Naturally occurring variants of citrus exocortis viroid in vegetable crops

Four viroids were isolated in crops of tomato, eggplant, carrot and turnip. They were detected only after inoculation of 'Rutgers' tomato with nucleic acid extracts from field plants and were characterized as variants of citrus exocortis viroid (CEVd). The size of the variants ranged from 371 to 374 nucleotides and they contained nucleotide exchanges and insertions affecting all structural domains. Symptoms were induced in tomato and carrot whereas eggplant and turnip responded as symptomless carriers. As the samples came from an area where no citrus is grown, it appears that these viroids have a reproductive cycle independent of this crop, and perhaps involving wild plants or other vegetables.     

Correlation of hop stunt viroid variants to cachexia and xyloporosis diseases of citrus

ABSTRACT Citrus viroid (CVd) group II is comprised of hop stunt viroid (HSVd)-related variants of 295 to 302 nucleotides. Included in this group are the cachexia-inducing agents citrus cachexia viroid (or CVd-IIb), CVd-IIc, Ca-903, and Ca-909 as well as the non-cachexia-inducing variant CVd-IIa. The cachexia indexing hosts 'Parson's Special' mandarin and 'Orlando' tangelo as well as Citrus macrophylla responded with symptoms of gumming, discoloration, and stem pitting when infected by CVd-IIb, CVd-IIc, or Ca-903. However, 'Palestine' sweet lime, the indicator host used to describe the xyloporosis disease, displayed a distinctly different fine-pitting reaction and no discoloration or gumming when infected by the same viroids. Cachexia-inducing variants contain a number of nucleotide changes more similar to hop-type HSVd sequences than to the citrus-type HSVd sequences, as typified by CVd-IIa. The nucleotide sequence of CVd-IIc was identical to CVd group II isolates common to trees expressing xyloporosis. Experimental evidence indicates that either CVd-IIb or CVd-IIc can cause citrus diseases known as cachexia and xyloporosis and that the two disease designations reflect the distinct responses of different indexing hosts to the same viroids.     

Molecular characterization and phylogenetic analysis of Citrus viroid I-LSS variants from citrus in Pakistan and China reveals their possible geographic origin

CVd-I-LSS (low sequence similarity), a variant of Citrus bent leaf viroid (CBLVd), was first discovered in Japan, and its distribution is currently limited to Japan and Iran. In the present study, seven CVd-I-LSS isolates were detected from different citrus hosts (Citrus sinensis, C. reticulata and C. limettioides) in Pakistan and China. Genetic diversity analysis of 49 cDNAs of CVd-I-LSS isolates showed that the Pakistan population was more diverse than that tested from Japan or China. Phylogenetic analysis clustered the predominant sequences examined into three main clades. Only sequences from the Pakistan isolates were found in all three clades, suggesting Pakistan may be the original source of CVd-I-LSS. Cultivar import records and the close phylogenetic relationship found between CVd-I-LSS from China and Japan suggested that the viroid isolated from China might have originated from Japan.     

Correction to: Biological,environmental and socioeconomic threats to citrus lime production

Journal of Plant Diseases and Protection - In the original publication, Narouei-Khandan et al.’s reference detail was incorrectly crosslinked with another article and missed to include in the...     

Molecular characterization and phylogenetic analysis of <Emphasis Type="Italic">Citrus viroid VI</Emphasis> variants from citrus in China

Citrus viroid VI (CVd-VI) is a viroid originally found from citrus and persimmon in Japan. We report here the identification and molecular characterization of CVd-VI from four growth regions of China. A total of 90 cDNA clones from nine citrus cultivars were sequenced. The sequence homologies of the Chinese CVd-VI and the reference sequence (NC_004359) vary from 94.2% to 97%. The sequence homologies among the Chinese isolates were up to 95.2%. Phylogenetic analysis of the 23 CVd-VI variants from China and Japan showed that they were grouped into two clades, one with 20 citrus variants and another with three persimmon variants, regardless of the geographic origins. Therefore, as with Hop stunt viroid, CVd-VI could also be divided into two types, citrus and persimmon types. Sequence alignment showed that most nucleotide changes between the two clades occurred in the P, V and T_L domains, and analysis indicated that these mutations influenced the predicted secondary structures under minimum energy.     

