Occurrence and genetic diversity analysis of apple stem pitting virus isolated from apples in China

Incidence and coat protein characterization of apple stem pitting virus isolates from Isparta province of Turkey

Turkey is one of the major countries in the world in terms of fruit growing due to its climate diversity and geographical features. Apple, which is one of these fruits, is grown intensively in Turkey, and especially the Mediterranean Region is important for apple agriculture. Today, viral diseases are considered as an important yield loss factor in apple farming. This study investigated the incidence and molecular characterization of apple stem pitting virus (ASPV) in Isparta province, a significant apple-producing region in Turkey. By using the DAS-ELISA and RT-PCR methods, ASPV infection was found in 7 out of 70 collected apple leaf samples. The partial nucleotid sequences of ASPV were obtained and registered in GenBank for accession numbers. The generated similarity matrix by using the representative isolates revealed that the new ASPV isolates shared 79–93% of their nucleotide sequences with GenBank reference acessions. The isolates collected in this research were clustered in group 1 of the phylogenetic tree that was created by selecting a specific number of isolates from GenBank and thought to be reliable in the phylogenetic differentiation of ASPV. This is the first study to examine the prevalence of ASPV in the Isparta region and its phylogeny. It is possible that the results of the research will contribute to a better understanding of the situation of ASPV in Turkey.

Seed Transmission of Three Viruses in Two Pear Rootstock Species Pyrus betulifolia and P. calleryana

Viral seed transmission causes the spread of many plant viral diseases. Pyrusbetulifolia and P. calleryana are important rootstock germplasms for pear production in China. This study revealed the widespread infection of apple stem grooving virus (ASGV), apple chlorotic leaf spot virus (ACLSV), and apple stem pitting virus (ASPV) in maternal trees of P. betulifolia and P. calleryana by nested multiplex reverse transcription-polymerase chain reaction (nmRT-PCR) assays. Seeds from eight P. betulifolia and two P. calleryana trees had positive rates of 15.9–73.9%, 0–21.2%, and 40.4% for ASGV, ASPV, and ACLSV, respectively. At the cotyledon and 6–8 true leaf stages, seedlings grown from seeds of infected trees gave positive rates of 5.4% and 9.3% for ASGV, 6.7% and 15.6% for ACLSV, and 0% and 2.7% for ASPV, respectively. Incidence in nursery P. betulifolia seedlings of 10.1%, 5.3%, and 3.5% were determined for ASGV, ACLSV, and ASPV, respectively. The nucleotide sequences of coat protein (CP) and movement protein coding genes of both ASGV and ASPV, and CP gene of ACLSV from maternal trees, seeds, and seedlings were analyzed. Sequence identities and phylogenetic comparison with corresponding sequences from GenBank demonstrated that molecular variation occurred within ASGV, ACLSV, and ASPV isolates, with most sequences determined here had close relationships with reported isolates infecting pear or formed independent clades. This is the first report on the seed transmission and the molecular characteristics of these viruses infecting two rootstock species. These findings provided important evidence in management effort for pear viral diseases.

Molecular Characterization of the Coat Protein Gene of Greek Apple Stem Pitting Virus Isolates: Evolution through Deletions, Insertions, and Recombination Events

A RT–PCR assay developed to amplify the full coat protein (CP) gene of apple stem pitting virus (ASPV) was evaluated using 180 Greek apple and pear samples and showed a broad detection range. This method was used to investigate the presence of ASPV in quince in Greece and showed a high incidence of 52%. The sequences of 14 isolates from various hosts with a distinct RFLP profile were determined. ASPV population genetics and the factors driving ASPV evolution were analyzed using the Greek ASPV sequences, novel sequences from Brazilian apple trees and Chinese botanical Pyrus species, and homologous sequences retrieved from GenBank. Fourteen variant types of Greek, Brazilian and botanical isolates, which differ in CP gene length and presence of indels, were identified. In addition, these analyses showed high intra- and inter-group variation among isolates from different countries and hosts, indicating the significant variability present in ASPV. Recombination events were detected in four isolates originating from Greek pear and quince and two from Brazilian apples. In a phylogenetic analysis, there was a tendency for isolates to cluster together based on CP gene length, the isolation host, and the detection method applied. Although there was no strict clustering based on geographical origin, most isolates from a given country tended to regroup in specific clusters. Interestingly, it was found that the phylogeny was correlated to the type, position, and pattern of indels, which represent hallmarks of specific lineages and indicate their possible role in virus diversification, rather than the CP size itself. Evidence of recombination between isolates from botanical and cultivated species and the clustering of isolates from botanical species and isolates from cultivated species suggest the existence of a possible undetermined transmission mechanism allowing the exchange of ASPV isolates between the cultivated and wild/ornamental hosts.

Cryotherapy by encapsulation-dehydration is effective for in vitro eradication of latent viruses from 'Marubakaido' apple rootstock

Apple stem pitting virus (ASPV), Apple chlorotic leaf spot virus (ACLSV) and Apple stem grooving virus (ASGV) are several major viral pathogens of apple trees, responsible for substantial damage to the world's apple industry. This study aimed to evaluate the effectiveness of the encapsulation-dehydration cryopreservation technique to eradicate these viral pathogens from in vitro shoot tips excised from 'Marubakaido' apple rootstock cultures. Axillary shoot tips were excised from in vitro cultures, encapsulated in alginate beads, precultured in MS salts, dehydrated in a laminar flow hood, immersed in liquid nitrogen, then warmed and recovered on medium. After LN exposure, in vitro rooting and acclimatization, recovered 'Marubakaido' plants exhibited 52% survival and 35% regrowth without callus formation. After 8 months of regrowth, PCR analyses revealed that all the plants were free of ACLSV and ASPV, but 2 out of 20 recovered plants were still infected with ASGV. This is the first report in Brazil of the application of cryotherapy to eradicate viral complexes in Malus. Cryotherapy can facilitate the production of virus-free plants by producing high quality plant material.