How to Manage Pests
UC Pest Management Guidelines
Purple spot is of major concern when new spears are emerging and being harvested. The pathogen produces elliptical, slightly sunken lesions 0.03 to 0.06 inch across and up to 0.125 inch long. Initially lesions are reddish-purple and later develop a tan-brown center, especially if the lesion is large. Lesions usually appear on the lower half of new spears and are very superficial. The internal tissue of the spear is not affected.
Purple spot is worse following cool, wet weather during spear emergence. The disease is usually most intense where debris from the previous year's fern growth is lying on the soil surface. The sexual stage, Pleospora allii, develops on this debris. Wounding of the spear is not necessary for infection; however, wounds created by blowing sand can increase the incidence of disease.
Once dry weather conditions develop, the disease subsides.
Good field sanitation is important in managing purple spot and other diseases. Last season's fern growth, which is the primary inoculum source, should be burned (where permitted), chopped and incorporated, or removed from the field before new spears emerge. Destroy volunteer asparagus within 400 yards of commercial asparagus fields. One of the best solutions is to incorporate the cut fern with a power driven rotary tiller two times, once in each direction. Chemical treatments are not recommended in California for this disease.
UC IPM Pest Management Guidelines: Asparagus