DMI-Fungicide Resistance in Venturia nashicola, the Causal Agent of Asian Pear Scab—How Reliable Are Mycelial Growth Tests in Culture?

Scab, caused by Venturia nashicola, is among the most serious diseases of Asian pears and control of this disease largely relies on sterol demethylation inhibitor (DMI) fungicides. However, pear growers have complained about field performance of DMIs since the mid-2000s. In this study, to evaluate pathogen sensitivity, mycelial growth tests and inoculation tests were conducted using DMI-amended culture medium and fungicide-sprayed potted pear trees, respectively. Results confirmed distribution of isolates resistant to fenarimol, hexaconazole, and difenoconazole in the field populations. Importantly, results from tests in culture did not fully correlate with those from tests in planta. Due to phenotypic instability of resistance and poor sporulation of this pathogen in culture, resistance is generally assessed by laborious and time-consuming inoculation with conidia collected from a field. To improve the result interpretation from in vitro tests, the isolates were genotyped: the CYP51 gene which encodes the target sterol 14α-demethylase was sequenced and various mutations have been detected in the coding sequence of DMI-resistant isolates. In addition to the detected single nucleotide polymorphisms, alternative mechanisms, not based on changes in the structure of the target protein, may also increase DMI resistance. Development of molecular methods for the diagnosis of DMI resistance seems to be challenging in V. nashicola.