A Novel Pear Scab (Venturia nashicola) Resistance Gene, Rvn3, from Interspecific Hybrid Pear (Pyrus pyrifolia × P. communis)

Asian pear scab is a fungal disease caused by Venturia nashicola. The identification of genes conferring scab resistance could facilitate the breeding of disease-resistant cultivars. Therefore, the present study aimed to identify a scab-resistance gene using an interspecific hybrid population ((Pyrus pyrifolia × P. communis) × P. pyrifolia). Artificial inoculation of V. nashicola was carried out for two years. The segregation ratio (1:1) of resistant to susceptible individuals indicated that resistance to V. nashicola was inherited from P. communis and controlled by a single dominant gene. Based on two years phenotypic data with the Kruskal–Wallis test and interval mapping, 12 common markers were significantly associated with scab resistance. A novel scab resistance gene, Rvn3, was mapped in linkage group 6 of the interspecific hybrid pear, and co-linearity between Rvn3 and one of the apple scab resistance genes, Rvi14, was confirmed. Notably, an insertion in pseudo-chromosome 6 of the interspecific hybrid cultivar showed homology with apple scab resistance genes. Hence, the newly discovered Rvn3 was considered an ortholog of the apple scab resistance gene. Since the mapping population used in the present study is a pseudo-BC1 population, pyramiding of multiple resistance genes to pseudo-BC1 could facilitate the breeding of pear cultivars with durable resistance.

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.

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

Scab, caused by Venturia nashicola is one of the most serious diseases of Asian pears including Japanese pear (Pyrus pyrifolia var. culta) and Chinese pears (P. bretschneideri and P. ussuriensis). Breeding of scab-resistant pear cultivars is essential to minimize the use of fungicides and the risk of fungicide resistance developing in the pathogen. A survey of pathogenic specialization in V. nashicola is needed to ensure durable scab resistance in cultivated pears. Race 1, 2, and 3 isolates of V. nashicola, each differing in pathogenicity to Japanese pear cv. Kousui and Asian pear strain Mamenashi 12, have been reported from Japan. In the present study, isolates collected from scabbed pears in China and Taiwan were classified as V. nashicola based on conidial size and mating ability. However, various isolates were found to have pathogenicity distinct from races 1, 2, and 3 in tests on seven differential host genotypes: Kousui; Mamenashi 12; Chinese pear cvs. Jingbaili, Yali, Linyuli, Nanguoli; and Taiwanese pear cv. Hengshanli. The new races were designated as races 4 to 7. Progenies characteristic of race 3 isolates were produced in a cross between race 1 and race 2 isolates, suggesting the possible role of sexual recombination in the emergence of novel races. Japanese pear cv. Kinchaku and cv. Xiangli of P. sinkiangensis (Korla fragrant pear grown in China) didn’t show visible symptoms after inoculation with any of the seven races. The broad scab resistance in Kinchaku and Xiangli makes them a promising genetic resource for resistance breeding programs.

Whole Genome Sequence Resource of the Asian Pear Scab Pathogen Venturia nashicola

Venturia nashicola, the cause of scab disease of Asian pears, is a host-specific, biotrophic fungus. It is restricted to Asia and is regarded as a quarantine threat outside this region. European pear displays nonhost resistance (NHR) to V. nashicola and Asian pears are nonhosts of V. pyrina (the cause of European pear scab disease). The host specificity of these two fungi is likely governed by differences in their effector arsenals, with a subset hypothesized to activate NHR. The Pyrus-Venturia pathosystem provides an opportunity to dissect the underlying genetics of nonhost interactions in this potentially more durable form of resistance. The V. nashicola genome will enable comparisons to other Venturia spp. genomes to identify effectors that potentially activate NHR in the pear scab pathosystem.

High Contiguity Whole Genome Sequence and Gene Annotation Resource for Two Venturia nashicola Isolates

Venturia nashicola is a fungal pathogen that causes Asian pear scab disease. This pathogen is of particular importance in Northeast Asian countries, where Asian pears are grown industrially. Scab disease in Asian pear is currently controlled by fungicide spraying and this situation calls for developing scab resistant cultivars. High-quality genome data are therefore required for in-depth comparative genome analysis of different isolates of V. nashicola and V. pyrina, a closely related species, which only infects European pear plants. Here, we report the high-contiguity whole genome assembly of two V. nashicola isolates, which is expected to enable genome comparisons for identification of the genes involved in host range determination of V. nashicola.