Incidence in Cyprus of citrus exocortis viroid and its mechanical transmission

A survey for citrus exocortis viroid (CEV) was conducted in commercial groves of the main citrus-producing areas of Cyprus during 1987–1990. Of 573 trees sampled, representing 25 species and varieties, 506 (88.3%) were found to be infected when indexed by grafting to seedlings of'Etrog' citron (Citrus medica) , selection Arizona 861. Levels of infection by CEV were lower in newer introductions than in introductions made before the 1940s. In greenhouse tests, CEV was readily transmitted from citron to healthy citron and gynura (Gynura aurantiaca) by knife-blade inoculation. Retransmission of CEV from infected gynura back to citron was successful.     

Identification of novel sources of resistance to Pea enation mosaic virus in chickpea germplasm

Chickpea can be seriously affected by Pea enation mosaic virus (PEMV) in the Pacific Northwest region of the USA and other areas of the world. Use of pesticides to manage PEMV vector transmission is ineffective and PEMV-resistant chickpeas have not been identified. Therefore, the Cicer core collection consisting of 499 wild Plant Introduction (PI) accessions and improved cultivars representing 25 countries and two chickpea phenotypes (Desi and Kabuli) was evaluated for resistance to PEMV based on a 1 to 5 scale where 1 = symptomless and 5 = severe mosaic and stunting. Eight accessions were considered resistant to PEMV based on mean disease severity values equal to or less than 1·9 in repeated greenhouse tests. All of the resistant accessions to PEMV were of the Desi type and all originated from Iran or India. PI 450763 was the only Kabuli accession that demonstrated tolerance to PEMV. PEMV resistance was not detected in commercial chickpea cultivars or advanced breeding lines currently grown in the USA and Canada. This is the first report of chickpea germplasm with resistance to PEMV.     

Detached leaf inoculation of germplasm for rapid screening of resistance to citrus canker and citrus bacterial spot

A detached leaf protocol for rapid screening of germplasm for resistance to citrus canker (Xanthomonas citri subsp. citri, Xcc) and citrus bacterial spot (Xanthomonas alfalfae subsp. citrumelonis, Xac) was developed to evaluate limited quantities of leaf material. Bacterial inocula of Xcc or Xac at 10⁴, 10⁵, or 10⁸ cfu ml⁻¹ were injection-infiltrated into the abaxial surface of disinfested, immature leaves of susceptible and resistant genotypes. Inoculated detached leaves were placed on the surface of 0.5% water agar plates and incubated at 28°C under a 12 h photoperiod. Likewise, inocula were infiltrated into attached leaves of greenhouse plants. At high inoculum concentrations of Xcc or Xac (10⁸ cfu ml⁻¹), resistant cultivars of kumquat developed a hypersensitive-like reaction within 3 days post inoculation (dpi). At 10⁵ cfu ml⁻¹, populations 14 dpi were <10⁴ per inoculation site. In canker-susceptible Citrus spp. (‘Duncan’ grapefruit and ‘Rough’ lemon), water-soaked areas occurred by 3 dpi and typical canker lesions developed by 7 to14 dpi. Concentration of Xcc recovered from inoculation sites was approximately 10⁵ cfu ml⁻¹ by 14 dpi. In citrus bacterial spot-susceptible citrus (‘Swingle’ citrumelo and grapefruit), symptoms developed within 7 dpi. Populations of Xac after inoculation at 10⁵ cfu ml⁻¹ were comparable to Xcc in susceptible hosts 14 dpi (>10⁵). The detached leaf assay is useful for the characterization and differentiation of lesion phenotype for each Xanthomonas pathogen permitting rapid screening of germplasm resistance based on the quantification of number of lesions and bacterial concentration.     

