Fungicide sensitivity profiles of Puccinia hemerocallidis to pyraclostrobin, flutolanil, and thiophanate-methyl

Fungicides are the most effective tools for managing daylily rust, caused by Puccinia hemerocallidis. However, repeated exposure to some fungicide active ingredients can increase risk of reduced sensitivity in target populations. The objective of this study was to determine the sensitivity profiles of P. hemerocallidis to pyraclostrobin, flutolanil, propiconazole and thiophanate-methyl. Thirty five isolates were used to determine the effective concentration of active ingredient to inhibit 50% spore germination (EC50). Isolates were most sensitive to pyraclostrobin with EC50 value ranges of 0.00013-0.00049 μg ml−1 and 0.00013-0.00052 μg ml−1 with mean EC50 values of 0.000284 μg ml−1 and 0.000285 μg ml−1 in experiments one and two, respectively. Isolates were least sensitive to thiophanate-methyl with value ranges of 0.00084-0.0204 μg ml−1 and 0.00082-0.0211 μg ml−1 with mean EC50 values of 0.0096 and 0.0094 μg ml−1. The greatest variation in sensitivity was observed with flutolanil, based on a 36- or 40-fold difference between the highest and lowest EC50 values observed in the two experiments. A dose response was not observed with any isolate to propiconazole. This is the first report documenting the sensitivity profile of the daylily rust pathogen P. hemerocallidis to fungicides.