Asparagus is a perennial crop that should have a productive life of 15 or more years. Asparagus spears grow upward through the soil from underground crowns. The spears are cut by hand every 2-5 days. After two to three months, spear harvest ceases and the spears are allowed to grow into ferns with a thick canopy of feathery leaves. During the fern stage, the plant produces and stores energy for the following year’s crop. Purple spot on asparagus was first reported in the U. S. in 1981. The main damage from purple spot is on fern growth. Fungal damage to the ferns results in defoliation of the needles which reduces the flow of carbohydrates to the roots and lowers next years yield.
“Purple spot of asparagus is an important fungal disease in many growing regions including Michigan, California, Washington, England, and New Zealand. The fungus overwinters as pseudothecia on the surface of asparagus fern debris and releases ascospores as primary inoculum in the spring.
Purplish lesions may subsequently develop on the spears, making them commercially unacceptable, especially for the fresh market… Following infection, tan to brown lesions may develop on the fern and premature defoliation may occur. Premature defoliation of the fern limits its photosynthetic ability, resulting in decreased carbohydrate reserves for the crown that may negatively affect yields in the following years.
Growers apply fungicides to manage purple spot on the fern after the last spears are harvested; fungicides are not applied to asparagus spears that will be harvested.”
Authors: Granke, L. L., and M. K. Hausbeck.
Affiliation: Department of Plant Pathology, Michigan State University
Title: Influence of Environment on Airborne Spore Concentrations and Severity of Asparagus Purple Spot.
Source: Plant Disease. 2010. 94:843-850.