L. P. Wang

Hypovirulence of the Phytopathogenic Fungus Botryosphaeria dothidea: Association with a Coinfecting Chrysovirus and a Partitivirus

ABSTRACT Botryosphaeria dothidea is an important pathogenic fungus causing fruit rot, leaf and stem ring spots and dieback, stem canker, stem death or stool mortality, and decline of pear trees. Seven double-stranded RNAs (dsRNAs; dsRNAs 1 to 7 with sizes of 3,654, 2,773, 2,597, 2,574, 1,823, 1,623, and 511 bp, respectively) were identified in an isolate of B. dothidea exhibiting attenuated growth and virulence and a sectoring phenotype. Characterization of the dsRNAs revealed that they belong to two dsRNA mycoviruses. The four largest dsRNAs (dsRNAs 1 to 4) are the genomic components of a novel member of the family Chrysoviridae (tentatively designated Botryosphaeria dothidea chrysovirus 1 [BdCV1]), a view supported by the morphology of the virions and phylogenetic analysis of the putative RNA-dependent RNA polymerases (RdRps). Two other dsRNAs (dsRNAs 5 and 6) are the genomic components of a novel member of the family Partitiviridae (tentatively designated Botryosphaeria dothidea partitivirus 1 [BdPV1]), which is placed in a clade distinct from other established partitivirus genera on the basis of the phylogenetic analysis of its RdRp. The smallest dsRNA, dsRNA7, seems to be a noncoding satellite RNA of BdPV1 on the basis of the conservation of its terminal sequences in BdPV1 genomic segments and its cosegregation with BdPV1 after horizontal transmission. This is the first report of a chrysovirus and a partitivirus infecting B. dothidea and of a chrysovirus associated with the hypovirulence of a phytopathogenic fungus. IMPORTANCE Our studies identified and characterized two novel mycoviruses, Botryosphaeria dothidea chrysovirus 1 (BdCV1) and Botryosphaeria dothidea partitivirus 1 (BdPV1), associated with the hypovirulence of an important fungus pathogenic to fruit trees. This is the first report of a chrysovirus and a partitivirus infecting B. dothidea and of a chrysovirus associated with the hypovirulence of a phytopathogenic fungus. BdCV1 appears to be a good candidate for the biological control of the serious disease induced by B. dothidea. Additionally, BdPV1 is placed in a clade distinct from the established genera. The BdCV1 capsid has two major structural proteins, and the capsid is distinct from that made up by a single polypeptide of the typical chrysoviruses. BdPV1 is the second partitivirus in which the putative capsid protein shares no significant identity with any mycovirus protein. A small accompanying dsRNA that is presumed to be a noncoding satellite RNA of BdPV1 is the first of its kind reported for a partitivirus.

Actinidia chlorotic ringspot‐associated virus: a novel emaravirus infecting kiwifruit plants

By integrating next-generation sequencing (NGS), bioinformatics, electron microscopy and conventional molecular biology tools, a new virus infecting kiwifruit vines has been identified and characterized. Being associated with double-membrane-bound bodies in infected tissues and having a genome composed of RNA segments, each one containing a single open reading frame in negative polarity, this virus shows the typical features of members of the genus Emaravirus. Five genomic RNA segments were identified. Additional molecular signatures in the viral RNAs and in the proteins they encode, together with data from phylogenetic analyses, support the proposal of creating a new species in the genus Emaravirus to classify the novel virus, which is tentatively named Actinidia chlorotic ringspot-associated virus (AcCRaV). Bioassays showed that AcCRaV is mechanically transmissible to Nicotiana benthamiana plants which, in turn, may develop chlorotic spots and ringspots. Field surveys disclosed the presence of AcCRaV in four different species of kiwifruit vines in five different provinces of central and western China, and support the association of the novel virus with symptoms of leaf chlorotic ringspots in Actinidia. Data on the molecular features of small RNAs of 21-24 nucleotides, derived from AcCRaV RNAs targeted by host RNA silencing mechanisms, are also reported, and possible molecular pathways involved in their biogenesis are discussed.

The first report of leaf spots in Aloe vera caused by Nigrospora oryzae in China.

