AN INDEX TO ASSESS AND MONITOR THE PROGRESSION OF WASTING DISEASE IN EELGRASS ZOSTERA-MARINA

An index based on a rapid visual determination of the relative amount of necrotic tissue on eelgrass shoots infected with wasting disease is described. The utility of the index for assessing and monitoring disease levels in natural and experimental populations of eelgrass is illustrated with examples from the field and a mesocosm experiment. Using the Wasting Index, monitoring of the disease along with environmental variables provided correlational data that aided understanding of the disease progression and linked disease severity with salinity. Once salinity increased above d threshold, disease spread rapidly and was inversely correlated with leaf area (infection phase). Following the initial peak of disease, the Wasting Index was positively correlated with leaf area (wasting phase). Declines in salinity below the threshold (caused by precipitation or spring runoff) allowed eelgrass recovery and reset the host-pathogen system.