Pathogenic Specialization of Venturia nashicola, Causal Agent of Asian Pear Scab, and Resistance of Pear Cultivars Kinchaku and Xiangli

Interspecificity of Scab Fungus Isolated from Wild Syrian Pear (Pyrus syriaca)

Scab on pear is caused by two pathogens, Venturia pyrina on European pear and V. nashicola on Asian pear. Five races of V. pyrina and seven races of V. nashicola have been reported thus far and pathological specialization occurs in both species. Among them, the five race isolates of V. pyrina were previously found from wild Syrian pear. In this study, mating and morphological characteristics of Venturia isolates from Syrian pear were compared with those of isolates from European and Japanese pear cultivated in Japan. The results from mating experiments showed that Syrian pear isolates were compatible with European pear isolates of V. pyrina to produce ascospores but were sterile with V. nashicola isolates in culture. Interestingly, however, the size and shape of conidia collected from naturally infected leaves of Syrian pear resembled those of V. nashicola. This finding may open the way to study coevolution between pear hosts and Venturia spp. in the future.

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.