CHALLENGES IN ASSESSING EFFICACY OF POLYOXIN-D ZINC SALT AGAINST COLLETOTRICHUM SPECIES

Polyoxin-D zinc salt was recently made available for disease control of small fruit pathogens in the United States, but very little is known about its efficacy against diseases other than gray mold. In this study, the inhibitory activity of polyoxin-D zinc salt against Colletotrichum species from peach, the causal agent of peach fruit anthracnose, was assessed based on mycelial growth and germ tube elongation assays on four growth media, and in detached fruit assays. EC50 values between isolates of different Colletotrichum species varied within and between media for both mycelial growth and germ tube elongation tests. Occasionally, variability in sensitivity to polyoxin-D zinc salt was also found between isolates within the same species on the same media. Polyoxin-D zinc salt applied on detached fruit at recommended label rates inhibited mycelial growth of C. nymphaeae (100%), C. fructicola (88.50%), C. siamense (78.97%), C. fioriniae subgroup 2 (75.47%), C. fioriniae-subgroup 1 (73.55%), and C. truncatum (58.90%). In our assays, polyoxin-D zinc salt was equal or superior in efficacy compared to penthiopyrad. The results indicate that the EC50 values determined for Colletotrichum species are strongly dependent on the medium used and that none of the media accurately reflected observations on detached fruit assays.