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.
Highlights
Asian pears such as Japanese pear (Pyrus pyrifolia var. culta) and Chinese pears (P. bretschneideri and P. ussuriensis), distinct from European pear (P. communis), are widely grown in East Asia [1]
Pear leaves or fruit naturally infected with scab were collected arbitrarily from demethylation inhibitor (DMI)-exposed commercial orchards located in Fukuoka and Saga prefectures, Japan, between 2005 and 2018, and 41 to 52 single-spore isolates obtained from each orchard
0.007 ± 0.016 mg/L, respectively, when tested using 56 stock isolates originating from an orchard in China that had not received DMI applications
Summary
Asian pears such as Japanese pear (Pyrus pyrifolia var. culta) and Chinese pears (P. bretschneideri and P. ussuriensis), distinct from European pear (P. communis), are widely grown in East Asia [1]. Scab, caused by the ascomycete Venturia nashicola [2,3], is among the most serious diseases of Asian pears [4] but is not known to occur outside Asia. V. nashicola is a quarantined pathogen in many countries [5,6]. To control this disease, growers largely rely on spray applications of fungicides because there are very few commercially acceptable cultivars with scab resistance [4,7,8]. The frequent use of benzimidazole (MBC) fungicides resulted in resistance development in the pathogen in the mid-1970s [9,10]. Various sterol demethylation inhibitor (DMI) fungicides including triflumizole, bitertanol, fenarimol, hexaconazole, fenbuconazole, difenoconazole and others, targeting the P450 sterol 14α-demethylase (CYP51)
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