lesions caused by the disease will appear and develop more rapidly when the host is wet.
SYMPTOMS
Purple-brown lesions that are sunken and oval-shaped, most commonly found on the lower stem.
Mature plants may have lesions on the branches and foliage.
Lesions may grow in size and eventually coalesce, causing heavy damage to the branches, foliage, and stem.
GENERAL MANAGEMENT
Volunteer asparagus should be removed from around asparagus fields.