Aloe vera L. var Chinese (Haw) Berg is a popular ornamental plant cultivated worldwide, whose extracts are used in cosmetics and medicine. Aloe plants are commonly affected by leaf spot disease caused by Alternaria alternata in Pakistan, India, and the United States (1). An outbreak of Alternaria leaf spot recently threatened aloe gel production and the value of ornamental commerce in Louisiana (1). During the summer of 2011, leaf spot symptoms were observed on A. vera plants growing in several greenhouses and ornamental gardens in Wuhan, Hubei Province, China. In two of the greenhouses, disease incidence reached 50 to 60%. The initial symptoms included chlorotic and brown spots that expanded to 2 to 4 mm in diameter and became darker with age. Lesions also developed on the tips of 30 to 50% of the leaves per plant. In severe infections, the lesions coalesced causing the entire leaf to become blighted and die. In September of 2012 and February of 2013, 10 symptomatic A. vera leaves were collected randomly from two greenhouses and gardens in Wuhan. A fungus was consistently recovered from approximately 80% of the tissue samples using conventional sterile protocols, and cultured on potato dextrose agar (PDA). The colonies were initially white, becoming grey to black, wool-like, and growing aerial mycelium covering the entire petri dish (9 cm in diameter) plate within 5 days when maintained in the dark at 25°C. The conidia were brown or black, spherical to subspherical, single celled (9 to 13 μm long × 11 to 15 μm wide), borne on hyaline vesicles at the tip of conidiophores. The conidiophores were short and rarely branched. These colonies were identified as Nigrospora oryzae based on the described morphological characteristics of N. oryzae (2). Genomic DNA was extracted from a representative isolate, LH-1, and the internal transcribed spacer region was amplified using primer pair ITS1/ITS4 (3). A 553-bp amplicon was obtained and sequenced. The resulting nucleotide sequence (GenBank Accession No. KC519728) had a high similarity of 99% to that of strain AHC-1 of N. oryzae (JQ864579). Pathogenicity tests for strain LH-1 were conducted in triplicate by placing agar pieces (5 mm in diameter) containing 5-day-old cultures on A. vera leaves. Four discs were placed on each punctured surface of each leaf. Noncolonized PDA agar pieces were inoculated as controls. Leaves were placed in moist chambers at 25°C with a 12-h photoperiod. After 3 days, the inoculated leaves showed symptoms similar to those observed in the greenhouses. N. oryzae was reisolated from these spots on the inoculated leaves. No visible symptoms developed on the control leaves. The pathogenicity tests were performed twice with the same results. Based on the results, N. oryzae was determined as a pathogen responsible for the leaf spots disease on A. vera. N. oryzae has been described as a leaf pathogen on fig (Ficus religiosa), cotton (Gossypium hirsutum) and Kentucky bluegrass (Poa pratensis) (4), and to our knowledge, this is the first report of N. oryae causing leaf spot disease on A. vera worldwide. References: (1) W. L. da Silva and R. Singh. Plant Dis. 86:1379, 2012. (2) M. B. Ellis. Dematiaceous Hyphomycetes, CAB, Kew, Surrey, England, 1971. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, 1990. (4) L. X. Zhang et al. Plant Dis. 96:1379, 2012.

Molecular and serological diversity in Apple chlorotic leaf spot virus from sand pear (Pyrus pyrifolia) in China

Apple chlorotic leaf spot virus (ACLSV) isolates from sand pear (Pyrus pyrifolia) were characterized by analyzing the sequences of their coat protein (CP) genes and serological reactivity of recombinant coat proteins (rCPs). The sequences of CP genes from 22 sand pear isolates showed a high divergence, with 87.3–100% identities at the nucleotide (nt) level and 92.7–100% identities at the amino acid (aa) level. Phylogenetic analysis on the aa sequence of CP showed that the analyzed ACLSV isolates fell into different clusters and all isolates from sand pear were grouped into a large cluster (I) which was then divided into two sub-clusters (A and B). Sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE), western blot and enzyme-linked immunosorbent assay (ELISA) analyses demonstrated that rCPs of eight ACLSV isolates (PP13, PP15-2, PP24, PP43, PE, PP54, PP56 and ACLSV-C) from two sub-clusters had different mobility rates and serological reactivity. The rCPs of five isolates grouped into the sub-cluster A showed stronger reactivity with antibodies against rCPs of a sand pear isolate ACLSV-BD and virions of a Japanese apple isolate P-205 than that with the antibody against a Chinese apple isolate ACLSV-C. Three isolates grouped into the sub-cluster B showed stronger reactivity with the antibody against ACLSV-C. The antigenic determinants of CPs from these eight isolates and isolates ACLSV-BD and P-205 were predicted. These results contribute to a further understanding of molecular diversity of the virus and its implication in serological detection.

