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

Abstract

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.

Reactive Oxygen Species and Salicylic Acid Mediate the Responses of Pear to Venturia nashicola Infection

“…As a common fungal disease in Asian pear production, scab is still controlled by sterol demethylation inhibitor (DMI) fungicides [48,49]. However, with the widespread use of fungicides, fungal resistance is increasing, and the problem of drug residues is particularly prominent.…”

In Vitro Evaluation of Azoxystrobin, Boscalid, Fentin-Hydroxide, Propiconazole, Pyraclostrobin Fungicides against Alternaria alternata Pathogen Isolated from Carya illinoinensis in South Africa

“…Ergosterol is the most common sterol in fungi because it is an important component of the fungal cell membrane and is essential for fungal growth [ 51 ]. However, the resistance to DMIs has been identified in some phytopathogenic fungal species of Aspergillus , Fusarium , Rhynchosporium, Penicillium [ 52 , 53 , 54 , 55 ], Venturia nashicola [ 56 ], Villosiclava virens [ 57 ], and Cercospora beticola [ 58 ] for their ability to substitute some amino acids, such as CYP51, in the target protein. Future preliminary studies on the molecular characterization of mutational genes, such as CYP51A , CYP51B , and CYP51C , found in filamentous fungi resulting in amino acid substitutions that alter the structure of the CYP51 protein [ 59 , 60 , 61 ] are worth investigating as a tool to better elucidate the resistance mechanism of A. alternata .…”

Emergence of Fungicide Sensitivity in Leptosphaeria maculans Isolates Collected from the Czech Republic to DMI Fungicides

“…In vitro fungicide sensitivity studies are used to determine changes in the sensitivity of fungal isolates to a particular active ingredient over time. Testing is typically carried out using mycelial growth assays where the growth of isolates on fungicide amended medium is compared to the growth on nonamended medium [42][43][44]. This method, though, is usually cumbersome and time-consuming [45].…”