First Incidence of Plum Pox Virus on Apricot Trees in China.

Plum pox disease, caused by Plum pox virus (PPV), is the most severe virus disease of plums, apricots, and peaches. The disease causes heavy losses for fruit growers and the international trade of propagation materials and fresh fruits. PPV was first reported in Bulgaria in 1917 (1). It is now widespread in Europe and has been reported from Cyprus, Syria, Egypt, India, Kazakhstan, Chile, the United States, and Canada. Leaves on symptomatic apricot trees (Prunus armeniaca cvs. Hong Mei and Bai Mei and a selected genotype) in the Hunan Province of China showed typical yellow rings and diffused chlorotic spots. Samples from three suspected trees were repeatedly analyzed using double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and reverse transcription-polymerase chain reaction (RT-PCR) in the summers of 2001-2003. PPV was detected in leaves, bark, and leaf buds of all three trees using ELISA with polyclonal and monoclonal antibodies provided by M. Navratil, Palacky University, Olomouc, Czech Republic (3). The results were confirmed using RT-PCR amplification of a 243-bp of the coat protein gene with a PPV-specific primer pair (2). BLAST analysis of two RT-PCR product sequences (GenBank Accession Nos. AY750961 and AY795603) showed 100% homology to multiple sequences of the PPV-D strain (GenBank Accession Nos. X81080, AF440743, and AF401295). The third sequence (GenBank Accession No. AY795602) had a C at position 112 rather than the T found in the other sequences. The ELISA, RT-PCR, and sequence results indicate that PPV-D was present in the apricot trees. To our knowledge, this is the first indication of PPV occurrence in China. This sporadic incidence of PPV on apricot trees requires addressing problems with the occurrence and spread of plum pox diseases in China and starting an eradication program. References: (1) D. Atanasoff. Annu. Univ. Sofia Fac. Agron. et Sylvic. 11:49, 1932. (2) T. Candresse et al. Phytopathology. 88:198, 1998. (3) I. Hilgert et al. Hybridoma. 12:215, 1993.

Variability and molecular typing of the woody-tree infecting prunus necrotic ringspot ilarvirus.

The 3'-part of the movement protein gene, the intergenic region and the complete coat protein gene of sixteen isolates of Prunus necrotic ringspot virus (PNRSV) from five different host species from the Czech Republic were sequenced in order to search for the bases of extensive variability of viroses caused by this pathogen. According to phylogenetic analyses all the 46 isolates sequenced to date split into three main groups, which correlated to a certain extend with their geographic origin. Modelled serological properties showed that all the new isolates belong to one serotype.

Testing of plum germplasm for sensitivity to plum pox.

A long-term orchard experiment with a broad assortment of plum cultivars aimed to screen their sensitivity to plum pox virus (PPV) was established in 1991. For this purpose, 207 cultivars to be artificially infected with PPV at a permanent site were chosen. The serotype M of PPV from a tree of cv. Domestic Prune, which had not been contaminated by other viruses, was used as a source of the infection. Three buds infected with PPV were budded on 1-year-old trees. In the course of experiment the following results were obtained. The highest transmission of PPV was recorded in the first year after infection, when 69.5% of positive trees were detected by enzyme-linked immunosorbent assay (ELISA). After 4 years, the absence of PPV was still detected in 11.2% of the cultivars. These were reinfected with the same source of PPV in 1996. In 1998, there were 92.9% of trees contaminated by sharka. Seven years after infection with PPV, a dieback of 41 trees took place. In the most cases a presence of an ilarvirus in the plant was detected. The PPV infection was not transferred further on cvs Bila trnecka, Francia Naranes, Large Sugar Prune, Reine Claude Diaphane, Renkloda Jandacek, Scoldus, Tarnina x Kirke, Valasska trnecka and K-4. There were 75% of trees fruited in 1997. Only 28 cultivars had no symptoms of PPV on fruits. A statistically significant relationship between the incidence of PPV after the artificial infection and a the presence of prunus necrotic ring spot virus (PNRSV). The presence of PNRSV reduced the transmission of PPV. Relationships between PPV and prune dwarf virus (PDV), and between PPV and (PDV + PNRSV) were not statistically significant.

Monoclonal antibodies suitable for plum pox virus determination.

Monoclonal antibodies (MAbs) to plum pox virus (PPV) were obtained after immunization of BALB/c mice with purified PPV-W isolate. Spleen cells from a mouse showing a high serum titer were used for fusion with Sp2/0-Ag14 myeloma cells. Culture supernatants were screened for specific antibody production against PPV-W isolate using indirect ELISA. A total of six stable hybridoma lines producing MAbs of IgG class were obtained. All four PPV isolates tested (W, A, D and M) can be distinguished by these MAbs. Two highly efficient antibodies were chosen for practical purposes, and their applicability in PPV diagnostics has been studied since 1989 in parallel with polyclonal antibodies and commercial kits (Boehringer, Bioreba). The experiments have proven comparable sensitivity with a detection limit ranging between 10 and 50 ng of virus per ml sample. In routine diagnostics of plums, peaches and apricots our MAbs ranked at least as high as the commercial kits.