First Report of Cherry necrotic rusty mottle virus on Stone Fruit Trees in China

During the growing seasons of 2010 through 2012, leaf tissues from 206 stone fruit trees, including one flowering cherry, three sour cherry, six nectarine (Prunus persica L. var. nucipersica Schneider), 14 apricot, 24 plum (P. domestica L.), 41 sweet cherry, and 117 peach [P. persica (L.) Batsch] trees, grown in six provinces of China, were randomly collected and tested for the CNRMV infection by RT-PCR. Out of those sampled trees, 37 showed shot holes and vein yellowing symptoms. Total RNA was extracted from leaves using the CTAB protocol reported by Li et al. (2). The primer pair CGRMV1/CGRMV2 (1) was used to amplify a fragment of 949 bp from CNRMV genome, which includes the CP gene (804 bp). PCR products with the expected size were detected in one sweet cherry, one apricot, one peach, one plum, and two sour cherry plants. However, no correlation between PCR data and symptom expression could be found. PCR products were cloned into the vector pMD18-T (TaKaRa, Dalian, China). Three independent clones from each isolate were sequenced by Genscript Corp., Nanjing, China, and sequences were deposited in the GenBank under accession nos. JX491635, JX491636, JX491637, JX648205, and JX648206. Results of sequence analysis showed that sequences of the five CNRMV isolates shared the highest nt (99.0 to 99.6%) and aa (98.9 to 100%) similarities with a cherry isolate from Germany (GenBank Accession No. AF237816). The sequence of one isolate from a peach tree (JX648205) was divergent and shared only 84.7 to 86.1% nt and 94.4 to 95.1% aa similarities with those cp sequences. Clones intra each isolate shared more than 99% nt similarities. To confirm CNRMV infection, seedlings of peach GF 305 were graft-inoculated with bud-woods from a peach and a sweet cherry tree, which was positive to CNRMV and also two other viruses: Cherry green ring mottle virus (CGRMV) and Plum bark necrosis stem pitting-associated virus (PBNSPaV), as tested by RT-PCR. Grafted seedlings were kept in an insectproof greenhouse and observed for symptom development. In May of the following year, some newly developed leaves of inoculated seedlings showed vein yellowing, ringspot, and shot hole symptoms. Results of Protein A sandwich (PAS)-ELISA using an antiserum raised against the recombinant CP of a CNRMV isolate (unpublished) and RT-PCR confirmed CNRMV infection in inoculated trees. In addition to CNRMV, tested seedlings were also found to be infected with CGRMV and PBNSPaV by RT-PCR. To our knowledge, this is the first report on the occurrence of CNRMV on stone fruit trees in China, and also the first record of the CNRMV infection in peach and plum plants. Given the economic importance of its hosts and the visible symptoms of the viral disease, it is important to prevent the virus spread by using virus-tested propagation materials. References: (1) R. Li and R. Mock. J. Virol. Methods 129:162, 2005. (2) R. Li et al. J. Virol. Methods 154:48, 2008.

First Report of Cherry green ring mottle virus on Cherry and Peach Grown in China

