A selective medium for the detection ofAlternaria dauci andAlternaria radicina

Abstract

A selective medium to detectAlternaria dauci andA. radicina on seed, plant debris and other substrates was developed. Growth and sporulation by most undesired organisms was reduced, but adequate mycelial growth and sporulation byA. dauci andA. radicina was maintained so they could be identified by their unusual mycelial growth or characteristic spores. The medium is based upon carrot leaf extract, which promotes profuse sporulation by both pathogens. Glucose, sodium polypectate and mineral salts further enhanced spore and mycelial production. Streptomycin sulfate and metalaxyl (or mefenoxam), combined with either benomyl or thiophanate-methyl, reduced growth and sporulation of unwanted organisms. There were strong interactions among the fungicides, bactericide, and most other medium components. There were also significant effects on the type of mycelial growth produced by some medium components. If the unique, dark mycelia produced by both pathogens is the desired detection method, the medium components must be optimized for eitherA. dauci orA. radicina because the two fungi responded differently in this regard. All medium configurations allowed both fungi to be identified by their characteristic spores. The sensitivity of theAlternaria dauci-radicina selective medium (ADRSM) to detectA. radicina on carrot seeds was similar to other methods presently in use, but ADRSM was more sensitive than the other methods for detectingA. dauci on infested carrot seeds.