国家种质广州甘薯圃病毒种类鉴定及分析

为明确引起国家种质广州甘薯资源圃中病毒病的病毒种类及优势种,为甘薯种质安全保存提供支持,2017年从甘薯资源圃中未脱毒更新的盆栽苗和大田苗中采集155份具有不同病毒病症状的甘薯资源样品,利用PCR和RT-PCR检测技术对这些样品进行了17种病毒的分子检测.155份样品均有病毒检出,包括甘薯羽状斑驳病毒Sweet pot...     

Survey on viroids infecting grapevine in Italy: identification and characterization of Australian grapevine viroid and Grapevine yellow speckle viroid 2

Five viroid species have been reported from grapevine. Hop stunt viroid (HSVd) and Grapevine yellow speckle viroid 1 (GYSVd-1) are distributed worldwide, whereas Grapevine yellow speckle viroid 2 (GYSVd-2), Australian grapevine viroid (AGVd) and Citrus exocortis viroid (CEVd) are found only sporadically. However, the presence of AGVd and GYSVd-2 in several countries, including China, Turkey and Tunisia, suggests a wider dissemination, possibly also in Europe, where AGVd has never been found and GYSVd-2 has been occasionally identified in Italy. Taking advantage of a multiplex RT-PCR assay recently developed for detecting simultaneously these five viroids, vines growing in Italy in commercial vineyards and germplasm collections were surveyed. Besides confirming the widespread presence of HSVd and GYSVd-1 in the field, GYSVd-2 and/or AGVd were identified in two grapevine table cultivars (Sultanina Bianca and Red Globe) from germplasm collections. Tests extended to vines cultivated in southern Italy confirmed the presence of both viroids, which were further characterized. No major sequence divergences between the AGVd and GYSVd-2 variants from Italy and those previously described from other countries were observed. Phylogenetic analysis supported the close relationships among AGVd variants from Italy, Tunisia and Australia. To our knowledge this is the first report of AGVd in Europe and the first molecular characterization of GYSVd-2 isolates from a European country.     

中国锦紫苏类病毒的检测及其分子生物学特征研究

 从无明显症状表现的11株锦紫苏叶片中抽提低分子量的RNA,经Return-PAGE、RT-PCR和Dot-blot hybridization检测,结果表明11株锦紫苏全部带有锦紫苏类病毒(Coleus blumei viroid,CBVd)。将部分PCR产物克隆到pGEM-3Zf(+)载体上并进行DNA序列测定。序列分析结果,所克隆的序列(GenBank登录号分别为DQ178395、DQ178396、DQ178397、DQ178398和DQ178399)与GenBank中报道的锦紫苏类病毒1号(CBVd-1)序列同源性为85.23%~99.20%。从市场上购买的锦紫苏种子经Return-PAGE和RT-PCR检测不携带锦紫苏类病毒。这是中国发生的锦紫苏类病毒的首次报道。     

Resistance to Pseudomonas syringae in a collection of pea germplasm under field and controlled conditions

A total of 242 Pisum accessions were screened for resistance to Pseudomonas syringae pv. pisi under controlled conditions. Resistance was found to all races, including race 6 and the recently described race 8. Fifty‐eight accessions were further tested for resistance to P. syringae pv. syringae under controlled conditions, with some highly resistant accessions identified. Finally, a set of 41 accessions were evaluated for resistance to P. syringae pv. pisi and pv. syringae under spring‐ and winter‐sowing field conditions. R2, R3 and R4 race‐specific resistance genes to P. syringae pv. pisi protected pea plants in the field. Resistance sources to race 6 identified under controlled conditions were ineffective in the field. Frost effects were also evaluated in relation to disease response. Results strongly suggest that frost tolerance is effective in lowering the disease effects caused by P. syringae pv. pisi and pv. syringae under frost‐stress conditions, even in the absence of disease resistance genes, although the highest degree of this protection is reached when frost tolerance and disease‐resistance genes are combined in the same genetic background.     

