Detection of Propiconazole Field Resistant Sclerotinia Homoeocarpa Isolates.

Dollar spot (Sclerotinia homoeocarpa) is a major turfgrass disease requiring multiple fungicide applications each year.  The demethylation inhibitor fungicide class (DMI) is the most frequently used class with confirmed resistance to S. homoeocarpa.  Resistance is expressed quantitatively and a clear correlation between in vitro sensitivity and reduced field efficacy has yet to be established for S. homoeocarpa.  The objective of this study was to determine the in vitro propiconazole (DMI) sensitivity of field resistant isolates of S. homoeocarpa.  Isolates were sampled from five locations (JTRF, WBGC, SMCC, HGC and HRCC) before propiconazole treatment and seven days after treatment (0.44 kg a.i. ha^-1).  Propiconazole sensitivity was determined by calculating the relative mycelium growth (RMG) percentage of all isolates on potato dextrose agar amended with 0.1, 0.3, 0.5, and 1.0 μg a.i. ml^-1 of propiconazole.  Isolates sampled from active S. homoeocarpa infection centers seven days after propiconazole treatment were considered field resistant isolates since propiconazole is labeled for a minimum of 14 days control.  Field resistant isolates were sampled from the WBGC, SMCC, HGC and HRCC sites, but not from JTRF.  Ninety five percent of field resistant isolates ranged from 50-100% RMG on 0.1 μg a.i. ml^-1 and 99% of field resistant isolates were capable of growth on 1.0 μg a.i. ml^-1.  Relative mycelium growth ranged from 0-40% on 0.1 μg a.i. ml^-1 for all isolates sampled from JTRF and no growth was observed on 1.0 μg a.i. ml^-1.  Results indicate that S. homoeocarpa isolates with RMG values above 50% on 0.1 μg a.i. ml^-1 of propiconazole or growth on 1.0 μg a.i. ml^-1 of propiconazole are capable of causing infection seven days after application in the field. This qualitative sensitivity assay may be useful for resistance monitoring of S. homoeocarpa populations.