Cherry green ring mottle virus (CGRMV; a member of the genus Foveavirus in the family Flexiviridae) has a single-stranded, positive-sense RNA genome of approximately 8.4 kb (4). The viral infection on several Prunus spp. has been mainly reported in Japan, New Zealand, and some countries in Africa, Europe, and North America (3). The virus can cause leaf yellowing on sour and tart cherry. Sweet cherry plants are symptomless hosts of the virus. During the growing season of 2010, leaf samples were collected randomly from one ornamental cherry (Prunus serrulata L.) and 26 sweet cherry (P. avium (L.) L.) plants grown in Shangdong and Henan provinces in northern China and 64 peach (P. persica L. Batsch) plants grown in Hubei Province in central China and tested for the presence of CGRMV by reverse transcription (RT)-PCR. Total RNA was extracted from leaves using the CTAB protocol reported by Li et al (2). Primer set, CGRMV1/CGRMV2 (1), was used for the amplification of a 949-bp fragment, which contains the complete CP gene of 807 bp. PCR products with the expected size were identified in one ornamental cherry, seven sweet cherry, and eight peach plants. Although some of sampled plants showed leaf chlorosis, we did not find the specific association between the symptom and CGRMV infection. The obtained PCR products were cloned into the vector pMD18-T (TaKaRa, Dalian, China). Three independent clones from each isolate were sequenced by Genscript Corp., Nanjing, China. Results showed that CP sequences from the Chinese CGRMV isolates shared 87.7 to 99.8% nucleotide and 93.3 to 100% deduced amino acid similarities, and clones intra each isolate shared more than 99% nt similarities. The CP gene sequences of two representative isolates from cherry (YT-Ch-1) and peach (Pe-HB-18) were submitted to GenBank with Accession Nos. HQ539656 and JF810672, respectively. The neighbor-joining phylogenetic trees generated with nucleotide and amino acid sequences of CP genes by Clustal X v1.8 revealed that all Chinese CGRMV isolates fell into two well-resolved clades. Most of the Chinese CGRMV isolates (12 of 16 isolates, including the isolate YT-Ch-1) were grouped in a large clade represented by isolate ITA5 (GenBank Accession No. AF533159). Four isolates from peach (including the isolate Pe-HB-18) clustered into another clade represented by isolate ITA6 (GenBank Accession No. AF533160). In July 2010, peach GF305 seedlings were inoculated by side grafting with budwoods from two CGRMV positive cherry plants. In May 2011, some newly developed leaves from all inoculated plants showed vein yellowing. The CGRMV infection in these inoculated peach GF305 plants was detected by RT-PCR and protein A sandwich-ELISA using antiserum raised against the recombinant CP of CGRMV isolate YT-Ch-1 (unpublished data). These results further confirmed the CGRMV infection on field cherry plants as detected by RT-PCR. To our knowledge, this is the first record of the presence of CGRMV in ornamental and sweet cherry and peach plants in China, which provides valuable information for further evaluating the sanitary status of the virus in sweet cherry and peach orchards in China. References: (1) R. Li and R. Mock. J. Virol. Methods 129:162, 2005. (2) R. Li et al. J. Virol. Methods 154:48, 2008. (3) K. G. Parker et al. USDA. Agric. Handb. No. 437:193, 1976. (4) Y. Zhang et al. J. Gen. Virol. 79:2275, 1998.

Virulence determination and molecular features of peach latent mosaic viroid isolates derived from phenotypically different peach leaves: a nucleotide polymorphism in L11 contributes to symptom alteration.

Symptoms of chlorosis along leaf edges (chlorosis-edge), along leaf veins (chlorosis-vein) and yellowing on peach leaves have been observed for a long history in the field, while the pathological factor(s) responsible for these symptoms remained unknown. Peach latent mosaic viroid (PLMVd) was detected in the leaves collected from three unique phenotypic peach trees showing above mentioned symptoms. The obtained PLMVd isolates were subjected to population structure analyses and biological assays to evaluate their pathogenicity on peach seedlings in an effort to elucidate the relationship between the PLMVd and the symptoms observed on peach trees in China. In addition, molecular features of PLMVd isolates were analyzed to obtain some insight into the structure-function relationships of this viroid. The results revealed that the symptoms of chlorosis-edge and yellowing were indeed incited by PLMVd, and a direct link between the nucleotide polymorphisms and the symptoms of yellowing and chlorosis-edge was established, i.e. residue U338 responsible for the yellowish symptom and C338 responsible for the chlorosis-edge symptom. This study provides an additional proof to endorse a previous proposal that PLMVd pathogenicity determinants reside in L11. The illustrative etiology of the disease, visualization of the symptoms progression and identification of the unique single nucleotide polymorphism possibly involved in the symptom induction will significantly increase understanding of the pathogenic mechanisms of PLMVd and will help in designing control strategies for the resulting disease.