燕麦种质资源抗白粉病鉴定及利用评价

采用田间自然感病的方法,于2009-2011年在甘肃省天水市甘谷县对128份燕麦品种进行了由白粉菌(Blumeria graminis f.sp.avenae)引起的燕麦白粉病田间抗性鉴定和评价,该地区属于白粉菌病害常发区。结果表明,所有供试材料均程度不同地感染燕麦白粉病,无免疫材料,2份材料‘MF9715’、‘4607’表现高抗;8份材料‘QO245-7’、‘白燕2号’、‘VAO-1’、‘709’、‘4663’、‘4641’、‘4628’和‘青永久307’表现中抗;其余118份材料表现中感、高感和极度感病。说明抗燕麦白粉病的材料严重匮乏,可利用的抗性种质资源相对更少。筛选出的高抗和中抗材料对燕麦白粉病有较好抗性,是今后可利用的抗性种质资源。     

86份贵协系小麦种质资源对条锈病的抗病性评价

小麦条锈病是发生于甘肃陇南麦区小麦生产上的最主要病害,利用和种植抗病品种(系)是防治该病害最经济有效且有利于保护环境的措施。2011-2013年度连续两年在甘肃省农业科学院植物保护研究所甘谷试验站田间对86份来自贵州大学的贵协系小麦种质资源材料进行了成株期抗条锈病鉴定,结果发现仅有‘贵协1’、‘贵协2’、‘贵协3’、‘贵协7’、‘贵协011-2-6’、‘贵协011-3-16’、‘贵协011-3-29’、‘贵协011-3-31’、‘贵协011-3-34’等9份材料表现抗病,且抗病性相对稳定,可作为一线抗源材料在陇南抗病育种中充分利用。     

Resistance to blast (Magnaporthe grisea) in a mini-core collection of finger millet germplasm

Blast caused by Pyricularia grisea [teleomorph: Magnaporthe grisea] is an economically important and widespread disease of finger millet in the world. Host resistance is the most economical and effective means of combating this disease as finger millet is predominantly grown by resource-poor and marginal farmers. At the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), we evaluated a finger millet mini-core collection of 80 germplasm accessions (about 1 % of the total germplasm collection representing major trait variability) for blast resistance both in the field and greenhouse. Field evaluation was done using a refined screening technique that included new improved rating scales for leaf, neck and finger infection. Sixty six of the 80 accessions showed combined resistance to leaf, neck and finger blast in two seasons (2009 and 2010) of field screening. A highly significant and positive correlation was found between neck and finger blast ratings (r?=?0.92), whereas small but significant correlations were found between leaf blast and neck blast (r?=?0.25) and between leaf blast and finger blast (r?=?0.30). These accessions were also screened for leaf blast resistance in the greenhouse by artificial inoculation of seedlings to confirm field observations. Fifty-eight of the 80 accessions were resistant to leaf blast in the greenhouse screen as well. These resistant accessions represented one wild (africana) and four cultivated races (vulgaris, plana, elongate and compacta) of finger millet that originated from 13 countries in Asia and Africa and exhibited considerable diversity for agronomic traits, such as maturity period, plant height and panicle type. These blast resistant accessions from the mini-core collection would be useful in finger millet disease resistance breeding programs.     

Characterization of new sources of resistance to Uromyces viciae-fabae in a germplasm collection of Vicia faba

A collection of 648 accessions of Vicia faba was screened for resistance to faba bean rust ( Uromyces viciae-fabae ). Two distinct types of resistance were identified, both resulting in reduced disease severity (DS) and area under the disease progress curve (AUDPC), but differing in the expression of hypersensitivity. One should be regarded as incomplete nonhypersensitive resistance and the other as incomplete resistance with late hypersensitivity. The hypersensitive resistance, which has not been reported before, was not dependent on temperature or plant age. These two types of resistance were characterized by three macroscopic components of resistance: increased latent period (LP), decreased colony size (CS) and a relatively reduced infection frequency (IF), both on seedlings and on adult plants. LP and CS were the components of nonhypersensitive resistance most highly correlated with DS and AUDPC measured under field conditions. The presence of necrosis was an additional component in the hypersensitive resistant response.